native/docs/device-sync.md

Device sync: contacts and calendars in Android's providers

Issue #34. Each signed-in account can opt in to device sync. Its address books then appear in Android's Contacts provider and its calendars in the Calendar provider, under an Android account of type "Bulwark". Google Contacts and Calendar, AOSP/Fossify Contacts, Etar, the dialer and Settings → Accounts all see them. Server changes reach the device, and creates, edits and deletes made in any Android app reach the server.

This document is for maintainers. It describes the design: the architecture, what every provider column holds, the mapping tables, the merge rules, the native API, the failure matrix, the known limitations and what was checked on devices. Behaviour it relies on was checked against Stalwart v0.16.23 (the server Bulwark targets) and AOSP's ContactsProvider, CalendarProvider and the calendar/contacts apps; the notable findings are quoted where they shape a rule.

Contents

1. Architecture
2. The Android shell
3. Device data model
4. A sync run
5. Change detection and merge rules
6. Uploads
7. Contacts mapping
8. Calendar mapping
9. Headless payload, report and SyncResult
10. Native module API
11. App integration
12. Failure matrix
13. Limits and performance
14. Testing
15. Decisions and limitations
16. Out of scope
17. Verification

Architecture

The Android sync framework drives everything. Kotlin is a thin shell; the sync engine is TypeScript that runs as a headless JS task and reaches the providers through a native module.

SyncManager ──onPerformSync──▶ DeviceSyncAdapter (Kotlin, SyncAdapterThread)
                                 │ posts to the main thread: start React Native if needed,
                                 │ start headless task BulwarkDeviceSync {runId, …}
                                 │ waits on a latch (≤ 9.5 min, 2.5 min for expedited jobs)
                                 ▼
                        runDeviceSync (TypeScript, JS thread)
                          │ JMAP: a detached JMAPClient per registry account
                          │ provider: NativeModules.BulwarkDeviceSync (query / applyBatch)
                          ▼
                        finishRun(runId, report) ──▶ latch released ──▶ SyncResult

Why this split:

• Auth, token refresh, mTLS, 429 back-off and session discovery stay in the existing JS client (src/api/jmap-client.ts). Refresh tokens rotate; a second owner outside JS's de-duplication could evict the account.
• Mappers and merge rules are pure TypeScript with no React Native import, so vitest covers them with golden fixtures and property tests. A test walks the import graph of every module in src/device-sync except the React Native boundary (native.ts, task.ts, jmap/client-port.ts and app/) and fails on react-native, expo, stores or jmap-client: the global mocks in src/test-setup.ts would otherwise hide such an import.
• Kotlin only translates: URIs, ContentProviderOperations, SyncResult. Its logic has JVM unit tests (android/app/src/test).

A pure-Kotlin engine was rejected: it would have to redeem rotating refresh tokens outside JS's de-duplication, re-implement session discovery with its origin rewrite, mTLS, 429 handling and the first-touch gate, read expo-secure-store's internal format, and start from zero test infrastructure.

There is one ReactHost and one Hermes runtime per process. A sync that runs while the UI is open runs in the UI's runtime (the task is started with allowedInForeground = true), and a sync that cold-started the process leaves its runtime for the UI to attach to. So the engine never touches the UI's jmapClient singleton or zustand stores it does not own, and a module-level mutex coordinates runs with app-side operations.

Code map

Path	What it holds
android/app/src/main/java/com/anonymous/bulwarkmobile/HeadlessJs.kt	starting React Native and a headless task; shared with the push job
android/app/src/main/java/com/anonymous/bulwarkmobile/sync/	authenticator, sync adapters, run registry, report mapping, native module, provider op translation, push router
android/app/src/main/res/xml/{authenticator,sync_contacts,sync_calendar,contacts}.xml	framework metadata
android/app/src/debug/java/…/sync/DeviceSyncDebugReceiver.kt	adb hooks for device tests (debug builds only)
src/device-sync/types.ts, android-columns.ts	contracts: bridge API, row model, ops, payload, report; provider column names
src/device-sync/planner.ts, wire.ts	the contract between the engine and the two planners; JMAP objects as Stalwart sends them
src/device-sync/native.ts	typed wrapper around NativeModules.BulwarkDeviceSync
src/device-sync/common/	pure helpers: identities and minted keys, canonical JSON, hashing, JSON Pointer patches
src/device-sync/contacts/	JSContact ↔ contacts rows, row matching, contact merge rules (pure)
src/device-sync/calendar/	JSCalendar ↔ calendar rows, RRULE, exceptions, zoned time, durations, merge rules (pure)
src/device-sync/jmap/	the JMAP side on a detached client: session accounts, /get, /changes, /query paging, /set, uid lookup, the auth rebuild
src/device-sync/engine/	orchestration: run entry, mutex, checkpoints, batches, SyncState, poison markers, default selection, teardown, report
src/device-sync/task.ts	the headless task entry (registered in index.ts) and the teardown the app calls
src/device-sync/app/	the app side: turning sync on and off, sign-out, reconciling Android's accounts, triggers, the chooser's collection list, the reminder filter
src/stores/device-sync-store.ts	per-account preferences and the last run status
src/components/settings/device-sync/	the "Sync to this device" sections, rendered by ContactsSettings.tsx and CalendarSettings.tsx

The Android shell

Account type and accounts

The account type is ${applicationId}.account, defined once in android/app/build.gradle with resValue and referenced as @string/device_sync_account_type from the XML and through R.string.device_sync_account_type from Kotlin. The package name may change before a store listing; nothing hard-codes it. The account label is "Bulwark" and its icon is the app icon.

One Android account exists per registry account (AccountEntry.id, username@hostname) that has device sync on for at least one authority. Its name is the account's login; when two registry accounts share a login on different servers, the second one is named after its registry id. The registry id is stored in the account's userData under registryId and is the only link back to the app: the name is for people. GET_ACCOUNTS is not needed: an authenticator always sees its own accounts, and addAccountExplicitly needs only a UID match on API 23+.

Other apps see the account (API 26+) when they hold READ_CONTACTS and we hold a granted WRITE_CONTACTS; Contacts apps that list accounts rely on that.

Authenticator

BulwarkAuthenticator and BulwarkAuthenticatorService:

• addAccount (Settings → Accounts → Add account) never creates an account without credentials. It returns an explicit Intent to MainActivity with bulwarkmobile://settings/contacts, where the user turns sync on. (The AccountManager only accepts an Intent that resolves inside our package.)
• getAccountRemovalAllowed allows removal. The providers then delete every row of the account and its SyncState; the app notices and turns device sync off for that account. It never recreates the account on its own.
• Everything else (getAuthToken, confirmCredentials, …) returns ERROR_CODE_UNSUPPORTED_OPERATION.

Sync adapters

Two AbstractThreadedSyncAdapter services (ContactsSyncService, CalendarSyncService), declared with supportsUploading="true", userVisible="true", allowParallelSyncs="false", and in the main process so they share the ReactHost with the UI. The contacts service also declares android.provider.CONTACTS_STRUCTURE → xml/contacts.xml, whose EditSchema lists exactly the data kinds in the contacts mapping. AOSP Contacts rejects the whole account type when that XML breaks its rules (every typed kind needs <Type> children, name needs all six supports* attributes, photo must be present, single-value kinds need maxOccurs="1"), and without an EditSchema it treats the account as read-only.

Exported services are safe: startSync/cancelSync ignore callers other than the system UID.

onPerformSync (see DeviceSyncAdapter.kt):

1. Reads registryId from the account's userData; a missing one is a hard error (databaseError).
2. Opens a run in DeviceSyncRuns: a random runId, a latch, a cancel flag. Headless task ids restart at 1 with every React context, so they never identify a run.
3. Posts to the main thread: HeadlessJs.withReadyReactContext starts React Native if needed (on the main thread only: the ReactHost is created lazily without a lock), then starts the headless task BulwarkDeviceSync with the payload and allowedInForeground = true. The task timeout is the adapter's deadline minus 20 s; the payload's deadline is 30 s earlier still.
4. Waits until the first of: finishRun, the headless task ending (JS settled without a report, timed out, or the context was reloaded), React refusing or failing to start (the boot task faults, e.g. a debug build without Metro), or the deadline: 9.5 minutes, or 2.5 minutes when the sync runs as an expedited job (schedule_as_expedited_job), inside JobScheduler's 10 and 3 minute guarantees. HeadlessJs stops waiting for React Native, and removes its listener, once a context is ready, the start task faults, or 10 minutes passed, so a context that comes up after the adapter gave up starts nothing.
5. Maps the report onto SyncResult; no report is a soft error.

onSyncCanceled interrupts the sync thread; the run is closed, and JS sees isRunCancelled(runId) == true (also for any run nobody waits for) at its next checkpoint. A run ends once: finishRun is accepted only while the run is still waiting, so a report that arrives after the deadline, a boot fault or the task's end is refused rather than half-applied. SyncManager holds a wake lock for as long as onPerformSync runs, so the adapter takes none.

SyncManager also cancels a sync that moves fewer than 10 bytes of network traffic in any 60-second window. The engine sends a Core/echo when a local phase has gone 40 s without a JMAP request.

Spike results

Checked before building anything else on it, on the API 34 and API 32 emulators with the debug build (commit ed2525e):

App state	Report received after
foreground (task runs next to the UI, AppState active)	319-334 ms
background (process alive)	320-327 ms
process killed (SyncManager starts it for the sync service; React starts headless, bundle from Metro)	16.2 s (API 34), 8.5 s (API 32)

An io report was retried with back-off, an auth report was not retried, too many deletions showed the system's "Attempted to delete too many Contacts." notification, and with Metro unreachable the adapter gave up after 2 s. Found on the way: SyncManager retries a failed sync at once when it made progress (madeSomeProgress), so hard errors are reported without progress; and without the runtime permission AbstractThreadedSyncAdapter fails the sync itself (databaseError) before onPerformSync runs, so the app must detect a revoked permission on its own.

Device data model

Identity

Row	Column	Value
raw contact	RawContacts.SOURCE_ID	<jmapAccountId>/<cardId>; null until created on the server
group	Groups.SOURCE_ID	<jmapAccountId>/<cardId> of a kind: "group" card
calendar	Calendars._SYNC_ID	<jmapAccountId>/<calendarId>
event (master or single)	Events._SYNC_ID	<jmapAccountId>/<eventId>, or ~pending/<uid> for a local event claimed for upload
event exception	Events._SYNC_ID + ORIGINAL_SYNC_ID	<jmapAccountId>/<eventId>#<recurrenceId> + the master's _SYNC_ID

JMAP ids use only [A-Za-z0-9_-] (RFC 8620 §1.2), so /, # and ~ never occur in them: ids stay unique across the JMAP accounts, shared ones included, that feed one Android account, and ~pending/… can never be a server id.

Identities are only unique for one server account's lifetime (Stalwart's ids come from small per-account counters). So the SyncState records which registry account and server origin the rows belong to, a run refuses to touch rows written for another one, and a merge treats a server object whose uid differs from its shadow's as a different object (delete and insert, never a patch).

Why a pending _SYNC_ID: CalendarProvider hard-deletes an app-deleted event that has no _SYNC_ID, so a local event deleted while its create was in flight would vanish without a trace and come back from the server. With a _SYNC_ID the delete is a soft delete (DELETED=1) the engine can act on. It also lets Etar create exceptions of a not-yet-uploaded series (it offers "this event" only when the master has a _SYNC_ID); the provider rewrites their ORIGINAL_SYNC_ID when the master's _SYNC_ID changes. Contacts need no such marker: raw contacts of a sync account are always soft-deleted.

A new local item is a raw contact without SOURCE_ID, or a master event (ORIGINAL_ID and ORIGINAL_SYNC_ID both null) whose _SYNC_ID is null or pending, or the newer of two masters sharing a _SYNC_ID (a CONTENT_EXCEPTION_URI split, see Recurrence). DIRTY is not the test: a raw contact inserted without data rows is not dirty. A row with ORIGINAL_* set is always an exception of its master and never uploads as an event of its own.

Contacts columns

Column	Holds
RawContacts.SOURCE_ID	identity (above)
RawContacts.SYNC1	the card's selected address books: <jmapAccountId>/<addressBookId> joined by ,
RawContacts.SYNC2	the shadow: the last server card, wire format, with each data: photo URI replaced by sha256:<hex> of its bytes, plus a ~memberOf member: the group cards whose memberships the rows held after our last write, so a removal on the device and an addition on the server can be told apart (stripped before any projection or patch), and ~noPhoto: true when the card's photo has no row on the device because it could not be written
RawContacts.SYNC3	the pending create: {"uid":…,"target":"<jmapAccountId>/<addressBookId>"}, written before the /set and kept until the identity is
RawContacts.SYNC4	poison marker {"fp":…,"type":…,"n":…,"until":…}
RawContacts.RAW_CONTACT_IS_READ_ONLY	1 when none of the card's address books grants mayWrite. Only written, and only along with other raw contact writes, so an echo stays write-free: ContactsProvider refuses the column in every projection and selection ("Invalid column", API 32 and 35). Whether device edits are put back is decided from the server's current rights on the stored card
Data.DATA_SYNC1	the entry key(s), e.g. emails:work1; see units
Data.DATA_SYNC2	photo rows: the hash of the server photo applied to the row
Data.DATA_SYNC3	the baseline: the row's mapped columns as the provider stored them after our last write (JSON; text over 1 KB as its hash)
Groups.SOURCE_ID, SYNC2, SYNC3, SYNC4	as for raw contacts, for group cards; GROUP_IS_READ_ONLY is written like RAW_CONTACT_IS_READ_ONLY, for groups in read-only books (SHOULD_SYNC is never written: an update carrying it makes ContactsProvider request a sync)
Settings	one row per account: UNGROUPED_VISIBLE=1, SHOULD_SYNC=1 (the default 0 hides every contact without a visible group)

Writing DIRTY=0, SOURCE_ID or SYNC1-4 does not bump RawContacts.VERSION; data-row inserts, updates and deletes do, whoever makes them. So VERSION is used only as an optimistic-lock token in asserts, and DIRTY for change detection. (A super-primary change in another account can bump our VERSION without DIRTY; the assert then fails once and the row is re-read.)

Calendar columns

Column	Holds
Calendars._SYNC_ID	identity
Calendars.CAL_SYNC1	unused: CalendarProvider sends it as the feed extra of the sync it requests whenever a calendar with SYNC_EVENTS=1 is inserted or changed, so anything there splits those requests apart
Calendars.CAL_SYNC2	the shadow: the last server Calendar object
Calendars.CAL_SYNC3	flags JSON: the readOnly reason (rights, subscription)
Events._SYNC_ID	identity
Events.SYNC_DATA1	masters and single events: the shadow, the full server event with its recurrenceOverrides
Events.SYNC_DATA2	exceptions: the recurrence id, exactly as the server's override key
Events.SYNC_DATA3	the pending create: {"uid":…,"target":"<jmapAccountId>/<calendarId>"}
Events.SYNC_DATA4	the baseline: mapped event columns, attendees and reminders as stored after our last write (JSON; text over 1 KB as its hash)
Events.SYNC_DATA5	poison marker
Events.UID_2445	the event's uid

Attendees and reminders have no sync columns, which is why their baseline sits on the event row. Events.MUTATORS (the packages that changed the event since our last write) tells which app made a device edit.

A description longer than 64 KB of UTF-8 is written truncated, marked in the baseline, and never uploaded: an event row carries the description, the shadow and the baseline, and one Binder transaction holds 1 MB.

SyncState

ContactsContract.SyncState and CalendarContract.SyncState hold one blob per Android account (CalendarProvider requires sync-adapter URIs for it): UTF-8 JSON, versioned.

{
  "v": 1,
  "owner": { "registryId": "usera@example.org@mail.example.org", "origin": "https://mail.example.org" },
  "accounts": {
    "c": {
      "collectionsState": "s12eq",
      "itemsState": "syufa",
      "selected": ["c/b"],
      "partial": [],
      "stale": [],
      "created": [],
      "reconcile": null,
      "groups": ["c/e12"],
      "taskOnly": []
    }
  },
  "deviceZone": "Europe/Berlin",
  "deviceZonePending": null,
  "reminderOwner": "device"
}

• owner: the registry account and the server origin the rows were written for. A run for another owner stops with internal, and a teardown uploads nothing for it; turning sync off and on again starts over.
• itemsState is the ContactCard or CalendarEvent state the rows describe. It is written in the same applyBatch as the last item group of a /changes page: the syncState op is the last op of that batch, is never yieldAllowed, and the native side refuses a batch that breaks either rule, so a provider yield can never commit the state ahead of its rows.
• No stored itemsState is newer than the shadow of an object changed before it: an object whose rows could not be written (a planner error, a provider refusal that re-planning did not fix) goes on stale in the same batch, is fetched again first in every run, and uploads nothing until it has been. The state still advances, so one bad object never blocks the pages after it. It also names the contacts whose membership a group card just written changed, until their own chunk ran.
• created lists the objects our uploads created after itemsState, written in the batch that stores their identity. The next run fetches them first, with stale: /changes from the stored state omits an object that was created and destroyed since (Stalwart does, as RFC 8620 allows), so a create that another client destroyed before the next download would otherwise stay on the device.
• reconcile marks a full reconcile in progress: { "from": <state taken first>, "phase": "ids" | "objects", "position": n, "after": <id>, "collections": [<keys>] }, written with every chunk of objects. after is the last id processed in the reconcile's sorted order: the next run lists the ids again and continues after it (phase and position are informative only). collections names the collections it lists, fixed when it starts; a collection selected while it runs is loaded after it, like any newly selected one. from: null asks for a reconcile that has not started (after cannotCalculateChanges, or when local deletions were discarded). Its deletions are applied only when the complete id list is known, and itemsState becomes from when it ends.
• selected names the collections whose rows are all on the device: a collection is added in the batch with its last loaded chunk, and removed in a batch of its own before its first row is dropped. partial names collections loaded or dropped only in part: added before the first chunk of a load or the first row a contacts drop removes (a calendar is dropped with its calendar row), removed with the last. Selected, such a collection is loaded again; deselected, it is dropped again.
• groups (contacts): the group cards present on the device, so a group an app hard-deleted (Fossify does) is recognised by its absence. It changes in the batch that inserts or deletes the group's row (such a batch ends with a state op, also on a retry), and a group a download finds unchanged is listed too.
• taskOnly lists calendars found to hold only tasks.
• deviceZone (calendar) is the zone floating events were written in; deviceZonePending is set while a zone-change pass runs and cleared, with deviceZone updated, in the batch of its last chunk.
• reminderOwner (calendar) is who the Reminders rows were written for (device or bulwark), so a run notices a changed choice; see Reminder owner.
• The providers delete the row with the account. A teardown clears it by writing ''.

A sync run

One run handles one Android account and one authority. A module-level mutex per (registry account, authority) serialises it with other runs and with app-side operations such as turning sync off; a run that cannot get it before its deadline reports cancelled with moreRecordsToGet.

1. Preflight, in this order:
   • The authority is still enabled for the Android account (getSyncAutomatically) → otherwise disabled.
   • The SyncState is read; without the runtime permission this first provider read fails with permission. A blob that cannot be understood (another version, garbage) counts as none: every JMAP account starts over with a full reconcile. Its owner must be this registry account (see Identity) → otherwise internal.
   • An upload-only sync (extras.upload) with nothing dirty, deleted or new for our account ends here with ok, before JMAP is opened: ContactsProvider and CalendarProvider schedule an upload sync for every account 30 s after any app write, other accounts' included. A pending reconcile, objects on stale or objects in created make it a full run. A raw contact or group deleted before a run claimed it (no SOURCE_ID, no pending create) does not count: it never reached the server, and the next run purges it.
   • new JMAPClient().loadAccount(registryId): false or AuthenticationError → rebuild once (another client may have rotated the refresh token); still failing → auth, plus one notification that deep-links to sign-in. Nothing local is touched. The SyncState's owner must also name this server's origin → otherwise internal.
   • The capability on at least one JMAP account → else unsupported.
2. Collections. AddressBook/get or Calendar/get (explicit properties) for the primary JMAP account of the capability (primaryAccounts[capability]) and every shared account that advertises it; calendar rows are inserted and updated to match. An account that refuses to list its collections (forbidden, accountNotFound, accountNotSupportedByMethod) is left out of the run: nothing downloads or uploads for it, its rows stay on the device untouched, and it is only logged, without an item error (Stalwart lists an account shared for mail only with every capability, and a refusal can pass). Newly selected collections are loaded after step 3 has brought the account up to date, restricted to them, so the stored state never skips changes of the other collections. Deselected collections are dropped in step 5.
3. Download.
   • Objects on stale, and the objects our last uploads created (created), are fetched again first.
   • With a stored itemsState: /changes with maxChanges: 256 and a hasMoreChanges loop; each page's changed ids are fetched with explicit properties in batches of maxObjectsInGet.
   • Without a state, or on cannotCalculateChanges: a full reconcile, checkpointed in reconcile (see SyncState). The state is taken first (/get with ids: []), then every selected collection's ids (/query, inAddressBook/inCalendar, limit 5000 with position paging; pages overlap by up to 100 ids, so a deletion between two pages skips nothing), then the objects in batches. Rows without an identity or with a pending one (and exceptions of pending masters) are never reconcile deletions. A reconcile that finds zero remote objects while the device holds more than 10 rows for that JMAP account stops with safetyAbort instead of wiping the device.
   • Contacts: a page's group cards are written before its contacts. When a group card's members changed, its old and new member contacts are fetched in the same page, before the page's state op: their membership rows change although their own cards did not. They go on stale in the batch that writes the group, so a stop before their chunk leaves them to be fetched first by the next run.
   • An object without rows first looks for a local item it may already be: a new row whose pending create targets a collection the object is in and whose pending uid is the object's uid (our own create whose identity never got written) is adopted: it takes the identity and shadow and is merged as a dirty item. A contact or group the user deleted meanwhile takes only the identity and shadow, and its deletion then uploads by id.
   • Each object is merged into its rows. An object that left every selected collection loses its rows only when they are clean; a dirty or deleted item is merged like any change first (an event into the calendar its rows are in; a deleted one's shadow follows the server, so its deletion is decided on the server's version), so its upload builds on the server's version, and its rows go once the upload is accepted. A full reconcile downloads such objects the same way.
   • An object whose rows could not be written (a planner error, a provider refusal that re-planning did not fix) goes on stale, and the page's state still advances.
   • Rows are written in chunks; the new state goes into the last chunk of each page, so every page is a checkpoint.
4. Upload, always after download so merges see the latest server truth; see Uploads.
5. After the upload. Deselected collections are dropped, and only their clean rows go: dirty items there were uploaded first (they are still valid objects on the server), and an item whose upload failed keeps its rows; its collection is no longer complete (not in selected, partial for contacts), so the next run drops it again, or loads it again when it is selected again. Rows of objects that left every selected collection go the same way. Calendar: then the device-zone pass and the reminder-owner pass rewrite clean events (see Timing and Reminder owner).
6. Report counts, conflicts, per-item errors and duration; persist the status for the settings UI; call finishRun before the task's promise settles (JS timers stop once the task is over). A run that wrote nothing either way, skipped nothing and reported no conflict, item error, note or failure (the upload sync Android starts after a manual one, the adapter's follow-up when changes arrived during a run, a manual sync right after another) keeps the previous status's conflicts, item errors and counts and moves only its time on; after a failed run it shows ok, without the failure's message. Any other run replaces the status, so a problem shows until the next run that does something.

/get always lists properties explicitly: with properties: null Stalwart leaves out JMAP-only fields such as useDefaultAlerts, and ids: null returns at most 500 objects without saying so (only collections, of which Stalwart allows 250 per account, are read with ids: null).

Checkpoints sit between chunks and phases. At each one the run checks isRunCancelled (and whether a teardown asked it to stop), its deadline and the time since the last JMAP request. Less than 30 s before the deadline it stops cleanly and reports moreRecordsToGet, which the adapter turns into fullSyncRequested (SyncManager ignores moreRecordsToGet itself). The step from a create's /set to its identity write is not interruptible, and a create batch starts only while at least two request timeouts (60 s) of budget remain before the deadline (deadlineMarginMs and createBudgetMs in DEFAULT_TUNING, engine/deps.ts). An expedited sync gets 2.5 minutes, of which the adapter keeps 50 s in reserve; when it first has to start React Native, little of the rest is left in which a create batch may start.

Failures stay contained. A planner that throws on one item affects only that item: a download puts the object on stale, an upload skips the item and reports it as plannerError. An account-level refusal of a whole /set holds back only that account (see Uploads).

Invariants

Every reviewer checks against these.

1. No server data loss. Uploads are JMAP patches (RFC 8620 §5.3, JSON Pointer with ~0/~1 escaping) computed per mapped property against the baseline. A property Android can't represent is never sent. A full-object replace is never sent, and neither is a destroy + create for what was an edit or a move on the device. Where Stalwart needs a whole map resent (deleting a map entry, adding the first entry), the map is rebuilt from the fresh shadow, so its other entries go back unchanged, and the request carries ifInState so it cannot overwrite a server change the merge did not see. What an editor did not model (a row it re-inserted without some column, an entry it dropped) is never read as a deletion.
2. No silent device-edit loss. A dirty row is cleared only after the server accepted the change, and only if it wasn't edited again during the upload: the clearing batch asserts RawContacts.VERSION for contacts and the read projection for events (every event row, with the attendees and reminders of the rows that are dirty: a clean row's DIRTY flag guards them, since CalendarProvider sets it on every app write to an event, its attendees or reminders), so check and clear are one provider transaction. A row whose mapped projection equals its baseline (only STARRED changed, say) is cleared without uploading. No dirty, deleted or pending row is removed locally (deselection, move-away, disable, logout, account reconciliation) before its upload succeeded or the user confirmed losing it.
3. Crash-safe and idempotent. Every step can be re-run. The JMAP state is written in the same applyBatch as the last chunk of rows it describes, as that batch's last op. Creates carry a client-minted uid, written to the row with the create's target before the /set; a download adopts a pending row by uid, and an upload looks the uid up and adopts what it finds in the create's target collection, so a retry never duplicates on either side.
4. No feedback loops. Adapter writes use CALLER_IS_SYNCADAPTER, which neither sets DIRTY nor schedules an upload sync, and never touch the columns that make a provider request a sync (CAL_SYNC1, Groups.SHOULD_SYNC). An echo of our own upload compares equal to the shadow and results in zero provider writes. A run that failed reports progress only for rows it actually wrote.
5. Scoped. Only rows of our own account type are read or written; the native side enforces it for every table.
6. Deletion safety. Local→server deletions above the threshold set tooManyDeletions; SYNC_EXTRAS_OVERRIDE_TOO_MANY_DELETIONS and SYNC_EXTRAS_DISCARD_LOCAL_DELETIONS are honoured. Deletions inferred from absence (groups) count toward the threshold. A full reconcile that returns zero remote objects while the device holds more than 10 aborts. Deleting a device item that belongs to several collections removes only the selected memberships.
7. Conflicts. See merge rules.
8. Limits respected: maxObjectsInGet/Set, maxCallsInRequest, one request in flight per run, /changes paging, provider batch sizes and yield points, the Binder budget, the headless timeout and the JS thread (yield between chunks).
9. iOS untouched. Everything is gated by Platform.OS === 'android' plus the presence of the native module.
10. No state ahead of a shadow. No stored itemsState is newer than the shadow of an object that changed before it (see stale).

Change detection and merge rules

Baselines

Mappings are lossy in both directions, and the providers normalise what they store: ContactsProvider splits a lone display name into components and fills a missing formatted address, CalendarProvider fills the organizer, zeroes all-day times and rewrites durations. Comparing rows against a fresh toDevice(shadow) would see edits nobody made.

So every row carries a baseline: its mapped columns exactly as the provider stored them after our last write, read back after each chunk (text over 1 KB as its hash). A column that differs from its baseline was written by someone else. A clean row's baseline is by definition its current content, so whenever the engine reads a clean row whose baseline differs, it rewrites the baseline; that also heals the crash window between a chunk and its read-back. Two exceptions:

• the read-back also reads DIRTY (and VERSION): a row that turned dirty in the meantime keeps its old baseline, so the user's edit still counts;
• timing and rule of a recurring master are never healed, and the rows of a split are not healed at all: a CONTENT_EXCEPTION_URI split changes them without DIRTY (see Recurrence).

Units

Changes are detected, merged and uploaded per unit:

• contacts: the name (the StructuredName row), each entry of a map (one data row: emails:work1, phones:p-a, addresses:home, …), the note, the photo, and the card's group memberships;
• events: groups of related columns (title, description, location, timing, rule, status, availability, privacy, color, calendar), each attendee (by email), the reminder set, each EXDATE and each exception row (recursively, the same units).

A unit's upload is the set of patch paths for the sub-fields whose columns changed against the baseline; sub-fields Android can't hold are never in it.

Row matching (contacts)

Row keys are hints, not identity. Fossify Contacts deletes and re-inserts every data row but the name on each save, dropping DATA_SYNC*, the columns it does not model (DEPARTMENT, labels, a website's type), IS_PRIMARY, and every nickname and organization but one; AOSP Contacts updates rows in place. So, per data kind:

1. Empty rows (an editor's blank nickname or note) are ignored. A Photo row is never empty: ContactsProvider keeps a photo within its 96 px thumbnail as the thumbnail alone, without PHOTO_FILE_ID, and the thumbnail does not read back.
2. Rows whose DATA_SYNC1 key is in the shadow keep that key.
3. Rows without a usable key are matched to the shadow's unmatched entries by the kind's primary value (email address case-insensitively, phone digits, URL, date + type, …); then, only when the numbers of unmatched rows and entries are equal, pairwise in order.
4. Rows still unmatched are new entries and get fresh keys: b + 8 random base36 characters (Stalwart parses numeric keys as array indexes and generates k<n> itself, so neither is used).
5. Shadow entries still without a row count as deleted on the device only when the kind still has rows matched by their key (an in-place editor removed one), or when the kind has no row left at all and some other kind that re-inserting editors rewrite still has rows matched by their key (the editor worked in place: AOSP deleting a contact's only email, note or birthday). When every row of the kind is keyless, an editor rewrote them all, and a missing entry may just be one it could not show: it stays on the server. A missing name row is never a deletion; a missing photo row always is (no editor re-inserts photo rows), unless the device never got the photo (~noPhoto).

A row matched by its key is compared column by column with its baseline; an emptied column is a real deletion. A row matched by value has no baseline: its primary value uploads when it differs from the shadow's projection, its TYPE and LABEL only for phones, emails, addresses and events (editors that re-insert rows write a constant type for websites), and never IS_PRIMARY; an empty column never clears anything.

A merge into a dirty contact never gives keys to a kind whose rows are all keyless: a keyed row there would later make the entries the editor could not show look deleted. Every write that leaves a contact clean (a download into a clean contact, an accepted upload, an upload with nothing to send, a revert) makes its rows match the card: rows are rewritten, re-inserted or deleted, not just given new baselines. Otherwise an entry Fossify could not show (a second nickname) would read as deleted at the next in-place edit; it also puts back a department Fossify dropped.

The name: "" and NULL are the same. name/full uploads when DISPLAY_NAME changed against the baseline and is not simply the provider's join of the current components (a typed display name); when only components changed, name/full is re-derived with them only if the shadow's full was the derived one (Stalwart never re-derives it), and left alone when it was custom ("Müller, Hans").

Lossy calendar editors

Events.MUTATORS names the app that changed the event. Fossify Calendar re-inserts every attendee as a required attendee and rebuilds the RRULE from its own model (FREQ, INTERVAL, COUNT or UNTIL, one BYDAY or BYMONTHDAY=-1, BYMONTH). For an event it changed, attendee type and relationship changes are ignored, and a rule change is applied part by part onto the shadow's rule: parts Fossify models and changed are patched, parts it dropped stay. Etar's move re-inserts every attendee but the user as a required attendee without a status; a pair takes those rows as they are, and the accepted write puts the server's back.

Key reconciliation (downloads)

Stalwart keeps the keys a JMAP client wrote. Cards and events written over CardDAV/CalDAV, or received as invitations, have no stored keys, and Stalwart generates them on every read: k1…kN by position for card entries and alerts, uuid5 of the address/name/URL for participants, locations and links. Such keys shift when an entry is inserted before them. So before merging a new server version, the engine maps the shadow's keys to the new keys by content (equal entries first, then equal primary values) and renames the rows' keys; only the remainder counts as added or removed. The first JMAP write to such an object freezes its keys.

Merge rules

For a dirty row whose object also changed on the server (base = shadow, local = rows, remote = new server object), per unit:

Local changed?	Remote changed?	Result
no	no	nothing
no	yes	write remote, new baseline
yes	no	keep local, still dirty; uploads later against the new shadow
yes	yes, to the same value	converged; new baseline
yes	yes, differently	server wins: write remote, new baseline, conflicts += 1

"Remote changed" compares the mapped projection of base and remote, so a server change to a sub-field Android doesn't show is not a conflict: the upload patches only the paths the user changed and the server keeps its own.

Whole-object rules:

• Edit on the device + delete on the server → the server delete wins; the rows are deleted and counted as a conflict. A move made on the device as a delete plus an insert (a pair not uploaded yet) is such an edit: the inserted row goes with the deleted one.
• Delete on the device + edit on the server → the device delete wins; the object is destroyed. A deletion is an explicit act; restoring the object would resurrect something the user removed. The download writes only the new version into the item's shadow, so the deletion is judged by the object's current collections: one the server meanwhile also filed in a collection this device does not sync only leaves the synced ones. Except when the edit moved the object out of every collection its rows were written for (contacts: SYNC1; events: the row's calendar) and into none the device syncs: the deletion is then moot, only the rows go, and the object stays where it is. Group rows record no address books, so a group card deleted on the device is destroyed even then.
• Delete on both → purge, no conflict.
• A server object whose uid differs from the shadow's is a different object: its rows are replaced, never patched.

Every download write into an existing item is guarded: the group asserts the DIRTY flag and VERSION (contacts) or projection (events) the plan was made from (for events, attendees and reminders only for the rows that are dirty), and ops that address one row by id expect exactly one row. Exception rows are deleted by id only. A failed assert re-reads and re-plans the item (up to 3 times); after that the item goes on stale. A batch that failed as a whole may have been committed up to a provider yield point: its downloads are then planned again from a new read rather than sent again, so a committed one becomes an echo.

Uploads

The upload runs phase by phase across the JMAP accounts: each account uploads its pairs (below), new rows are claimed, each account resolves its series splits, then every account's creates go up, then every account's updates, then every account's deletions, then (contacts) groups deleted by absence and group memberships. So a move between accounts never destroys before it created. Changes go in /set calls of at most maxObjectsInSet objects (creates: 50) and about 1 MB of JSON (Stalwart splits bigger calls into several commits, which can apply partially), one request in flight, and one sendSchedulingMessages value per call (it is a request-level flag).

• ifInState. Every /set carries ifInState = the item state the merge was based on (then the newState of our previous /set). A stateMismatch (or Stalwart's serverUnavailable, a concurrent write that failed the whole call) means the server changed since the download: the run downloads that account again and repeats the phase. When /changes from the stored state then shows nothing new, the next /set uses the type's current state instead: Stalwart can name one point of its history two ways (a /changes state s… against the type's state n before the first item change), and ifInState compares them as text. After three retries the run reports io without progress (SyncManager would otherwise retry at once, without back-off).
• Account refusals. A method error on a whole /set that concerns the account (accountReadOnly, forbidden, accountNotFound, accountNotSupportedByMethod) holds back only that account: its items wait, the report lists the account, and the other accounts go on.
• Splits. A CONTENT_EXCEPTION_URI split (two masters sharing a _SYNC_ID, see Recurrence) is resolved before the creates: the older master's capped rule is planned while the split is still visible, the newer row is claimed as a new event, then the rule is sent. The new event is created with the other creates.
• Pairs. Some apps turn an edit into a delete plus an insert: Etar moves an event to another calendar that way, and turns a recurring series into a single event (or edits "this and following" from its first instance) the same way. A deleted row and a new row of the same run whose projections match apart from the calendar or the rule (attendees by address, the user's own left out: Etar's copy of a meeting has no row for the user, or one as its organizer when the user was not listed) are uploaded as a patch of the existing object (calendarIds swapped; recurrenceRule: null with its overrides removed); the new row takes the identity and the exception rows of the old one, which is purged. Rows of different JMAP accounts never pair: that move is a create in the target account, identity written, and later in the run a destroy in the source, whose echo finds no rows left to change. Unpaired rows are ordinary creates and deletions. Pairs go up before any new row is claimed: the new row takes the old row's identity instead of a uid of its own, and a paired deletion is a move, which the deletion threshold does not count. When an app changes the rows between the patch and the identity write, they are read and paired again; rows that no longer pair do not get the identity: the object is fetched again first next run, and the rows upload as a create and a deletion. The new row's baseline is the old row's, with what the patch changed (the calendar; for a series turned single the rule and its timing as a single event), so anything else saved with the move or made before it, a reminder say, keeps the row dirty and uploads right after the pair as an ordinary update; the pair's accepted write then waits for that upload. Such a change keeps the device's value even where the server changed the same field meanwhile: the download of the deleted row refreshes only its shadow, so no merge sees the conflict. Attendee rows of the new row are the app's re-insert (see Lossy calendar editors): only their addresses count, for the pairing. A new row without reminder rows is taken to have copied none, not to have deleted them. The old row's exception rows (Etar copies only those it never synced) move to the new row with their pending edits; a series turned single has none left.
• New rows (see Identity for what counts as new):
  1. Claim: write the pending create (SYNC3 / SYNC_DATA3: the uid and the target collection; for events also UID_2445 and _SYNC_ID = ~pending/<uid>). A row keeps a uid it already carries unless another row, synced or pending, uses the same uid (a CONTENT_EXCEPTION_URI split clones UID_2445, an app's "duplicate" copies it): then it gets a fresh one, and the engine loads every master that shares a new master's UID_2445 so the planner sees the collision. A server object with that uid does not count, because it may be this row's own earlier create. The target is fixed at the claim: contacts go to the address book chosen in the settings if it is selected and writable, else to the primary account's default book (or its first by sort order) among the selected writable ones; with none, the item is skipped and reported as noWritableAddressBook in every run. Events go to the calendar of their row. A claim whose collection is gone from the server (its account was listed and no longer has it) is made again for contacts and groups, since nothing of an earlier create can exist there; an event's create into a calendar that is gone is not sent and is reported as collectionGone until the user moves the event to another calendar.
  2. Look the uid up: contacts in the target address book (card uids are unique per book), events in the whole account (event uids are unique per account). A hit in the create's target collection is adopted: the row takes its identity and is merged as a dirty item. A hit elsewhere (an event with the same uid in another calendar) is not this row's create, and adopting it would overwrite an unrelated event: the item is poisoned as uidConflict.
  3. Otherwise create.
  4. invalidProperties on uid means the object exists: Stalwart's duplicate check is synchronous but the uid query reads an asynchronous search index, so a just-created object can be invisible to the query. The card error names the existing id; for events the lookup is retried with back-off and, failing that, left to the next run's download, which adopts the row by uid.
  5. From the /set to the identity write nothing is interruptible, and a create batch starts only with at least two request timeouts (60 s) of budget left.
• Exception rows (ORIGINAL_* set) upload only as recurrenceOverrides/<key> of their master; those of a new master go up inside its create.
• Dirty rows: the patch from the changed units. An empty patch clears DIRTY without a request.
• Deleted rows (DELETED=1): an object that belongs to several collections loses only the selected memberships (addressBookIds/<id>: null / calendarIds/<id>: null) and is destroyed only when none would remain. Then the rows are purged with sync-adapter deletes (for events also their exceptions, by id: a sync-adapter delete removes only the row itself). notFound counts as done. A deleted row without identity but with a pending create is looked up first and destroyed if found. One without a pending create (a contact or group created and deleted on the device between two runs: ContactsProvider soft-deletes those too) never reached the server: the claim step purges it, nothing uploads, and it never counts as a waiting change. A deletion is decided on the server's current version (the download merges a deleted item's shadow first): when that holds none of the collections the rows were written for and none the device syncs, it is moot and only the rows are purged. A collection deselected on the device since still counts as the rows' own, and an object moved into another collection the device syncs is not moot either: those deletions go up.
• Afterwards: a /get of every touched id (a separate request for creates: Stalwart does not resolve result references to /set). The result becomes the new shadow, and the rows' baselines are refreshed from it, in one group per item that first asserts the VERSION/projection read before the upload, clears DIRTY and clears a poison marker the item carried. If the assert fails (edited again meanwhile), the shadow and identity are still written, the baselines of the uploaded units are set to the values that were uploaded, and DIRTY stays: the newer edit uploads next time, and so does a revert to the old value. For contacts these baseline writes address their data rows with where _id = ?, so a row deleted meanwhile does not fail the group. A group card accepted for a membership patch keeps a rename the device made that the card does not show: only its shadow is written, and the rename uploads next.
• SetErrors:
  • notFound on update → treated as a remote delete (the rows go, and for a pair the new row too); on a destroy it counts as done; on a create the item is poisoned;
  • forbidden in a call that asked for scheduling messages → the item is retried once without them, and the report notes "invitations not sent" once that retry is accepted (Stalwart refuses scheduling when it is disabled, when the account has no calendar address, or without the scheduling permission). forbidden otherwise: on a create the item is poisoned (a revert would delete the user's new item); on an update or a deletion the object is read-only on the server after all, and the report lists the item as forbidden: its rows are rewritten in place from the server's current version, fetched for it, so device-only columns (a star, a ringtone, joins, row ids) stay; a refused deletion brings the item back as a new copy, and the rows are removed when the object is gone or outside the selection;
  • stateMismatch → as above;
  • invalidProperties on uid → as above;
  • invalidProperties, invalidPatch, tooLarge, overQuota and the rest → the item is poisoned.
• Poison markers. The engine writes the marker (SYNC4 / SYNC_DATA5, {"fp":…,"type":…,"n":…,"until":…}): fp is a fingerprint of what the item uploads from (its flags, shadow and mapped rows; for events also the attendees, reminders and exception rows), until the end of a back-off of 1 h that doubles up to 24 h while the item stays the same. The engine checks the marker before planUpload: until the back-off ends or the item changes, the item is skipped and the report lists it. The marker is cleared once an upload of the item is accepted. The planners check the same marker with the same fingerprint content, a duplicate of the engine's check.

Deletion threshold

More than 50 local deletions of an authority in one run, and more than 20 % of that authority's synced objects, set tooManyDeletions unless the run carries overrideTooManyDeletions. Deletions inferred from absence (groups) count. No deletion of that run is uploaded; everything else proceeds. The user resolves it in Android's too-many-deletions notification, or with "Review deletions" in the app's sync settings, which asks whether to delete the items on the server too ("Delete on server", a sync with overrideTooManyDeletions) or to bring them back ("Bring them back", a sync with discardLocalDeletions). With discardLocalDeletions, the affected JMAP accounts first get a reconcile marker in a batch of its own, then the deleted rows are purged, so the next runs download the objects again whatever their extras; a crash in between only asks the question again. A teardown applies the same threshold: deletions above it stay on the device and count as waiting changes, so the app asks before turning sync off (see Lifecycle).

Contacts mapping

JSContact (RFC 9553) as Stalwart stores it, to ContactsContract. Anything not listed is preserved through patching. Data rows are written with both the display name and the components (the provider recomputes whichever is missing), and every entry row stores its key(s) in DATA_SYNC1. Text is cut to what ContactsProvider keeps (10 KiB; phone numbers 1000 characters), and a column that was cut never uploads.

Name (StructuredName, one row, key name)

JSContact	Android
name.full	DISPLAY_NAME; when absent, deriveFullName(components) is written and full is not uploaded unless the user changes it
component title (also prefix)	PREFIX
component given	GIVEN_NAME
component given2 (also middle, additional)	MIDDLE_NAME
component surname (+ surname2, joined with a space)	FAMILY_NAME
component generation, credential (also suffix)	SUFFIX
component phonetic of given / given2 / surname	PHONETIC_GIVEN_NAME / PHONETIC_MIDDLE_NAME / PHONETIC_FAMILY_NAME

Upload: a changed column replaces the matching components (the first of their kind; surname2 is dropped only when FAMILY_NAME changed), keeps every other component (separators, repeats, unknown kinds) in place, and sends the whole name/components array (JSON Pointer can't address array elements). When name/full goes along is in row matching: a typed display name uploads; a recomputed one re-derives full only when the server's was derived too. A name the server holds without components uploads whole: the components from the row and full = the display name, so retyping "Ada" as "Ada King" on the device sends both.

Entries

JSContact	Android kind	Columns
nicknames/<k>	Nickname	NAME, TYPE=DEFAULT
emails/<k>	Email	ADDRESS; TYPE from contexts: private→HOME, work→WORK, none→OTHER, label→CUSTOM+LABEL (mobile→MOBILE, which uploads that label); pref=1→IS_PRIMARY
phones/<k>	Phone	NUMBER; TYPE from features + contexts (table below); label→CUSTOM+LABEL; pref=1→IS_PRIMARY
addresses/<k>	StructuredPostal	components → STREET, POBOX, NEIGHBORHOOD, CITY, REGION, POSTCODE, COUNTRY (table below), full→FORMATTED_ADDRESS (a card without full gets a one-line address joined from the components, so the provider leaves the row as written); contexts → TYPE HOME/WORK/OTHER, label→CUSTOM
organizations/<k> + titles pointing at it	Organization	name→COMPANY, units joined with , →DEPARTMENT, title with kind: title→TITLE, title with kind: role→JOB_DESCRIPTION; key `organizations:
links/<k>	Website	URL ← uri; contexts private→HOME, work→WORK, else OTHER; label homepage/blog/profile/ftp→HOMEPAGE/BLOG/PROFILE/FTP (they upload that label), any other label→CUSTOM
anniversaries/<k>	Event	START_DATE: YYYY-MM-DD, --MM-DD without year (partialDateToString); kind birth→BIRTHDAY, wedding→ANNIVERSARY, other→OTHER, anything else → CUSTOM + its kind as LABEL; a custom type uploads its label, lowercased, as the kind
relatedTo/<uri>	Relation	NAME ← the key (a urn:uuid: of a synced card shows that card's name), TYPE from the relation set (table below)
first entry of notes	Note	NOTE ← note
first media entry with kind: "photo"	Photo	full-size bytes into PHOTO; the provider scales them, stores the display photo and sets PHOTO_FILE_ID
group cards (kind: "group")	Groups	TITLE ← name.full; GROUP_VISIBLE=1; members keys are uids of the member cards
membership	GroupMembership	one row per group card whose members holds this card's uid, written with GROUP_ROW_ID (data1, the group's row id); GROUP_SOURCE_ID is only read

Phone types (first match wins; reverse: the table read right to left, and TYPE_OTHER → no features):

Features / contexts	Phone.TYPE
mobile + work	WORK_MOBILE
mobile	MOBILE (read: also cell)
fax + work	FAX_WORK
fax + private	FAX_HOME
fax	OTHER_FAX
pager + work	WORK_PAGER
pager	PAGER
textphone	TTY_TDD
main-number + work	COMPANY_MAIN
main-number	MAIN
work (voice)	WORK
private (voice)	HOME
anything else	OTHER

CALLBACK, CAR, ISDN, RADIO, TELEX and ASSISTANT upload as voice with the label callback, car, isdn, radio, telex or assistant, and MMS as text with the label mms; such a label next to that feature reads back as the type (a custom label uploads without the feature and stays CUSTOM).

Postal components (reverse: an edited field replaces all components it was built from with one component of the first kind, keeps the rest):

Android	JSContact component kinds, joined in order
STREET	name (street), number, building, floor, apartment, room, block, direction, landmark (with separators)
POBOX	postOfficeBox
NEIGHBORHOOD	district, subdistrict
CITY	locality
REGION	region
POSTCODE	postcode
COUNTRY	country

Legacy flat address fields (street, locality, …) are read when there are no components and never written. When an editor wrote only the formatted address (AOSP Contacts edits it as one line), the provider copies it into STREET and empties the other parts. An unchanged line uploads nothing. A changed line uploads as full with the parts it implies, read back along the old line: each line or comma-separated segment of the old one must be some of the old parts in a row, and a segment whose change is confined to one part (the parts around it keep their words) rewrites that part as an edit of its field would. When the layout changed or a change spans two parts, the address keeps the whole line as its street (name) component. Either way full and the components agree; an address without components would show empty in the app's editor and in CardDAV clients.

Relations: friend↔FRIEND, spouse↔SPOUSE, child↔CHILD, parent↔PARENT, sibling→RELATIVE, kin→RELATIVE, co-worker/colleague→CUSTOM with the type as label, agent→ASSISTANT, emergency→CUSTOM. Device types without an RFC 9553 equivalent (BROTHER, SISTER, MOTHER, FATHER, MANAGER, ASSISTANT, DOMESTIC_PARTNER, PARTNER, REFERRED_BY) upload as the nearest RFC type (sibling, parent, colleague, agent, spouse, spouse, contact) and are only sent when the user changed the type. RELATIVE uploads as kin; a custom type uploads its label, lowercased, as the relation type. Only a row matched by its key can rename a relation (relatedTo is keyed by its value, so another name is another entry): a re-inserted row showing another name may just show a related card that was renamed since.

For emails, phones, addresses and websites, changing only the label of a custom type uploads only label; the contexts the entry had stay.

Deleting an entry resends its whole map (Stalwart turns map/key: null into a valueless vCard line or, for relatedTo, ignores it). Only addresses, members and keywords take a null delete cleanly.

Preserve-only (never written, never uploaded): speakToAs, preferredLanguages, cryptoKeys, directories, calendars, schedulingAddresses, onlineServices, keywords, personalInfo, localizations, language, notes after the first, media other than the first photo, and kind (an org card syncs like a person; group cards are groups). Device-only columns (STARRED, custom ringtone, send-to-voicemail, IS_SUPER_PRIMARY) never upload.

Photos: a data: URI is decoded; a blob-backed media entry is downloaded with the detached client. Upload: the display photo is read back, scaled to at most 512 px, JPEG, and sent as a data:image/jpeg;base64,… URI in media/<key>/uri, matching the app's picker. DATA_SYNC2 holds the hash of the server photo last applied, the baseline holds PHOTO_FILE_ID; a new file id means the user changed the photo. A photo within the 96 px thumbnail has no file id, so replacing it on the device with another photo that small is not seen. Photos travel in batches of their own (Binder budget). A photo larger than 700,000 base64 characters (about 512 KiB) is not written: a contact's rows travel in one provider batch, one Binder transaction of at most 1 MB. Such a photo, and one whose bytes could not be fetched, keeps an existing row and counts as unchanged; without a row the shadow's ~noPhoto records it, so the missing row is not read as a deletion, and the photo stays on the server until the device sets one of its own.

Groups: group cards are written before the contacts of the same download, and membership rows name their group by row id (GROUP_ROW_ID, looked up by the group's SOURCE_ID), never by GROUP_SOURCE_ID: a membership naming an unknown GROUP_SOURCE_ID makes the provider create an empty group. A membership whose group has no row is not written. A contact's memberships are merged from three sets: what the rows held after our last write (~memberOf in the shadow), the rows now, and the group cards that list the card's uid now; both sides can only flip a membership the same way, so memberships never conflict. Membership edits, also those of a contact created on the device (after its create), upload after the deletions as patches of the group cards' members (members/<uid>: true or null; the whole map for a group without members). Edits for a group on stale wait until it has been fetched again. A contact stays dirty until the server's group cards show its membership edits (the clean and accepted writes keep DIRTY); once the run's membership patches are accepted, a contact with nothing else to upload is cleaned in the same run. A membership edit the server can't take, of a group in read-only address books or of another JMAP account, does not wait: the contact's next clean or accepted write puts it back to what the server has, like an edit of a read-only contact, and when that was the contact's only change the upload counts it as skipped and the report lists the contact as groupNotWritable.

A group deleted on the device is found by DELETED, or by its absence from the rows while SyncState.groups still lists it (Fossify hard-deletes groups through a sync-adapter URI); both count toward the deletion threshold. An absent group whose card is no longer a group, or is in no address book that syncs, is only dropped from the list: it is not one the device showed.

A contact or group whose address books are all read-only takes no device changes: an edit (a group's rename) is put back from the shadow, a deletion is purged and fetched again, and the report lists either as readOnly. Joining a writable group is no edit of the contact: it uploads through that group's card.

Calendar mapping

Calendar rows

JMAP Calendar	Android Calendars
id	_SYNC_ID = <jmapAccountId>/<id>
name	NAME, CALENDAR_DISPLAY_NAME; a shared account's calendars show <name> (<account name>) (the JMAP account's name, unless it is one of the user's addresses), the primary account's get no suffix
color	CALENDAR_COLOR (CSS color → ARGB; absent → the app's default palette colour)
myRights	CALENDAR_ACCESS_LEVEL: mayWriteAll → OWNER (700); mayWriteOwn → CONTRIBUTOR (500); mayRSVP → RESPOND (300); else READ (200); subscribed iCal feed → READ
the user's calendar address	OWNER_ACCOUNT (never null: Etar crashes on it)
timeZone	CALENDAR_TIME_ZONE (device zone when null)
—	SYNC_EVENTS=1 (the default 0 hides every instance; written again when an app turns it off), VISIBLE=1 on insert only (local afterwards), ALLOWED_REMINDERS="1,2", ALLOWED_AVAILABILITY="0,1", ALLOWED_ATTENDEE_TYPES="0,1,2,3", MAX_REMINDERS=5 (0 when Bulwark owns reminders), CAN_ORGANIZER_RESPOND=0, CAN_PARTIALLY_UPDATE=0

Not synced: the virtual birthday calendar (client-side only), calendars without mayReadItems, and task-only calendars: a calendar whose first 50 objects are all @type: "Task" (isTaskLikeObject), re-checked on each full reconcile.

Deleting a calendar row deletes its events (provider trigger); the engine first uploads the calendar's dirty events.

A read-only calendar (without mayWriteAll and mayWriteOwn, or a subscribed feed) takes no device changes: a dirty event is rewritten from its shadow, a deleted one is purged and fetched again, a new one is removed. The one exception is the user's own answer, to the series (participants/<id>/participationStatus) or to occurrences (their overrides' participants, changed in nothing but the user's status), which uploads when the calendar grants mayRSVP. Stalwart 0.16.23 refuses it anyway (forbidden: its /set requires write rights in a shared calendar); the rows are then rewritten from the server and the item is reported (forbidden). Edits of an event Stalwart holds only as an instance (no series) are reverted too, in any calendar. Every change put back this way is listed in the run's report: readOnly (an edit, a deletion or a new event in a read-only calendar, or a move into one), rsvpRefused (an answer where the calendar takes none), instanceOnly, and crossAccountMove for an event an app moved in place to another account's calendar.

Timing

JSCalendar	Android
timed, timeZone set	DTSTART = instant of start in timeZone; EVENT_TIMEZONE = timeZone
floating (timeZone absent)	as timed, in the device zone; SyncState.deviceZone records it. When the device zone changes, clean floating events are rewritten so they keep their wall time
showWithoutTime	ALL_DAY=1, DTSTART = UTC midnight of the date, EVENT_TIMEZONE="UTC"
non-recurring	DTEND = start + duration (days nominal in the zone, the rest exact); DURATION null
recurring	DURATION = P<n>D (all-day) or P<seconds>S (timed), DTEND null
exception row	DTEND (the provider drops DURATION from exceptions)

Android's Duration parser takes [+-]P then <digits><W|D|H|M|S> (with M always minutes, no fractions, no Y); all-day durations must be P<n>D or the provider throws. Server durations are parsed in full (weeks included, P1W comes from CalDAV DTEND). All-day events are at least one day long on the device: a duration with a time part is rounded up to whole days, a zero one becomes one day, and neither is uploaded unless the timing changes.

Upload of timing: only when a timing column differs from the baseline. The local start is recomputed in the event's zone (floating events stay floating, all-day events stay dates), and the duration from DTEND/DURATION with whole days nominal. EVENT_TIMEZONE must be an IANA id: an unknown id silently becomes GMT on Android, and makes the event floating on Stalwart; zones are validated with Intl before either write.

DST edges. Stalwart silently drops a start, an until or an override key whose local time falls in a DST gap or overlap of the zone. Checked on the local server: an event created at 2026-10-25T02:30 or 2026-03-29T02:30 Europe/Berlin comes back without start and timeZone, and a daily rule with until: 2026-10-25T02:30 comes back without until, i.e. unbounded. So before an upload:

• a local time in a gap moves forward by the gap (RFC 5545 semantics);
• a non-recurring event whose start falls in an overlap is sent in the fixed-offset zone of its instant (Etc/GMT-2 for 02:30 CEST; Etc/UTC when the offset is not a whole hour): same instant, same wall time, only the zone name changes;
• a recurring event whose start or an override key falls in an overlap is not uploaded: the item stays dirty and is reported as dstAmbiguous;
• an until in a gap or overlap moves one hour later (better than Stalwart dropping it and the series running forever).

Recurrence

JSCalendar	Android
recurrenceRule	RRULE: FREQ, INTERVAL (>1), COUNT, UNTIL (UTC …Z for timed events, YYYYMMDD for all-day), every BY* part, BYSETPOS, WKST (when not Monday)
rscale other than gregorian, skip, leap months (byMonth with L)	not representable: the rule is written without them, the event's timing and rule are never uploaded, and an item whose only changes are to them is reported as ruleNotRepresentable
excludedRecurrenceRule	not supported by Stalwart; an EXRULE written by a device app is ignored
override {excluded: true}	EXDATE: comma list of UTC YYYYMMDDTHHMMSSZ, or YYYYMMDD for all-day (no prefix means UTC)
any other override, including {}	an exception row: master ⊕ override (applied as a PatchObject); ORIGINAL_SYNC_ID, ORIGINAL_INSTANCE_TIME = the key's instant in the master's zone, ORIGINAL_ALL_DAY; SYNC_DATA2 = the key
—	RDATE is never written

Why no RDATE: an infinite rule plus RDATE cannot be inserted (the provider tries to expand it to infinity), and RDATE-only events lose their DTSTART instance. An exception row whose original instance the rule does not generate simply shows as an extra event, which is what an extra JMAP instance is.

AOSP expands BYSETPOS only for monthly rules with BYDAY, and walks at most 2000 periods from DTSTART (a daily series older than about 5.5 years shows no instances); both are listed under limitations.

Device-side changes map back as follows:

On the device	Upload
exception row inserted (Etar "this event": ORIGINAL_SYNC_ID + ORIGINAL_INSTANCE_TIME, DIRTY=1) or edited	recurrenceOverrides/<key>; the key is ORIGINAL_INSTANCE_TIME in the master's zone (a date at midnight for all-day). A new override carries the master's title, locations, participants and sequence (Stalwart stores a bare override as a separate event otherwise); an existing one is patched one level deep (recurrenceOverrides/<key>/<property>), never deeper: Stalwart turns a deeper pointer into a partial override (other attendees vanish for that instance)
exception row with STATUS_CANCELED (Etar and Fossify "delete this event"), a deleted exception row, or an EXDATE added to the master	recurrenceOverrides/<key>: {excluded: true}
EXDATE removed	recurrenceOverrides/<key>: null
master RRULE capped with UNTIL/COUNT + a new event without identity (Etar "this and following")	the master uploads only recurrenceRule (whole object) with the new until/count and removes the overrides after the new end (recurrenceOverrides/<key>: null; for a COUNT the end is the start of the last instance, CalendarProvider's LAST_DATE − DURATION); the new series is a row of its own, claimed and created as a new event with a fresh uid; exclusions Etar copied from the old series' EXDATE that fall before the new series' start are left out
master start moved (Etar "all events")	start/duration/timeZone, and every override key moved by the same local offset (overrides whose start equalled their old key move too), sent as the whole recurrenceOverrides map
two masters with the same _SYNC_ID (a CONTENT_EXCEPTION_URI split clones _SYNC_ID, SYNC_DATA* and UID_2445, and caps the old master's rule without DIRTY)	both masters are always loaded together. The download merges into the older master as a dirty item, leaves the newer one alone and heals no baseline of either. The upload resolves the split before the creates: the older master's current RRULE is uploaded, and the newer row is claimed as a new event with a fresh uid; that claim points the source's exception rows' ORIGINAL_SYNC_ID back at the source and sets the source DIRTY=1, so its capped rule uploads
a new master carrying another master's UID_2445 (an app's "duplicate" copies it)	a clone without a source: claimed with a fresh uid and created as a new event, never adopting the other master's object (the engine loads every master that shares a new master's UID_2445)
event moved to another calendar, or a series turned into a single event (Etar deletes and re-inserts both)	paired with the deleted row and uploaded as a patch of the same object (see Uploads); never across JMAP accounts, where the move is a create in the target account and a destroy in the source
CALENDAR_ID changed in place to a calendar of another JMAP account	not uploaded: the row is put back into its calendar, rewritten from the shadow and reported as crossAccountMove (changes made with it upload, and their accepted write puts it back)
an RSVP for one instance (Etar goes through CONTENT_EXCEPTION_URI)	the override's whole participants map with the user's new status. An exception row's status uploads only when the user organizes the event: Etar also puts STATUS_CONFIRMED on the instance, and Stalwart refuses an attendee's change to an event's status

On an exception row an app created, an empty title, description, location or colour, or no attendee or reminder rows, means "inherit from the series", never a deletion. An override never carries privacy: Stalwart drops it from overrides.

On an exception row device sync wrote, clearing the description or the location clears it for that occurrence. An override without them shows the series' ones, so while the series has a description or location the override gets the empty text, or one location without a name (Stalwart writes no LOCATION line for an empty map, which would read back as the series' locations); otherwise the override's own value is removed. A cleared colour gets the empty colour while the series has one (calcard keeps an empty COLOR; the device shows the calendar's colour). Removing every reminder of one occurrence can be stored only while the series has no alerts of its own: the override leaves the calendar's defaults (useDefaultAlerts: false) and has none. Otherwise the occurrence would show the series' alerts (an override is its own VEVENT, and an empty map writes no VALARM), so the change is put back and reported as remindersNotRepresentable; when the occurrence has other changes, those upload and their accepted write puts the reminders back.

The first override of an event with none sends the whole recurrenceOverrides map (a patch below a missing property fails).

Other properties

JSCalendar	Android
title	TITLE (exception rows always carry the master's title: Fossify Calendar treats an exception with an empty title as deleted)
description	DESCRIPTION; descriptionContentType: text/html is written as plain text and uploaded only when edited, then as text/plain
first entry of locations (server order)	EVENT_LOCATION ← name; edit patches locations/<k>/name, a new one adds locations/<new>, clearing deletes the entry
status confirmed / tentative / cancelled	STATUS 1 / 0 / 2; absent → 1. Never written NULL (the provider throws). A NULL STATUS (CalendarProvider has no default; Google Calendar leaves it NULL on the events and exceptions it inserts) sets no status: a create leaves status out, an override carries none, and it is never a change or a reason not to pair
freeBusyStatus busy / free	AVAILABILITY 0 / 1; device TENTATIVE (2) uploads as busy
privacy absent / public / private / secret	ACCESS_LEVEL DEFAULT / PUBLIC / PRIVATE / CONFIDENTIAL
color	EVENT_COLOR (CSS name or #rrggbb → ARGB; upload #rrggbb); no Colors table in v1
participants with a mailto: calendarAddress	Attendees rows: ATTENDEE_EMAIL (the user's own address normalised to exactly OWNER_ACCOUNT: Android matches "me" case-sensitively), ATTENDEE_NAME, ATTENDEE_RELATIONSHIP (owner or organizer address → ORGANIZER, else ATTENDEE), ATTENDEE_TYPE (optional role → OPTIONAL, kind resource/location → RESOURCE, else REQUIRED), ATTENDEE_STATUS (accepted 1, declined 2, needs-action 3, tentative 4, delegated/other 0); participant ids stay in the shadow
organizerCalendarAddress	ORGANIZER when the event has participants; otherwise unset (the provider uses the owner, which keeps the event editable)
—	HAS_ATTENDEE_DATA=1; SELF_ATTENDEE_STATUS is never written (the provider derives it and throws on update)
alerts with an offset before start (relativeTo start or absent, offset ≤ 0)	Reminders: MINUTES = −offset, METHOD = EMAIL for action: email, else ALERT
alerts relative to the end, absolute alerts, offsets after the start	preserved, not shown
useDefaultAlerts: true	the calendar's defaultAlertsWithTime/WithoutTime become the reminders (as the app's own scheduler does); a device edit uploads useDefaultAlerts: false plus explicit alerts; an edit of one occurrence's reminders does so in that occurrence's override only (Stalwart keeps an override's useDefaultAlerts as a JSPROP), so the series and its other occurrences stay on the defaults
uid	UID_2445
keywords, categories, priority, virtualLocations, links, relatedTo, sequence, locale, replyTo, locations after the first, participants without a mailto: address	preserved only

Attendees upload: the user's own RSVP (their attendee row's status) patches participants/<id>/participationStatus, with sendSchedulingMessages: true like the app's rsvpEvent; on an exception row it goes into the override's whole participants map. Added attendees get new participant keys with roles: {attendee: true}, participationStatus: needs-action and expectReply: true; removed ones are deleted (participants/<id>: null is clean on Stalwart).

Reminders upload: the representable alerts are replaced by the device's reminders, matched by (minutes, method) to keep existing alert keys; the others stay. Only when the reminder owner is the device calendar app (below).

Scheduling messages

Stalwart sends iTIP only when a /set asks for it (sendSchedulingMessages, default false), whereas CalDAV clients get implicit scheduling on every change. Device sync follows CalDAV's lead for changes a calendar app would notify about:

• the user is the organizer and the event has other participants: true when title, description, timing, rule, overrides, location, status or attendees changed, on a create, and on destroy (CANCEL); reminders, colour, availability and privacy alone send nothing. An event without an organizer counts as the user's: once it has attendees, Stalwart makes the account's first address its organizer;
• the user is an attendee: true for their own RSVP change and on destroy (Stalwart sends REPLY DECLINED); other edits are stored for the user only;
• events in the past: Stalwart sends nothing anyway.

Reminder owner

When calendar sync is turned on while calendarNotificationsEnabled is on, the app asks who should remind the user, per account:

• the calendar app (default when Bulwark's reminders are off): Reminders rows are written, and Bulwark's own scheduler skips events of the synced calendars of that account (tasks keep their reminders) while the device calendar receives them. It does not while the account's calendar sync is paused in Android settings, Android's auto-sync is off (Android then runs manual syncs only, and the periodic, pushed and app-triggered syncs are not manual) or the calendar permission is revoked, nor before the app read Android's side: a double reminder is better than a missed one. The app reads it when the scheduler starts and each time it schedules, and schedules again when the answer changed;
• Bulwark: no Reminders rows; MAX_REMINDERS=0 on the account's calendars so apps offer no reminder editor; reminder rows an app adds anyway are neither uploaded nor deleted.

Changing the choice requests a calendar sync. The calendar SyncState records who the rows were written for (reminderOwner), so that run notices the change: its uploads still work for the old owner, so pending reminder edits go up first. After the upload it rewrites the Reminders rows of that account's clean events together with their baselines, one group per event that asserts the projection it read, and sets the calendars' MAX_REMINDERS; no server writes. A reminder edit that has not reached the server is never overwritten.

Headless payload, report and SyncResult

Payload (RunPayload in src/device-sync/types.ts): { runId, accountName, registryId, authority, extras: { manual, upload, expedited, ignoreBackoff, overrideTooManyDeletions, discardLocalDeletions }, deadline }; only booleans cross from the framework's extras.

Report (RunReport): { v: 1, runId, authority, outcome, message?, startedAt, durationMs, stats: { downloaded: {created, updated, deleted}, uploaded: {created, updated, deleted}, entries, skipped }, conflicts, itemErrors[], tooManyDeletions?: {count, threshold}, delayUntil?, moreRecordsToGet? }.

Outcome	Meaning	SyncResult (SyncReports.kt)
ok	synced; item errors may be listed	stats only
io	network or server trouble	numIoExceptions=1 (soft: back-off; at once if progress was made)
internal	an unexpected exception	as io
auth	credentials rejected after a rebuild	numAuthExceptions=1, no progress (hard)
permission	runtime permission missing	databaseError, no progress (hard)
unsupported	no capability	databaseError, no progress
safetyAbort	empty server, full device	databaseError, no progress
tooManyDeletions	above the threshold	tooManyDeletions=true, numDeletes = pending deletions (the system's notification offers "delete" and "undo")
cancelled	framework cancel or out of time	stats only
disabled	sync is off	nothing

A 429 ends the run with io and delayUntil from its Retry-After (epoch seconds; 60 s when the server sends none). delayUntil and moreRecordsToGet (→ fullSyncRequested) pass through. A run without a report is a soft error. The stats count only rows actually written and objects actually sent, so a run that repeated a page without writing anything is not "progress" to SyncManager. A change pushed while a run is active (SyncManager drops a requestSync that matches a running sync) flags the run; the adapter reads the flag after the run is closed and then sets fullSyncRequested. The push of our own upload arrives while the run is still open, so every run that uploads gets one follow-up sync (see Triggers).

Native module API

NativeModules.BulwarkDeviceSync (DeviceSyncNativeModule in types.ts). Large payloads cross as JSON strings, which the bridge moves far faster than nested maps. Every provider call is scoped to one account of our type.

Method	Does
getInfo()	{ accountType, sdkInt }
listAccounts()	our accounts with their registryId
ensureAccount(name, registryId)	adds the account (both authorities syncable, automatic off); true when created, false when it exists for the same registry id or without one (it is adopted); rejects (conflict) when an account of that name belongs to another registry id
removeAccount(name)	removes it; the providers drop its rows
getSyncSettings(name)	master auto-sync, and per authority: syncable, automatic, periodic seconds, active, pending
setSyncEnabled(name, authority, on)	syncable = 1 and setSyncAutomatically
setPeriodicSync(name, authority, seconds)	replaces our periodic syncs (including the framework's default daily one); 0 removes them; the framework clamps to 15 min
requestSync(name, authority, optionsJson)	{manual, expedited, upload, overrideTooManyDeletions, discardLocalDeletions} as SYNC_EXTRAS_*; constant extras so repeated requests coalesce; flags an active run for fullSyncRequested
openAccountSettings(name)	the account's sync screen (android.settings.ACCOUNT_SYNC_SETTINGS with the Account as the account extra and in :settings:show_fragment_args: the Settings app of Android 12L and later passes only those arguments on to the screen, which closes at once without an account), else SYNC_SETTINGS filtered to our type
finishRun(runId, reportJson) / isRunCancelled(runId)	the run handshake
setPushRoutes(json)	{ jmapAccountId: [{ accountName, authorities }] } for the push router, in SharedPreferences
showSyncProblem(name, title, text, uri, channelName) / clearSyncProblem(name)	the account's one sync-problem notification; a tap opens uri (bulwarkmobile:// only) in the app; channelName names the channel in the app's language
query(name, authority, queryJson)	ProviderQuery → ProviderRows
applyBatch(name, authority, opsJson)	ProviderOp[] → BatchResult; failures resolve as {ok: false, reason}
readSyncState(name, authority)	the SyncState blob as text
readPhoto(name, rawContactId, maxPx)	the display photo (or thumbnail), scaled, as JPEG base64 + file id

Provider calls run one at a time on a thread of the module's own (a batch can hold the provider for seconds at a contended yield point), through an unstable provider client, so a provider process that dies fails the call instead of this process. query, readSyncState and readPhoto reject with code scope, permission or provider.

Events: BulwarkDeviceSync:accountsChanged (an account of our type was added or removed; JS re-lists). Events without a JS listener are dropped, so they are hints; the UI re-reads state on mount and on foreground.

Scoping, enforced in Kotlin for every op (android/app/src/main/java/com/anonymous/bulwarkmobile/sync/ProviderScope.kt):

• URIs always carry caller_is_syncadapter=true, account_name, account_type (CalendarProvider accepts no other query parameters).
• Queries and updates/deletes by where get the account condition ANDed in:
  • account_name/account_type columns for raw_contacts, groups, settings, calendars and colors;
  • data rows are filtered by ContactsProvider itself through the account URI parameters (their account columns are never selected on: Android 17 drops them from the Data view for apps targeting API 37);
  • events get calendar_id IN the account's calendars (CalendarProvider updates events on the Events table, which has no account columns);
  • attendees, reminders and extended properties get event_id IN the account's events among those the op touches (resolved first; the providers may compile selections strictly, so no subqueries).
• Selections and sort orders are checked before the account condition is ANDed on as (where) AND (scope): their parentheses must balance outside quoted literals (1) OR (1 would close the scope's own), and they may not end the statement, carry a comment or use SELECT, UNION, ATTACH, DETACH or PRAGMA (scope otherwise). The engine sends only constant selections with ? arguments; the check guards against bugs.
• Ops by id verify the row belongs to the account first (scope otherwise) and expect exactly one row unless they set expectCount, so a row that vanished fails with assert. id and where together address the id row if it matches. On attendees, reminders and extended properties the id column is table-qualified (Attendees._id), since CalendarProvider reads attendees through a join where a bare _id is ambiguous; the engine therefore never writes _id into a where there. Extended properties are updated through their item URI, so by id only.
• Inserts into data/attendees/reminders/extended_properties/events must name a parent (a back-reference to an insert of the same batch, or an id); an insert or update naming a parent, exception master (original_id) or group (data1 of a membership row) checks it: gone fails with assert (re-read and re-plan), another account's with scope. An update of a data row by id that writes data1 without restating the mimetype has the row's mimetype looked up, so a membership's group is checked there too; such a write by where is refused (scope). A back-reference must point to an earlier insert of the right table; settings inserts return no id.
• Inserts into the tables with account columns (raw contacts, groups, settings, calendars, colors) get account_name and account_type added, so calendar inserts carry them too. Otherwise account_name/account_type/data_set may only be written to restate the account, on the tables that have them (scope otherwise).
• Tables are an enumerated set per authority; anything else is refused (scope).
• Blobs ({ b64 }) are only accepted for data.data15 (photos). data.data14, a photo row's file id, is never written (scope): the provider sets it, and another account's file id would let readPhoto read that account's photo.
• A syncState op must be the last op of its batch and not yieldAllowed, so a provider yield never commits the state ahead of the rows (refused as scope otherwise).
• Batches: for contacts, at most 499 ops up to and including each yield point (ContactsProvider counts an op before resetting at a yield point and throws at 500), so the SyncState op goes in a batch of its own after a group of 499 ops or more; about 500 KB per call as estimateBatchBytes weighs it (2 bytes per character of the ops' JSON plus 300 bytes per op: the Parcel carries UTF-16 strings); TransactionTooLargeException resolves as tooLarge so JS splits the batch. An item too big for one batch comes as a chain of groups (OpGroup.next): the engine applies them in order, each in a batch of its own; a group that fails an assert has the item planned again from what the earlier ones wrote, any other failure gives it up (on stale). OperationApplicationException resolves as assert (except the provider's own "too many operations" and failed inserts, provider), SecurityException as permission. The whole batch is also refused (scope) once the Android account is gone, so no rows outlive it.

What the providers do with values, which the planners account for:

• contacts data1..data14 are TEXT columns: a number written there reads back as a string, so mappers normalise types before comparing;
• Events.dirty and Calendars.dirty are NULL after a sync-adapter insert: planners write dirty = 0 on every insert and select dirty rows with dirty = 1.

App integration

Store

src/stores/device-sync-store.ts, keyed by AccountEntry.id, persisted under device-sync:v1 with createPersistStorage({ writeDelayMs: 0 }) (not the 1-second delayed storage) and not part of the settings export. Per account: contactsSelection / calendarSelection (explicit choices only; unlisted collections follow the default: those of the capability's primary account on, every other account's off (also one the server marks as personal), and the webmail's "Trusted Senders" address book off too: it is the allow-list of senders whose images load, not an address book people expect in their phone's contacts), newContactsAddressBook, reminderOwner, intervalSeconds per authority, the last RunStatus per authority, androidAccountName (the Android account it uses), knownStates (the latest states the engine synced, per JMAP account and type, for the echo skip), enabled (what the user turned on in the app), removedInAndroidSettings, suspended and primaryJmapAccounts. Android's getSyncAutomatically stays the source of truth for whether an authority syncs (see Lifecycle). Headless runs await hydration before any set() (zustand 5 writes on every set(), even before hydration, and would overwrite the stored preferences).

Settings sections

Android only (supportsDeviceSync in platform-capabilities.ts and the native module present). Settings → Contacts and Settings → Calendar each get a "Sync to this device" section with a row per signed-in account:

• an on/off switch (reflects getSyncAutomatically; "Paused in Android settings" when the account exists but the authority is off there; a hint when the device's master auto-sync is off);
• a chooser (sheet) for address books or calendars, with the primary account's on and shared ones (and Trusted Senders) off by default, read-only ones marked, and a warning when another signed-in account already syncs the same shared collection;
• contacts: the address book for new contacts;
• calendar: who reminds you (calendar app or Bulwark);
• the interval: 15 min, 30 min, 1 h (default), 6 h, manual;
• "Sync now" (requestSync with manual + expedited);
• the last sync time (its "… ago" refreshed every 30 s while the section is open) and its result, or the error with an action ("Sign in again", "Grant access", "Review deletions", which asks whether to delete the items on the server too or bring them back; see Deletion threshold);
• "Android account settings" (openAccountSettings).

Turning sync on asks for READ/WRITE_CONTACTS or READ/WRITE_CALENDAR after a rationale; denied → sync stays off and the section says why, with a link to the app's system settings when Android no longer asks.

Lifecycle

• Enable: permissions → ensureAccount → setSyncEnabled(true) → setPeriodicSync(interval) → requestSync(manual); push routes and push types are refreshed.
• Disable an authority: automatic sync goes off first (setSyncEnabled(false)), so no queued sync downloads everything again right after the rows are gone. Then the engine's teardown runs under the run mutex; a sync of that account and authority that holds it is asked to stop at its next checkpoint. The teardown uploads the authority's dirty, deleted and new items (bounded to 30 s; it is not a sync run, so the "disabled" preflight does not stop it), with the deletion threshold applied, and never uploads rows written for another registry account or server (SyncState owner). It downloads nothing first: its uploads carry the stored state (ifInState), so a server change makes it download before it decides; for an account whose first sync never ended (no stored state) only new items go up, and the rest counts as waiting. Then it deletes the authority's rows and clears the SyncState by writing ''. When items could not be uploaded (offline, errors, deletions above the threshold), nothing is deleted and the app asks: "N changes made on this device haven't reached the server. Turn off anyway?" Turning off anyway still makes one short upload attempt (5 s) before the rows go; "Keep syncing" turns automatic sync back on. With nothing waiting the rows go without a question: the server keeps everything. When both authorities are off the Android account is removed. Turning calendar sync off also forgets who reminds of synced events, so the question comes again the next time. Turning sync on, off and back on for one app account runs one step at a time: a turn-on made while a turn-off still tears down waits for it, then sets everything up again.
• Logout (auth-store.logout, logoutAll, removeAccount): the same as disabling both, before the credentials go, with the same question when changes would be lost; "stay signed in" keeps the accounts syncing. Several accounts are released in parallel, a toast says it is working, and an internal error never blocks signing out (the next reconcile suspends what was left). The release waits for a turn-on or turn-off of the account that is under way (they run one at a time per account); one asked for while it runs comes after it and does nothing once the account was released.
• Registry evictions (the app drops an account on its own, e.g. after an AuthenticationError): reconciling Android accounts against the registry at launch, on foreground and whenever accounts of our type change finds an Android account whose registry id is gone. It is not removed: automatic sync is turned off and one notification says it stopped syncing ("Sign in again"; it gives no count of waiting changes, which would need a provider read). Signing in again resumes exactly the authorities that synced; the section lists the account with "Sign in again" and "Remove from this device", and removing it there or in Android Settings drops it. An empty or unreadable registry is never acted on.
• Account removed in Android Settings: detected by the accounts listener and on foreground; the store marks it, the switches show off, and the app does not recreate the account.
• The store keeps what the user turned on in the app (enabled) next to Android's switches, which stay the source of truth: an authority switched on in Android Settings is adopted as on, one switched off there shows as "Paused in Android settings" (not "Off"), and "Sync now" is disabled while paused, since a manual run would only report disabled. suspended marks an evicted account, primaryJmapAccounts the primary JMAP account per authority (push routes and the trigger filter).
• An Android account that exists without a registry id (created, then the app died before writing it) is adopted by the next ensureAccount; one with another registry id is refused (conflict) and the app uses the registry id as the account name instead. Any other failure to add the account fails the turn-on.
• Uninstall: Android removes the accounts of an uninstalled authenticator, and the providers delete their rows.
• Permission revoked: Android fails the sync before our code runs (databaseError), so the section checks the permission itself and shows "Grant access".

Triggers

• Periodic sync per the user's interval (default 1 h: push covers freshness).
• Device edits: the providers schedule upload syncs 30 s after any app write (supportsUploading).
• App running: SSE StateChange for AddressBook/ContactCard or Calendar/CalendarEvent on a synced account (onStateChangeType in state-change-bus.ts; the stream serves the active account), the app's own successful contact/calendar mutations (store actions), and returning to the foreground → requestSync for that authority. The first request of an account and authority arms a 5 s timer and later ones ride along (the timer is not restarted), so an edit and the StateChange echoing it make one sync. A StateChange whose state equals the one the engine last recorded is our own echo and is skipped, also when the engine recorded it while the timer ran. Foreground requests go out at most once a minute per account and authority, and the launch counts as one. Edits that change no device row (tasks, calendar sharing, the default collections) request nothing. These are ordinary requests, which Android drops while the account is paused.
• A push with no readable changed map (older relays), or one that names no type, starts the mail task as before and requests no syncs; periodic syncs cover those setups. An account entry that is not an object may be mail too; the other entries are still routed.
• Every run that uploads gets one follow-up sync: the server's push of our own change arrives while the run is open, and SyncManager drops a request matching the running sync, so the run asks for another one when it ends. The follow-up finds nothing to do, so it stays one.
• App closed: while an account has device sync on, its push subscription adds ContactCard, AddressBook (contacts) and CalendarEvent, Calendar (calendar) to EmailDelivery, per account, when the account's session advertises urn:ietf:params:jmap:contacts or urn:ietf:params:jmap:calendars. A server that refuses a subscription write carrying them gets it again with EmailDelivery alone, so mail push never depends on device sync. Turning sync on or off updates the subscription right away; for a signed-in account other than the one the UI serves, through a client of its own, never the UI's singleton. The relay forwards the StateChange's changed map verbatim (FCM data key changed; only its first account key is repeated as accountId). A native router in the FCM and UnifiedPush services walks every account key of changed, maps contact and calendar types to requestSync through the routes JS stored (setPushRoutes; the device-sync layer refreshes them whenever the device sync preferences change), and starts the mail push task only when the push may carry mail: anything but a StateChange (an EmailPush), a map it can't read or that names no type, an account entry it can't read, or a type that may mean new mail in some account's map: EmailDelivery, or Email/Mailbox, which new mail changes too (a subscription made by an older build listens to them until the launch-time resync replaces its types, and a server without EmailDelivery has only them). A guard at the top of pushBackgroundTask (carriesNoMail) applies the same rules, tested with the same cases, so a contact or calendar change can never take the legacy "notify the newest unread mail" path.

Failure matrix

Failure	What happens	Recovery
No network	io → back-off (30 s ×2, max 1 h)	automatic
Network lost mid-upload	the /set may have applied: a create's row is adopted by uid on the next download or lookup, patches re-apply idempotently	automatic
Process killed mid-sync	no report: soft error; the state was only advanced with complete chunks	next run repeats the page; echo writes are no-ops
OAuth token expired	the client refreshes; a rotated token held by another client → rebuild once	automatic, else auth + notification
Password revoked	auth (hard, no retry) + "Sign in again"	user signs in; the next trigger syncs
Permission revoked	Android fails the sync (databaseError)	"Grant access" in settings
cannotCalculateChanges	full reconcile of that JMAP account	automatic
Server SetError on an item	notFound on an update → local delete (a pair's new row too); forbidden with scheduling → retried without it ("invitations not sent" once that is accepted); forbidden on an update → rows rewritten in place from the server's current version, on a deletion → the item comes back from the server, both reported as forbidden; forbidden on a create, and the rest → poisoned with back-off, listed in the report	automatic / user fixes the item
A device change the server can't take (a read-only collection, an answer where none is allowed, an instance without its series, an in-place move to another account's calendar, a membership of a group the device can't change, every reminder removed from one occurrence of a series with alerts of its own)	put back on the device without a request, listed in the report as readOnly, rsvpRefused, instanceOnly, crossAccountMove, groupNotWritable or remindersNotRepresentable	—
A whole /set refused for an account (accountReadOnly, forbidden, accountNotFound, accountNotSupportedByMethod)	that account's items wait, the report lists the account; the other accounts go on	automatic once the account takes writes
A shared account refuses to list its address books or calendars (forbidden, accountNotFound, accountNotSupportedByMethod)	the account is left out of the run and only logged; its rows stay untouched	automatic once it lists them again
A planner throws on an item	download: the object goes on stale; upload: the item is skipped (plannerError); the rest of the run goes on	fixed in code; listed in the report
stateMismatch or serverUnavailable on a /set	re-download, retry up to 3 times, then io without progress	automatic (back-off)
429	io with delayUntil = Retry-After (60 s without one)	automatic
Too many local deletions	tooManyDeletions + system notification	user picks delete or undo (in the notification, or "Review deletions" in the settings)
Empty server on a full reconcile	safetyAbort, device untouched	user checks the server; turning sync off and on starts over
Edit during an upload	clearing assert fails → DIRTY stays, shadow updated	next run uploads the newer edit
JS crash or reload mid-run	task ends without report → soft error	automatic
React fails to start (debug without Metro)	boot fault → soft error in ~2 s	automatic
Account removed in Settings	providers drop rows; app turns sync off	user turns it on again
Logout / turning sync off	pending changes uploaded first; if that fails (or deletions exceed the threshold) the user is asked before anything is deleted	—
Provider refuses a batch (assert/tooLarge/provider)	per-item retry / split; an item that still fails goes on stale (the state still advances) and uploads nothing until it was fetched again	automatic, or listed in the report
App drops the account from its registry (e.g. after an auth error)	Android account kept, sync off, one "Sign in again" notification without a count of waiting changes; the section lists the account	user signs in again, or removes it from the device
Rows written for another server or registry account (SyncState owner)	the run stops with internal; the settings show its message	turning sync off and on starts over; the teardown uploads nothing for the other owner and asks before its changes are lost
DST-ambiguous start on a recurring event, unrepresentable rule	item skipped and reported with a reason (dstAmbiguous, ruleNotRepresentable); device shows it, never uploads its timing	limitation

Limits and performance

• JMAP: maxObjectsInGet/Set (500 on Stalwart), maxCallsInRequest (16), maxConcurrentRequests (4, shared with the UI: the engine keeps one request in flight), maxSizeRequest (10 MB; the engine targets 1 MB), queries of at most 5000 ids per call, /changes pages of 256 (Stalwart caps at 5000), cards and events of at most 512 KiB.
• Provider: ≤ 499 ops between yield points in ContactsProvider (the engine packs at most 400 per batch; CalendarProvider has no such limit), about 500 KB per applyBatch as estimateBatchBytes weighs it, photos in their own batch (photos over about 512 KiB stay on the server); 50 raw contacts or 50 events per chunk.
• A recurring meeting with many attendees on many occurrences is big: 25 attendees on 52 overrides weigh about 2 MB of provider ops on its first download, more than one Binder transaction (1 MB). Such an event is written as a chain of groups of at most 500 KB and 498 ops each: a new one master first (with its identity), then its exception rows, then its shadow (one column, about 0.56 MB here); a write into an existing one exception rows first and the master with its new shadow last. A run cut off between two groups is finished by the next one, planned from the old shadow. An event whose shadow alone is over a transaction does not sync (tooLarge, reported).
• JS thread: a setTimeout(0) yield between chunks, so the UI keeps rendering when the app is open (a teardown, which the UI starts, yields without timers: React Native pauses them while the app is in the background and no headless task runs); JSON crosses the bridge as strings.
• Time: 9.5 min per run (2.5 min expedited), checkpoints between chunks, continuation through fullSyncRequested. Checkpoints stop 30 s before the payload's deadline and a create batch needs 60 s left (see A sync run), so an expedited sync that cold-starts React Native has little room for creates.
• Contacts: the first card with a relation to name, or the first change of a group's members, reads every synced contact's shadow (SYNC2) once per run to index the cards by uid.

The 2,000 + 2,000 scale run is in Verification.

Testing

• Pure units (vitest): mappers with golden fixtures, one field per test, and round-trip properties (server → rows → projection equals the server's mapped subset; unmapped data survives edits); RRULE, EXDATE, durations, time zones, floating, all-day, DST edges of rules, exceptions, splits, reminders, attendees; merge rules; the import-graph purity test.

• Engine (vitest): src/device-sync/__tests__/fakes/fake-provider.ts, an in-memory ProviderPort with DIRTY/DELETED/VERSION, soft/hard deletes, asserts, back-references, yield points (and a batch committed up to a yield point before it fails), STATUS without a default, photos within the 96 px thumbnail kept as the thumbnail alone, and a Binder budget per batch (estimateBatchBytes, a photo by its bytes), with user-edit helpers (AOSP-style in-place edits, Fossify-style delete+reinsert, Etar's move); and a fake JMAP server for ContactCard/CalendarEvent/AddressBook/Calendar with /changes, cannotCalculateChanges, ifInState, SetError injection, the async uid index and Stalwart's key behaviour and its dropping of empty maps, inside overrides too. Crash injection at every checkpoint; poison items; the deletion guards; echo suppression; an edit during upload.

• Kotlin (JUnit, gradlew :app:testDebugUnitTest): report mapping, op and URI translation, scoping.

• Device (API 34 and 32 emulators, local Stalwart only): the scenarios in Verification, checked on three sides: the provider (adb shell content query …), the server (JMAP calls) and the apps' UI. Debug builds create accounts without UI:

  adb shell am broadcast -n com.anonymous.bulwarkmobile/.sync.DeviceSyncDebugReceiver \
    --es cmd ensure --es name <name> --es registry <AccountEntry.id> --es enable contacts,calendar
  adb shell requestsync -n <name> -t com.anonymous.bulwarkmobile.account -a com.android.contacts --manual
  

Decisions and limitations

Decisions made where the brief left a choice (safe defaults):

• Default reminder owner when Bulwark's reminders are off: the calendar app.
• Delete on the device + edit on the server: the delete wins, unless the edit moved the object out of the device's collections.
• Deletion threshold: > 50 and > 20 % of the authority's objects.
• Full-reconcile safety: more than 10 device rows against an empty server.
• Turning a Contacts/Calendar toggle off in Android Settings pauses sync and keeps the data, like Google accounts; turning it off in the app also removes the data.
• Events that are only in unselected calendars, and cards only in unselected address books, are not on the device; an event in several calendars lives in the first selected one (by id), a move on the device patches only that membership, and a deletion removes only the selected memberships.
• A shared collection that another signed-in account already syncs gets a warning in the chooser, not a refusal.
• An app account dropped from the registry keeps its Android account (sync off) until the user signs in again or removes it: its device changes would otherwise be lost.
• A shared account that refuses to list its address books or calendars is left out of the run and only logged, not reported: Stalwart lists an account shared for mail only with every capability, so the warning would never go away. Its rows stay on the device, since a refusal can pass.
• The user's own address for "me" and OWNER_ACCOUNT: the session login when it is an email address, else the first identity's address; aliases from identities also count as "me".

Known limitations:

• Google Contacts may not show or edit third-party accounts (closed source; DAVx5 reports the same); AOSP-based and Fossify Contacts do.
• Fossify Contacts re-inserts rows on save: a field or entry it does not model (a second nickname or organization, a department, a label) is kept on the server, but an entry deleted or a field cleared in Fossify is not deleted or cleared on the server, and comes back on the device with the contact's next clean write. Fossify Calendar's rule edits upload only in the parts its editor models.
• Android shows extra instances for BYSETPOS rules other than monthly-BYDAY, and none for a daily series older than about 5.5 years (AOSP's 2000-period expansion cap).
• Rules with rscale/skip/leap months: shown approximately, timing never uploaded. EXRULE is not supported by Stalwart.
• Recurring events whose start or override falls in a DST overlap cannot be uploaded (Stalwart drops ambiguous local times).
• An UNTIL in a DST gap or overlap moves one hour later, so an Etar "this and following" split exactly at an instance inside the repeated hour can include that instance in the old series again (better than Stalwart dropping the UNTIL).
• A single occurrence that Stalwart holds only as an instance (no master) is shown but read-only: an edit is put back and reported as instanceOnly.
• Stalwart refuses every change to an event in a calendar shared without write rights, even the user's own answer to an invitation (to the series or to one occurrence). Where the calendar grants RSVP the sync sends the answer; after the refusal (forbidden, also on the retry without scheduling messages) it rewrites the rows from the server's version, so the answer is put back on the device, and the report lists the item as forbidden. Where the calendar grants no RSVP, the answer is put back without a request and reported as rsvpRefused.
• When the organizer of an event is one of the user's own identities (for example a shared mailbox the user may send as, which Stalwart lists among the user's identities), the device shows the user as the organizer: no answer buttons, and the user's own attendee entry is folded into the organizer's row. Every identity address counts as "me" (calendarAddresses in src/device-sync/jmap/session.ts, the address keys in src/device-sync/calendar/attendees.ts).
• Moving an event to a calendar of another JMAP account uploads as a create in the target account and a destroy in the source (a new uid; participants are notified accordingly). An app that moves it by changing CALENDAR_ID in place gets the row put back at the next sync (reported as crossAccountMove).
• Fossify Calendar shows the occurrences of a zoned weekly series one hour off after a daylight-saving change (it expands the rule without the zone's DST change); Google Calendar and Etar show them right. The rows are correct.
• An event whose shadow alone is over one provider transaction does not sync (tooLarge, reported). A big event the server replaced under a reused id (another uid) is written as one group and can fail the same way.
• A move made on the device whose patch fails with a poisoning error (invalidPatch, tooLarge) leaves the event twice on the server until the back-off ends; then the move ends as a create plus a destroy (a new uid). A forbidden (a move out of a calendar the user can't write, say) ends as a copy in the target calendar while the original stays; the refusal is reported.
• When the device changed a field with a move or before it, and the server changed the same field meanwhile, the device's value wins: the pair does not see the conflict (see Uploads).
• Google Calendar has no move: its "Copy to" plus a deletion pairs only when both reach the same sync (within the 30 s before the upload sync, say); otherwise the copy uploads as a new event (a new uid, without the original's unmapped properties). Seen on the API 34 emulator.
• Google Calendar copies the series' reminders onto an occurrence it edits, so that occurrence keeps its own reminders afterwards. A new exception row an app inserts without reminders inherits the series' reminders, also when the user removed them all in the app's "this event" editor. Removing every reminder of one occurrence of a series with reminders of its own can't be stored on Stalwart: it is put back and reported (remindersNotRepresentable).
• Etar writes an answer saved together with a move to the old row, so the answer is lost with that row.
• A group card deleted on the device that another client moved out of the synced address books is destroyed anyway: group rows record no address books (see Merge rules).
• Descriptions over 64 KB are shown truncated and are not uploaded when edited.
• A shared address book or calendar that two signed-in accounts can both see appears once per account when both sync it (each Android account holds its own copy).
• Floating events follow the device zone; alerts relative to the end, absolute alerts and alerts after the start are kept on the server but not shown.
• Photos travel as 512 px JPEG data: URIs; a larger original is replaced by its scaled copy only when the user changes the photo on the device. A photo over about 512 KiB is not shown on the device (it stays on the server), and replacing a photo within the 96 px thumbnail by another one that small on the device is not uploaded.
• AOSP Contacts edits an address as one formatted line. The sync reads the parts back along the old line; when the edit changes the line's layout or spans two parts, the server's address keeps the whole line as its street: full and the components agree, but city, postcode and country are no longer separate parts.
• The settings' status line shows how many items could not be synced, never conflicts: those resolve on their own (the server wins), and the stored run counts them.
• Read-only address books and calendars and subscribed calendars were not verified on a device (the test server had none); their handling is covered by tests only.

Open:

• Contacts: a custom name/full is kept even when a component edit makes it stale; a group title cleared on the device is not uploaded; clearing company and department leaves an organization entry without a name on the server.
• Calendar: a recurrence id in a repeated hour uses the first instant (RFC 5545) for ORIGINAL_INSTANCE_TIME; to be checked against CalendarProvider's instance expansion on a device.

Out of scope

Possible follow-ups:

• iOS. iOS has no third-party sync-provider API for Contacts or Calendar. The realistic route is a CalDAV/CardDAV configuration profile, which Stalwart already serves.
• Tasks through the OpenTasks provider.
• A contacts Directory provider for server-side search.
• Importing device-local contacts into Bulwark.
• An account-scoped Colors table so calendar apps can offer event colours.
• Re-basing old daily/hourly series past AOSP's 2000-period expansion cap.

Verification

Checked on Android emulators against a local Stalwart 0.16.23, each time on three sides (provider rows, server objects, the apps' screens): contacts on API 32 with AOSP Contacts and Fossify Contacts, calendars on API 34 with Google Calendar, Etar and Fossify Calendar. What these runs found in other apps or in Stalwart is under known limitations.

• Contacts. Every mapped field of a card (name parts, nickname, organization with department and title, typed and labelled emails, phones and addresses, a website, a birthday without year, an anniversary, a relation, a note, a photo) shows in both apps. An edit in AOSP Contacts (in place) and one in Fossify Contacts (delete and re-insert) upload only the changed entries and leave the rest of the card alone; the next clean write puts back on the device what Fossify dropped (a department, the primary flag, the address parts). A contact created in AOSP Contacts goes up once; deletions go both ways; a conflict on different fields keeps both changes, one on the same field takes the server's and is counted. A photo within the 96 px thumbnail keeps one row through server edits and stays on the server after a device edit.
• Calendar. Timed events in two zones, a two-day all-day event, a floating event, a weekly series with an excluded and a moved occurrence, two alerts and a meeting show right in Google Calendar and Etar. An edit, a new event and a deletion in Google Calendar go up, and the edit keeps the event's unmapped properties (keywords, locale, priority, links). Etar's "only this event", deleting one occurrence and "this and all future events", and "only this occurrence" in Fossify Calendar and Google Calendar become the right overrides, exclusions and series, with no instance duplicated or lost. "Yes" and "Maybe" in Google Calendar change only the user's participationStatus. Tasks and a tasks-only calendar stay off the device; switching the reminder owner both ways rewrites only the reminders; a calendar deselected and selected again leaves and comes back without server writes; an event created in a shared account's writable calendar lands in that account; a change of the device zone rewrites only floating events.
• No duplicates. With the process killed, or the network cut, between a create's /set and its identity write (30 contacts or 20 events at a time), the next run adopts every row by uid, and server and device hold each item once.
• Deletion threshold. 60 contacts deleted at once stay on the server until the user chooses in Android's notification: "undo" brings them back on the device, "delete" deletes them on the server.
• Settings and lifecycle. The switch, the chooser, "Sync now", "Paused in Android settings", "Grant access" after the permissions were revoked, and "Android account settings" (API 32 and 34) work as described. Turning sync off with an edit waiting offline asks first, and "Keep syncing" keeps the edit, which goes up once the network is back; turning sync off and on removes and restores the rows without server writes or duplicates; an account removed in Android Settings is not added back by the app; signing out removes the rows and the Android account.
• Covered by tests only: contact groups and memberships; photos too large for the device; meetings too big for one transaction; moves between calendars and a series turned into a single event (Etar's delete and insert, meetings included); events and exceptions Google Calendar creates without a status; a device create the server deletes before the next download; a deletion of an object another client moved away meanwhile; a contact created and deleted between two syncs; changes put back because the server can't take them, an occurrence's cleared colour and removed reminders among them; an address edited as one line in AOSP Contacts; the status kept after a run that did nothing; the "Review deletions" question; Bulwark's reminders while the device calendar can't sync; the push rules; and signing out during a turn-on.

Scale, with a debug build (JavaScript from Metro in dev mode) on an API 34 emulator given 4 GB of memory: 2,000 cards and 2,000 events (100 of them recurring, with overrides) on top of 152 cards and 35 events. Times are the adapter's, from the start of the sync to the report; memory is the peak PSS of the app process.

Run	Contacts	Calendar	Peak PSS
First sync, contacts alone	2,000 inserts, 57.6 s	—	464 MB
First sync, calendar alone	—	2,000 inserts + 68 exception rows, 38.7 s	450 MB
First sync, both at once (they share the JS thread)	117.1 s	117.8 s	455 MB
Both at once, while scrolling the mail list	150.3 s	154.0 s	433 MB
1 card and 1 event changed on the server	0.83 s	1.03 s	448 MB
No changes	0.38 s	0.70 s	436 MB
Both collections deleted on the server	2,000 deletes, 27-36 s	2,000 deletes, 36-41 s	452 MB

The app used about 412 MB before the runs (a debug build with its dev tooling); syncing added 20-50 MB at the peak. No run crashed or caused an ANR. During the first syncs, scrolling the mail list stayed as smooth as without them (2.85 % janky frames against 3.05 %), and switching between the app's tabs took as long as without a sync, except once, when the app's own Contacts screen loaded the 2,000 new cards at the same time. The sync shares the JS thread with the app's screens (it peaked at 96 %), so a first sync takes longer while the app is in use (74 s instead of 57.6 s for the contacts). A release build and a real phone were not measured.