neon/docs/rfcs/035-safekeeper-dynamic-membership-change.md

Safekeeper dynamic membership change

To quickly recover from safekeeper node failures and do rebalancing we need to be able to change set of safekeepers the timeline resides on. The procedure must be safe (not lose committed log) regardless of safekeepers and compute state. It should be able to progress if any majority of old safekeeper set, any majority of new safekeeper set and compute are up and connected. This is known as a consensus membership change. It always involves two phases: 1) switch old majority to old + new configuration, preventing commits without acknowledge from the new set 2) bootstrap the new set by ensuring majority of the new set has all data which ever could have been committed before the first phase completed; after that switch is safe to finish. Without two phases switch to the new set which quorum might not intersect with quorum of the old set (and typical case of ABC -> ABD switch is an example of that, because quorums AC and BD don't intersect). Furthermore, procedure is typically carried out by the consensus leader, and so enumeration of configurations which establishes order between them is done through consensus log.

In our case consensus leader is compute (walproposer), and we don't want to wake up all computes for the change. Neither we want to fully reimplement the leader logic second time outside compute. Because of that the proposed algorithm relies for issuing configurations on the external fault tolerant (distributed) strongly consisent storage with simple API: CAS (compare-and-swap) on the single key. Properly configured postgres suits this.

In the system consensus is implemented at the timeline level, so algorithm below applies to the single timeline.

Algorithm

Definitions

A configuration is

struct Configuration {
    generation: Generation, // a number uniquely identifying configuration
    sk_set: Vec<NodeId>, // current safekeeper set
    new_sk_set: Optional<Vec<NodeId>>,
}

Configuration with new_set present is used for the intermediate step during the change and called joint configuration. Generations establish order of generations: we say c1 is higher than c2 if c1.generation > c2.generation.

Persistently stored data changes

Safekeeper starts storing its current configuration in the control file. Update of is atomic, so in-memory value always matches the persistent one.

External CAS providing storage (let's call it configuration storage here) also stores configuration for each timeline. It is initialized with generation 1 and initial set of safekeepers during timeline creation. Executed CAS on it must never be lost.

Compute <-> safekeeper protocol changes

ProposerGreeting message carries walproposer's configuration if it is already established (see below), else null. AcceptorGreeting message carries safekeeper's current Configuration. All further messages (VoteRequest, VoteResponse, ProposerElected, AppendRequest, AppendResponse) carry generation number, of walproposer in case of wp->sk message or of safekeeper in case of sk->wp message.

Safekeeper changes

Basic rule: once safekeeper observes configuration higher than his own it immediately switches to it. It must refuse all messages with lower generation that his. It also refuses messages if it is not member of the current generation (that is, of either sk_set of sk_new_set), though it is likely not unsafe to process them (walproposer should ignore result anyway).

If there is non null configuration in ProposerGreeting and it is higher than current safekeeper one, safekeeper switches to it.

Safekeeper sends its current configuration in its first message to walproposer AcceptorGreeting. It refuses all other walproposer messages if the configuration generation in them is less than its current one. Namely, it refuses to vote, to truncate WAL in handle_elected and to accept WAL. In response it sends its current configuration generation to let walproposer know.

Safekeeper gets PUT /v1/tenants/{tenant_id}/timelines/{timeline_id}/configuration accepting Configuration. Safekeeper switches to the given conf it is higher than its current one and ignores it otherwise. In any case it replies with

struct ConfigurationSwitchResponse {
    conf: Configuration,
    term: Term,
    last_log_term: Term,
    flush_lsn: Lsn,
}

Compute (walproposer) changes

Basic rule is that joint configuration requires votes from majorities in the both set and new_sk_set.

Compute receives list of safekeepers to connect to from the control plane as currently and tries to communicate with all of them. However, the list does not define consensus members. Instead, on start walproposer tracks highest configuration it receives from AcceptorGreetings. Once it assembles greetings from majority of sk_set and majority of new_sk_set (if it is present), it establishes this configuration as its own and moves to voting.

It should stop talking to safekeepers not listed in the configuration at this point, though it is not unsafe to continue doing so.

To be elected it must receive votes from both majorites if new_sk_set is present. Similarly, to commit WAL it must receive flush acknowledge from both majorities.

If walproposer hears from safekeeper configuration higher than his own (i.e. refusal to accept due to configuration change) it simply restarts.

Change algorithm

The following algorithm can be executed anywhere having access to configuration storage and safekeepers. It is safe to interrupt / restart it and run multiple instances of it concurrently, though likely one of them won't make progress then. It accepts desired_set: Vec<NodeId> as input.

Algorithm will refuse to make the change if it encounters previous interrupted change attempt, but in this case it will try to finish it.

It will eventually converge if old majority, new majority and configuration storage are reachable.

1. Fetch current timeline configuration from the configuration storage.
2. If it is already joint one and new_set is different from desired_set refuse to change. However, assign join conf to (in memory) var join_conf and proceed to step 4 to finish the ongoing change.
3. Else, create joint joint_conf: Configuration: increment current conf number n and put desired_set to new_sk_set. Persist it in the configuration storage by doing CAS on the current generation: change happens only if current configuration number is still n. Apart from guaranteeing uniqueness of configurations, CAS linearizes them, ensuring that new configuration is created only following the previous one when we know that the transition is safe. Failed CAS aborts the procedure.
4. Call PUT configuration on safekeepers from the current set, delivering them joint_conf. Collecting responses from majority is required to proceed. If any response returned generation higher than joint_conf.generation, abort (another switch raced us). Otherwise, choose max <last_log_term, flush_lsn> among responses and establish it as (in memory) sync_position. Also choose max term and establish it as (in memory) sync_term. We can't finish the switch until majority of the new set catches up to this sync_position because data before it could be committed without ack from the new set. Similarly, we'll bump term on new majority to sync_term so that two computes with the same term are never elected.
5. Initialize timeline on safekeeper(s) from new_sk_set where it doesn't exist yet by doing pull_timeline from the majority of the current set. Doing that on majority of new_sk_set is enough to proceed, but it is reasonable to ensure that all new_sk_set members are initialized -- if some of them are down why are we migrating there?
6. Call POST bump_term(sync_term) on safekeepers from the new set. Success on majority is enough.
7. Repeatedly call PUT configuration on safekeepers from the new set, delivering them joint_conf and collecting their positions. This will switch them to the joint_conf which generally won't be needed because pull_timeline already includes it and plus additionally would be broadcast by compute. More importantly, we may proceed to the next step only when <last_log_term, flush_lsn> on the majority of the new set reached sync_position. Similarly, on the happy path no waiting is not needed because pull_timeline already includes it. However, we should double check to be safe. For example, timeline could have been created earlier e.g. manually or after try-to-migrate, abort, try-to-migrate-again sequence.
8. Create new_conf: Configuration incrementing join_conf generation and having new safekeeper set as sk_set and None new_sk_set. Write it to configuration storage under one more CAS.
9. Call PUT configuration on safekeepers from the new set, delivering them new_conf. It is enough to deliver it to the majority of the new set; the rest can be updated by compute.

I haven't put huge effort to make the description above very precise, because it is natural language prone to interpretations anyway. Instead I'd like to make TLA+ spec of it.

Description above focuses on safety. To make the flow practical and live, here a few more considerations.

1. It makes sense to ping new set to ensure it we are migrating to live node(s) before step 3.
2. If e.g. accidentally wrong new sk set has been specified, before CAS in step 6 is completed it is safe to rollback to the old conf with one more CAS.
3. On step 4 timeline might be already created on members of the new set for various reasons; the simplest is the procedure restart. There are more complicated scenarious like mentioned in step 5. Deleting and re-doing pull_timeline is generally unsafe without involving generations, so seems simpler to treat existing timeline as success. However, this also has a disadvantage: you might imagine an surpassingly unlikely schedule where condition in the step 5 is never reached until compute is (re)awaken up to synchronize new member(s). I don't think we'll observe this in practice, but can add waking up compute if needed.
4. In the end timeline should be locally deleted on the safekeeper(s) which are in the old set but not in the new one, unless they are unreachable. To be safe this also should be done under generation number (deletion proceeds only if current configuration is <= than one in request and safekeeper is not memeber of it).
5. If current conf fetched on step 1 is already not joint and members equal to desired_set, jump to step 7, using it as new_conf.

Implementation

The procedure ought to be driven from somewhere. Obvious candidates are control plane and storage_controller; and as each of them already has db we don't want yet another storage. I propose to manage safekeepers in storage_controller because 1) since it is in rust it simplifies simulation testing (more on this below) 2) it already manages pageservers.

This assumes that migration will be fully usable only after we migrate all tenants/timelines to storage_controller. It is discussible whether we want also to manage pageserver attachments for all of these, but likely we do.

This requires us to define storcon <-> cplane interface and changes.

storage_controller <-> control plane interface and changes

First of all, control plane should change storing safekeepers per timeline instead of per tenant because we can't migrate tenants atomically.

The important question is how updated configuration is delivered from storage_controller to control plane to provide it to computes. As always, there are two options, pull and push. Let's do it the same push as with pageserver /notify-attach because 1) it keeps storage_controller out of critical compute start path 2) uniformity. It makes storage_controller responsible for retrying notifying control plane until it succeeds.

It is not needed for the control plane to fully know the Configuration. It is enough for it to only to be aware of the list of safekeepers in the latest configuration to supply it to compute, plus associated generation number to protect from stale update requests and to also pass it to compute.

So, cplane /notify-safekeepers for the timeline can accept JSON like

{
   tenant_id: String,
   timeline_id: String,
   generation: u32,
   safekeepers: Vec<SafekeeperId>,
}

where SafekeeperId is

{
   node_id: u64,
   host: String
}

In principle host is redundant, but may be useful for observability.

The request updates list of safekeepers in the db if the provided conf generation is higher (the cplane db should also store generations for this). Similarly to /notify-attach, it should update db which makes the call successful, and then try to schedule apply_config if possible, it is ok if not. storage_controller should rate limit calling the endpoint, but likely this won't be needed, as migration throughput is limited by pull_timeline.

Timeline (branch) creation in cplane should call storage_controller POST tenant/:tenant_id/timeline like it currently does for sharded tenants. Response should be augmented with safekeepers_generation and safekeepers fields like described in /notify-safekeepers above. Initially (currently) these fields may be absent; in this case cplane chooses safekeepers on its own like it currently does. The call should be retried until succeeds.

Timeline deletion and tenant deletion in cplane should call appropriate storage_controller endpoints like it currently does for sharded tenants. The calls should be retried until they succeed.

When compute receives safekeepers list from control plane it needs to know the generation to checked whether it should be updated (note that compute may get safekeeper list from either cplane or safekeepers). Currently neon.safekeepers GUC is just a comma separates list of host:port. Let's prefix it with g#<generation>: to this end, so it will look like

g#42:safekeeper-0.eu-central-1.aws.neon.tech:6401,safekeeper-2.eu-central-1.aws.neon.tech:6401,safekeeper-1.eu-central-1.aws.neon.tech:6401

To summarize, list of cplane changes:

• per tenant -> per timeline safekeepers management and addition of int safekeeper_generation field.
• /notify-safekeepers endpoint.
• Branch creation call may return list of safekeepers and when it is present cplane should adopt it instead of choosing on its own like it does currently.
• neon.safekeepers GUC should be prefixed with g#<generation>:.

storage_controller implementation

If desired, we may continue using current 'load everything on startup and keep in memory' approach: single timeline shouldn't take more than 100 bytes (it's 16 byte tenant_id, 16 byte timeline_id, int generation, vec of ~3 safekeeper ids plus some flags), so 10^6 of timelines shouldn't take more than 100MB.

Similar to pageserver attachment Intents storage_controller would have in-memory MigrationRequest (or its absense) for each timeline and pool of tasks trying to make these request reality; this ensures one instance of storage_controller won't do several migrations on the same timeline concurrently. In the first version it is simpler to have more manual control and no retries, i.e. migration failure removes the request. Later we can build retries and automatic scheduling/migration around. MigrationRequest is

enum MigrationRequest {
    To(Vec<NodeId>),
    FinishPending,
}

FinishPending requests to run the procedure to ensure state is clean: current configuration is not joint and majority of safekeepers are aware of it, but do not attempt to migrate anywhere. If current configuration fetched on step 1 is not joint it jumps to step 7. It should be run at startup for all timelines (but similarly, in the first version it is ok to trigger it manually).

Schema

safekeepers table mirroring current nodes should be added, except that for scheduling_policy: it is enough to have at least in the beginning only 3 fields: 1) active 2) paused (initially means only not assign new tlis there 3) decomissioned (node is removed).

timelines table:

table! {
    // timeline_id is primary key
    timelines (tenant_id, timeline_id) {
        timeline_id -> Varchar,
        tenant_id -> Varchar,
        start_lsn -> pg_lsn,
        generation -> Int4,
        sk_set -> Array<Int4>, // list of safekeeper ids
        new_sk_set -> Nullable<Array<Int8>>, // list of safekeeper ids, null if not joint conf
        cplane_notified_generation -> Int4,
        deleted_at -> Nullable<Timestamptz>,
    }
}

start_lsn is needed to create timeline on safekeepers properly, see below. We might also want to add ancestor_timeline_id to preserve the hierarchy, but for this RFC it is not needed.

API

Node management is similar to pageserver:

1. POST /control/v1/safekeepers inserts safekeeper.
2. GET /control/v1/safekeepers lists safekeepers.
3. GET /control/v1/safekeepers/:node_id gets safekeeper.
4. PUT /control/v1/safekepers/:node_id/status changes status to e.g. offline or decomissioned. Initially it is simpler not to schedule any migrations here.

Safekeeper deploy scripts should register safekeeper at storage_contorller as they currently do with cplane, under the same id.

Timeline creation/deletion will work through already existing POST and DELETE tenant/:tenant_id/timeline. Cplane is expected to retry both until they succeed. See next section on the implementation details.

We don't want to block timeline creation/deletion when one safekeeper is down. Currently this is crutched by compute implicitly creating timeline on any safekeeper it is connected to. This creates ugly timeline state on safekeeper when timeline is created, but start LSN is not defined yet. Next section describes dealing with this.

Tenant deletion repeats timeline deletion for all timelines.

Migration API: the first version is the simplest and the most imperative:

1. PUT /control/v1/safekeepers/migrate schedules MigrationRequests to move all timelines from one safekeeper to another. It accepts json

{
    "src_sk": u32,
    "dst_sk": u32,
    "limit": Optional<u32>,
}

Returns list of scheduled requests.

2. PUT /control/v1/tenant/:tenant_id/timeline/:timeline_id/safekeeper_migrate schedules MigrationRequest to move single timeline to given set of safekeepers:

{
    "desired_set": Vec<u32>,
}

Returns scheduled request.

Similar call should be added for the tenant.

It would be great to have some way of subscribing to the results (apart from looking at logs/metrics).

3. GET /control/v1/tenant/:tenant_id/timeline/:timeline_id/ should return current in memory state of the timeline and pending MigrationRequest, if any.

4. PUT /control/v1/tenant/:tenant_id/timeline/:timeline_id/safekeeper_migrate_abort tries to abort the migration by switching configuration from the joint to the one with (previous) sk_set under CAS (incrementing generation as always).

API implementation and reconciliation

For timeline creation/deletion we want to preserve the basic assumption that unreachable minority (1 sk of 3) doesn't block their completion, but eventually we want to finish creation/deletion on nodes which missed it (unless they are removed). Similarly for migration; it may and should finish even though excluded members missed their exclusion. And of course e.g. such pending exclusion on node C after migration ABC -> ABD must not prevent next migration ABD -> ABE. As another example, if some node missed timeline creation it clearly must not block migration from it. Hence it is natural to have per safekeeper background reconciler which retries these ops until they succeed. There are 3 possible operation types, and the type is defined by timeline state (membership configuration and whether it is deleted) and safekeeper id: we may need to create timeline on sk (node added), locally delete it (node excluded, somewhat similar to detach) or globally delete it (timeline is deleted).

Next, on storage controller restart in principle these pending operations can be figured out by comparing safekeepers state against storcon state. But it seems better to me to materialize them in the database; it is not expensive, avoids these startup scans which themselves can fail etc and makes it very easy to see outstanding work directly at the source of truth -- the db. So we can add table safekeeper_timeline_pending_ops

table! {
    // timeline_id, sk_id is primary key
    safekeeper_timeline_pending_ops (sk_id, tenant_id, timeline_id) {
        sk_id -> int8,
        tenant_id -> Varchar,
        timeline_id -> Varchar,
        generation -> Int4,
        op_type -> Varchar,
    }
}

op_type can be include (seed from peers and ensure generation is up to date), exclude (remove locally) and delete. Field is actually not strictly needed as it can be computed from current configuration, but gives more explicit observability.

generation is necessary there because after op is done reconciler must remove it and not remove another row with higher gen which in theory might appear.

Any insert of row should overwrite (remove) all rows with the same sk and timeline id but lower generation as next op makes previous obsolete. Insertion of op_type delete overwrites all rows.

About exclude: rather than adding explicit safekeeper http endpoint, it is reasonable to reuse membership switch endpoint: if safekeeper is not member of the configuration it locally removes the timeline on the switch. In this case 404 should also be considered an 'ok' answer by the caller.

So, main loop of per sk reconcile reads safekeeper_timeline_pending_ops joined with timeline configuration to get current conf (with generation n) for the safekeeper and does the jobs, infinitely retrying failures:

1. If node is member (include):

• Check if timeline exists on it, if not, call pull_timeline on it from other members
• Call switch configuration to the current

2. If node is not member (exclude):

• Call switch configuration to the current, 404 is ok.

3. If timeline is deleted (delete), call delete.

In cases 1 and 2 remove safekeeper_timeline_pending_ops for the sk and timeline with generation <= n if op_type is not delete. In case 3 also remove safekeeper_timeline_pending_ops entry + remove timelines entry if there is nothing left in safekeeper_timeline_pending_ops for the timeline.

Let's consider in details how APIs can be implemented from this angle.

Timeline creation. It is assumed that cplane retries it until success, so all actions must be idempotent. Now, a tricky point here is timeline start LSN. For the initial (tenant creation) call cplane doesn't know it. However, setting start_lsn on safekeepers during creation is a good thing -- it provides a guarantee that walproposer can always find a common point in WAL histories of safekeeper and its own, and so absense of it would be a clear sign of corruption. The following sequence works:

1. Create timeline (or observe that it exists) on pageserver, figuring out last_record_lsn in response.

2. Choose safekeepers and insert (ON CONFLICT DO NOTHING) timeline row into the db. Note that last_record_lsn returned on the previous step is movable as it changes once ingestion starts, insert must not overwrite it (as well as other fields like membership conf). On the contrary, start_lsn used in the next step must be set to the value in the db. cplane_notified_generation can be set to 1 (initial generation) in insert to avoid notifying cplane about initial conf as cplane will receive it in timeline creation request anyway.

3. Issue timeline creation calls to at least majority of safekeepers. Using majority here is not necessary but handy because it guarantees that any live majority will have at least one sk with created timeline and so reconciliation task can use pull_timeline shared with migration instead of create timeline special init case. OFC if timeline is already exists call is ignored.

4. For minority of safekeepers which could have missed creation insert entries to safekeeper_timeline_pending_ops. We won't miss this insertion because response to cplane is sent only after it has happened, and cplane retries the call until 200 response.

   There is a small question how request handler (timeline creation in this case) would interact with per sk reconciler. As always I prefer to do the simplest possible thing and here it seems to be just waking it up so it re-reads the db for work to do. Passing work in memory is faster, but that shouldn't matter, and path to scan db for work will exist anyway, simpler to reuse it.

For pg version / wal segment size: while we may persist them in timelines table, it is not necessary as initial creation at step 3 can take them from pageserver or cplane creation call and later pull_timeline will carry them around.

Timeline migration.

1. CAS to the db to create joint conf, and in the same transaction create safekeeper_timeline_pending_ops include entries to initialize new members as well as deliver this conf to current ones; poke per sk reconcilers to work on it. Also any conf change should also poke cplane notifier task(s).
2. Once it becomes possible per alg description above, get out of joint conf with another CAS. Task should get wakeups from per sk reconcilers because conf switch is required for advancement; however retries should be sleep based as well as LSN advancement might be needed, though in happy path it isn't. To see whether further transition is possible on wakup migration executor polls safekeepers per the algorithm. CAS creating new conf with only new members should again insert entries to safekeeper_timeline_pending_ops to switch them there, as well as exclude rows to remove timeline from old members.

Timeline deletion: just set deleted_at on the timeline row and insert safekeeper_timeline_pending_ops entries in the same xact, the rest is done by per sk reconcilers.

When node is removed (set to decomissioned), safekeeper_timeline_pending_ops for it must be cleared in the same transaction.

One more task pool should infinitely retry notifying control plane about changed safekeeper sets (trying making cplane_notified_generation equal generation).

Dealing with multiple instances of storage_controller

Operations described above executed concurrently might create some errors but do not prevent progress, so while we normally don't want to run multiple instances of storage_controller it is fine to have it temporarily, e.g. during redeploy.

To harden against some controller instance creating some work in safekeeper_timeline_pending_ops and then disappearing without anyone pickup up the job per sk reconcilers apart from explicit wakups should scan for work periodically. It is possible to remove that though if all db updates are protected with leadership token/term -- then such scans are needed only after leadership is acquired.

Any interactions with db update in-memory controller state, e.g. if migration request failed because different one is in progress, controller remembers that and tries to finish it.

Testing

neon_local should be switched to use storage_controller, playing role of control plane.

There should be following layers of tests:

1. Model checked TLA+ spec specifies the algorithm and verifies its basic safety.

2. To cover real code and at the same time test many schedules we should have simulation tests. For that, configuration storage, storage_controller <-> safekeeper communication and pull_timeline need to be mocked and main switch procedure wrapped to as a node (thread) in simulation tests, using these mocks. Test would inject migrations like it currently injects safekeeper/walproposer restars. Main assert is the same -- committed WAL must not be lost.

3. Since simulation testing injects at relatively high level points (not syscalls), it omits some code, in particular pull_timeline. Thus it is better to have basic tests covering whole system as well. Extended version of test_restarts_under_load would do: start background load and do migration under it, then restart endpoint and check that no reported commits had been lost. I'd also add one more creating classic network split scenario, with one compute talking to AC and another to BD while migration from nodes ABC to ABD happens.

4. Simple e2e test should ensure that full flow including cplane notification works.

Order of implementation and rollout

Note that

• Control plane parts and integration with it is fully independent from everything else (tests would use simulation and neon_local).
• It is reasonable to make compute <-> safekeepers protocol change independent of enabling generations.
• There is a lot of infra work making storage_controller aware of timelines and safekeepers and its impl/rollout should be separate from migration itself.
• Initially walproposer can just stop working while it observes joint configuration. Such window would be typically very short anyway.
• Obviously we want to test the whole thing thoroughly on staging and only then gradually enable in prod.

Let's have the following implementation bits for gradual rollout:

• compute gets neon.safekeepers_proto_version flag. Initially both compute and safekeepers will be able to talk both versions so that we can delay force restart of them and for simplicity of rollback in case it is needed.
• storcon gets -set-safekeepers config option disabled by default. Timeline creation request chooses safekeepers (and returns them in response to cplane) only when it is set to true.
• control_plane [see above](storage_controller-<->-control-plane interface-and-changes) prefixes neon.safekeepers GUC with generation number. When it is 0 (or prefix not present at all), walproposer behaves as currently, committing on the provided safekeeper list -- generations are disabled. If it is non 0 it follows this RFC rules.
• We provide a script for manual migration to storage controller. It selects timeline(s) from control plane (specified or all of them) db and calls special import endpoint on storage controller which is very similar to timeline creation: it inserts into the db, sets configuration to initial on the safekeepers, calls cplane notify-safekeepers.

Then the rollout for a region would be:

• Current situation: safekeepers are choosen by control_plane.
• We manually migrate some timelines, test moving them around.
• Then we enable --set-safekeepers so that all new timelines are on storage controller.
• Finally migrate all existing timelines using the script (no compute should be speaking old proto version at this point).

Until all timelines are managed by storcon we'd need to use current ad hoc script to migrate if needed. To keep state clean, all storage controller managed timelines must be migrated before that, or controller db and configurations state of safekeepers dropped manually.

Very rough implementation order:

• Add concept of configurations to safekeepers (including control file), implement v3 protocol.
• Implement walproposer changes, including protocol.
• Implement storconn part. Use it in neon_local (and pytest).
• Make cplane store safekeepers per timeline instead of per tenant.
• Implement cplane/storcon integration. Route branch creation/deletion through storcon. Then we can test migration of new branches.
• Finally import existing branches. Then we can drop cplane safekeeper selection code. Gradually enable configurations at computes and safekeepers. Before that, all computes must talk only v3 protocol version.

Integration with evicted timelines

Currently, pull_timeline doesn't work correctly with evicted timelines because copy would point to original partial file. To fix let's just do s3 copy of the file. It is a bit stupid as generally unnecessary work, but it makes sense to implement proper migration before doing smarter timeline archival. Issue

Possible optimizations

Steps above suggest walproposer restart (with re-election) and thus reconnection to safekeepers. Since by bumping term on new majority we ensure that leader terms are unique even across generation switches it is possible to preserve connections. However, it is more complicated, reconnection is very fast and it is much more important to avoid compute restart than millisecond order of write stall.

Multiple joint consensus: algorithm above rejects attempt to change membership while another attempt is in progress. It is possible to overlay them and AFAIK Aurora does this but similarly I don't think this is needed.

Misc

We should use Compute <-> safekeeper protocol change to include other (long yearned) modifications:

• send data in network order without putting whole structs to be arch independent
• remove term_start_lsn from AppendRequest
• add horizon to TermHistory
• add to ProposerGreeting number of connection from this wp to sk