greptimedb/src/mito2/AGENTS.md

mito2 — Agent & Contributor Guide

Navigation aid for src/mito2. Keep it short and point to code; do not duplicate the code here. Paths are relative to the repo root.

Repo-wide rules that apply here: .agents/architecture-invariants.md.

What this crate does

Mito2 is GreptimeDB's primary time-series region storage engine. It owns the write path (memtable + WAL), flushing memtables to Parquet SST files, TWCS/windowed compaction, and the read path (multi-level merge + dedup with snapshot isolation). It implements the RegionEngine trait from store-api.

Module map

Module	Path	Purpose
engine	src/mito2/src/engine.rs	MitoEngine (the RegionEngine impl) and request dispatch
worker	src/mito2/src/worker.rs, src/mito2/src/worker/	Per-region worker loop; write/alter/flush handlers
region	src/mito2/src/region.rs, src/mito2/src/region/version.rs	MitoRegion state and copy-on-write VersionControl snapshots
request	src/mito2/src/request.rs	WriteRequest/RegionRequest types and result channels
wal	src/mito2/src/wal.rs	Write-ahead log wrapper over log-store
memtable	src/mito2/src/memtable/	In-memory write buffers (time-series / bulk / partition)
flush	src/mito2/src/flush.rs	FlushScheduler, WriteBufferManager, memtable → SST
compaction	src/mito2/src/compaction/	Compaction scheduler (scheduler.rs + scheduler/), TWCS picker, strict-window manual picker, compactor, memory control
access_layer	src/mito2/src/access_layer.rs	SST read/write over the object store
sst	src/mito2/src/sst/	Parquet format, file metadata, index layout
series_index	src/mito2/src/series_index/	Series index writer/searcher, bucket planning, catalogs, worker-owned reconciliation (maintenance.rs), shared approximate usage (task.rs), catalogs, and index lifecycle
read	src/mito2/src/read/	ScanRegion, merge, dedup, projection, streaming
manifest	src/mito2/src/manifest/	RegionManifestManager, manifest actions/edits
cache	src/mito2/src/cache.rs	Write/file/page caches
gc	src/mito2/src/gc.rs, src/mito2/src/gc/	Dropped-file cleanup worker
schedule	src/mito2/src/schedule/	Local/remote background job scheduling
remap_manifest	src/mito2/src/remap_manifest.rs	Manifest path remapping for region copy/migration
config	src/mito2/src/config.rs	MitoConfig tuning knobs
temp_file_sweeper	src/mito2/src/temp_file_sweeper.rs	Reclaims temporary files a crashed writer left in a region, triggered when the region becomes a writable leader or a create opens an existing region (under the hdfs-object-store feature)
test_util	src/mito2/src/test_util.rs	TestEnv and builders (under the test feature)

Write path

MitoEngine::handle_request (engine.rs), or handle_batch_put_requests for the puts of one request to several regions (one message per worker) → worker loop (worker/handle_write.rs) → sequence + WAL assembly (region_write_ctx.rs) → wal.rs → memtable (memtable/) → when buffer pressure trips, flush.rs writes SSTs via access_layer.rs and appends a RegionEdit to the manifest (manifest/manager.rs).

Read path

MitoEngine::handle_query (engine.rs) → read/scan_region.rs takes an immutable Version (region/version.rs) → scans memtables and Parquet SSTs (sst/parquet.rs) → merges (read/) and dedups by sequence → projected, filtered RecordBatch stream.

Internal series merges use the query runtime pool forwarded through QueryScanContext; reader and output-batch accounting uses the separate scan memory tracker. Direct scanner callers default to an unbounded operator pool.

Public surface

• Entry: MitoEngine in src/mito2/src/engine.rs, built via MitoEngineBuilder.
• Trait: impl RegionEngine for MitoEngine (store-api's region engine contract).
• Consumed by datanode (sends RegionRequests) and the query layer (scans).

When you change X, also touch Y

• Manifest format (manifest/action.rs): affects crash recovery and follower replay. Keep it backward compatible.
• SST/Parquet layout (sst/): readers must stay compatible with existing files.
• Series-index disk limit: tasks estimate usage from entry sizes in open-region snapshots and defer new builds when the shared estimate is full. Cleanup still prunes obsolete range entries and expires series indexes to reclaim published usage. Concurrent builds may overshoot; closed-region files are not counted. Series handles and the SST purger own file deletion.
• Series-index coverage (series_index/catalog.rs): SeriesIndexEntry stores compaction-window width and SST summaries keyed by aligned start in both catalogs and Parquet footers.
• Request types (request.rs): usually tied to proto definitions consumed by datanode.
• WAL/memtable encoding (wal/, memtable/): breaks replay if changed incompatibly.

Testing

cargo nextest run -p mito2

Tests live next to the code as *_test.rs (e.g. src/mito2/src/engine/flush_test.rs). TestEnv in test_util.rs spins up an engine over an in-process object store.

Gotchas

• Sequence numbers are strictly increasing per region; dedup and snapshot reads depend on this. Do not change assignment lightly.
• Manifest version is monotonic — never reset or skip it.
• Lock ordering: take the manifest lock before updating version_control; the reverse deadlocks against concurrent flush/compaction.
• All region I/O runs on tokio workers; never block_on inside a worker.
• Loaded regions have resident compaction state, including Idle and handed-off DDLs. Publish create/open/catchup results through the worker's register_region; only close/drop unregisters them. See compaction/scheduler/state.rs.
• Local compaction units use the snapshot-free CompactionContext. Retaining a full CompactionRegion in a unit pins other units' inputs and delays SST purge.
• In flat batches, equal __primary_key dictionary keys imply the same series, but not the reverse: wrap_pk_binary_to_dict gives every row its own key. Compare key bytes when grouping rows by series.

Maintenance contract

Update this file when you add/rename a top-level module, change the write/read path entry points, or alter a persisted format (manifest, SST, WAL).