Glossary

Bucket

The object-storage container (S3 bucket, GCS bucket, Azure container, or local directory) that holds all of an engine’s state. One engine is one bucket, or one prefix of one.

Claim

A put-if-absent write with read-back of the winner. It implements a unique constraint: exactly one caller wins, and losers converge on the winner’s value.

Commit

An immutable object in a log holding an ordered batch of events. It is numbered densely: commit N+1 exists only after commit N.

Compare-and-swap

CAS

A conditional write that succeeds only if the object still carries the etag the writer last observed.

Cooperative write

A write into a lease’s state from outside the lease, which does not fence the holder. The holder observes it on its next renew or update.

Epoch

A lease’s monotonic fence token, incremented on every acquisition. Together with the key and the holder, it identifies one ownership period.

Etag

An opaque, backend-native version identifier of an object that changes on every write: an S3 or Azure etag, a GCS generation, or a content MD5 on the filesystem.

Fencing

Rejecting writes from a holder whose ownership has ended. CairnDB fences through storage conditions (etag plus epoch), so even a paused process cannot write stale state.

Group commit

Batching the events appended while a PUT is in flight into the next commit. It amortizes round-trips without weakening the durable ack.

Lease

An expiring, fenced ownership document {epoch, holder, deadline_at, state}.

Log

A named, append-only, densely numbered sequence of commits: CairnDB’s write-ahead log. Order is total within a log.

Projection

A local, read-only SQLite database derived from a log by deterministic replay through handlers.

Put-if-absent

A conditional write that succeeds only if the key does not exist yet.

Revalidate hook

A committer callback that re-checks pending events against the commits that won a race, and keeps, modifies, or rejects each event.

Schema version

Either an event’s payload version (Event.schema_version) or a projection’s table layout version (Projection version, which selects snapshots/v{version}/).

Sequence number

An event’s position in its log: (commit, index), rendered canonically as 000000000042:000003.

Snapshot

A fully built projection database through a given commit, stored immutably in the bucket to bound client startup and replay.