# corium-log
Durable, append-only transaction logs with replay and range scans.
## What it does
The transaction log is Corium's source of truth — indexes are a deterministic
fold of it. This crate defines:
- **`TxRecord`** — one committed transaction: monotonic `t`, `tx_instant`
timestamp, and the asserted/retracted `datoms`.
- **`TransactionLog`** — the append/replay/range-scan trait: durably append the
next transaction, replay from the start, and scan a `t` range.
- Log chunk format plus append-only file implementations, including the
per-lease-version split used for HA (a deposed writer's stale appends are
discarded during merged replay).
## Dependencies
- `corium-core` — for `Datom`, `EntityId`, and value encoding.
- `thiserror` for errors; `tempfile` (dev) for tests.
Pure, synchronous library code — no async or network dependencies.
## Architecture
Logs are append-only and replayable: given the log, the entire index state is
reconstructible, which is what makes peer crash/restart lossless and GC safe.
Records are length-framed, value-encoded with `corium-core`'s codec, and
protected by a per-record CRC32C over the frame header and payload. Replay
rejects a fully written frame whose contents changed, while a partial trailing
write is discarded as an unacknowledged torn tail. The format bit in the length
word lets upgraded readers continue to replay legacy length-only frames; every
new append is checksummed. For high availability the log is split into
per-lease-version files; merged replay orders them by lease version and drops
appends from a fenced-out writer, so a standby that takes over never replays a
deposed transactor's uncommitted tail. See
[`docs/design/log-and-transactor.md`](../../docs/design/log-and-transactor.md).