Transaction Messages

Messages managing transactions on a single VChannel.

• BeginTxn: Opens a new transaction session, allocating a TxnID and establishing a keepalive window.
• CommitTxn: Commits an open transaction, finalizing all body messages.
• RollbackTxn: Aborts an open transaction, discarding all body messages.
• Txn: Synthetic message — complete committed transaction assembled on consumer side, never directly appended.

Key Invariants

• BeginTxn must precede all body messages and CommitTxn/RollbackTxn for the same transaction.
• Body messages must arrive within the keepalive window, otherwise the transaction expires. After expiration, no messages for that transaction will appear in the WAL.
• CommitTxn/RollbackTxn must come after all body messages have completed (inFlightCount == 0).
• The consumer assembles Begin + body messages into a single ImmutableTxnMessage using CommitTxn's TimeTick and LastConfirmedMessageID as the transaction's overall TimeTick and LastConfirmedMessageID.
• On RollbackTxn, the consumer discards the corresponding Begin + body messages.

BeginTxn@tt=10 → Insert@tt=11 → Insert@tt=12 → CommitTxn@tt=15
Consumer sees: ImmutableTxnMessage@tt=15 (contains all body messages)

BeginTxn@tt=10 → Insert@tt=11 → RollbackTxn@tt=15
Consumer sees: (nothing — body messages discarded)