The other half of the rebuilt-peer story, and only reachable once 0.1.2 made
back-fill work: with the wiped node repopulated and writing again, its writes
never reached its peer.
last_applied_remote_seq is a watermark in the PEER's seq space, and a rebuilt
peer is a new database whose oplog numbers from 1. Two things then went wrong,
and both had to be fixed — the first alone leaves data stuck:
- The healthy node kept advertising the OLD peer's watermark, and the sending
side seeds its pump from exactly that number, so the rebuilt node skipped its
entire oplog. The watermark (and the observed peer clock) is now reset when
the peer's node id changes. last_acked_seq is deliberately NOT reset: it
describes our own oplog's pruned horizon and is what tells a rebuilt peer it
needs a snapshot — clearing it would turn 0.1.2's back-fill back off.
- A peer claiming to have applied more of our stream than we have ever produced
can only be remembering a previous incarnation of us. That claim is now
rejected in favour of starting from the beginning; LWW makes the re-send
harmless. This mirrors the clamp RecordPeerAckAsync already applies to acks.
Found on the OtOpcUa docker-dev rig, where the healthy node held watermark 10
while the rebuilt peer's oplog was seqs 1-3 and its last_acked stayed 0 — the
peer never accepted a single one. Covered by a test that converges with one
peer, replaces it with a fresh database, and requires a write on the new peer
to arrive: RED before this fix, green after. 149/149 pass.
Version 0.1.3.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Joseph Doherty
3b7489a7b0