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
A converged pair prunes its oplog on ack, so "no oplog rows at all" is the
steady state of a healthy pair — not an anomaly. ComputeSnapshotRequiredAsync
measured the peer's gap against the oldest surviving oplog row and treated an
empty oplog as "no gap possible", which made that steady state the one state
from which a wiped peer could never be healed: it reconnected with an empty
database and a zero watermark, was told it needed no snapshot, and stayed
empty indefinitely.
With an empty oplog the oldest seq we could still stream is one past
last_acked_seq — ack-pruning is the only thing that empties the oplog without
also flagging needs_snapshot, and it deletes exactly seq <= last_acked_seq.
Measuring the gap against that heals the wiped peer and leaves every other
case identical: a converged peer's watermark equals last_acked_seq (no
snapshot), and a pair that has never written anything has both at zero.
Found by the OtOpcUa LocalDb Phase 1 live gate (check 4), which recorded it as
a documented limitation. Covered by a test that reproduces the live-gate
scenario through the real session stack: RED before this fix (the wiped side
never converges, 15 s timeout), green after. 148/148 pass.
Version 0.1.2.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Joseph Doherty