## Problem
`LogicalReplicationClient` uses a Redlock leader lock to guarantee a
single active consumer per Postgres logical replication slot. The lock
resource was keyed on the client `name`:
```
logical-replication-client:${this.options.name}
```
A slot permits exactly one consumer, so the lock's job is to serialize
consumers **of a given slot**. Keying it on `name` breaks that whenever
two clients target the same slot with different names — most notably
across a rolling deploy where the client `name` changes but `slotName`
does not. Both acquire *distinct* locks, both consider themselves
leader, and the second to reach `START_REPLICATION` hits `replication
slot "<slot>" is active for PID <n>`. Because that query was
fire-and-forget and its failure was only logged (no retry), the consumer
stopped and replication stalled until the process was restarted.
## Fix
**1. Key the leader lock on `slotName`** — the actual single-consumer
resource:
```
logical-replication-client:${this.options.slotName}
```
Consumers of the same slot now contend on the same lock and hand off
cleanly across restarts/deploys; different slots stay independent.
`name` is kept for logging and the pg `application_name`.
**2. Self-healing resubscribe** (`resubscribeOnFailure`, opt-in) —
instead of logging-and-dying, a client re-subscribes with exponential
backoff after a lost election or a failed `START_REPLICATION`, so a
rolling deploy self-heals: the incoming pod retries until the draining
pod releases the slot, then takes over. Safety:
- `#cleanupAttempt()` unconditionally ends the pg client (freeing the
walsender) and releases the leader lock before rescheduling — retries
never leak connections/locks.
- `shutdown()` sets an intentional-stop latch re-checked after every
`await` in `subscribe()` (and aborts the lock-acquire spin), so a
resubscribe can never race or outlive an intentional shutdown.
- Backoff resets only on genuine stream start, so a permanently stuck
slot backs off to the ceiling and logs loudly rather than tight-looping;
an epoch guard neutralises stale `START_REPLICATION` catches.
Runs- and sessions-replication opt in and use `shutdown()` for all
intentional stops.
**3. Observability** — the admin runs-replication status route probed
the old name-keyed Redis key (would report `leader:false` for every
source after fix#1); now probes the slot-keyed key.
## Tests
`internal-packages/replication/src/client.test.ts` (real Postgres +
Redis containers):
- same-slot/different-name → second client must not double-lead or race
into "slot is active" (the regression)
- a failing `START_REPLICATION` retry loop must not leak connections or
locks
- `shutdown()` during an in-flight `subscribe()` must not leave a zombie
leader
- `subscribe()` after `shutdown()` re-arms `resubscribeOnFailure`
- self-heals once the leader releases the slot
Plus the multi-source wiring test updated to the slot-keyed lock keys.
## Rollout
With the self-healing resubscribe, this ships as a **plain rolling
deploy** — the incoming pods retry across the one-time lock-key
transition and take over once the old pods drain (a brief replication
stall that the durable slot replays on reconnect — no data loss). No
stop-before-start required.
Daniel Sutton
c9f427e21f