Prod runs 3 uvicorn workers, but the in-process subscription notifier was
started in every worker's lifespan (it was written assuming a single-worker
deploy). Its sweep fetches recent-forecast/archive data from Open-Meteo on a
15-min timer independent of any request, so running it 3× tripled the
background upstream load against a shared quota — the source of the overnight
429/503 rate-limit errors in logs/errors, and 3× redundant subscription scans.
Elect one worker to own the notifier via a non-blocking exclusive flock on a
lockfile (THERMOGRAPH_SINGLETON_LOCK): the first worker wins and holds the fd
for its lifetime; others stand down; the OS releases the lock if the leader
dies, so a restart re-elects cleanly. Unset (single worker / dev / tests)
always wins, so behavior there is unchanged. Mirrors the shared-metrics store's
env-selected cross-worker coordination.
The neighbor warmer stays per-worker on purpose: each worker drains its own
request-fed queue, so gating it would leave non-leader queues undrained.
- backend/singleton.py: flock-based leader election (claim()).
- backend/app.py: gate notify.start() on singleton.claim().
- deploy/thermograph.service: default THERMOGRAPH_SINGLETON_LOCK to
data/notifier.lock (needs the unit reinstalled on prod to take effect).
- deploy/thermograph.env.example: document it alongside WORKERS.
- Tests: leader election — single-worker default, first-wins, idempotent
re-claim, second-holder stands down, re-election after release. 183 passed.
Co-authored-by: root <root@vmi3417050.contaboserver.net>
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