An audit for blocking work on the event loop, written up rather than acted on —
the chart changes in flight land first.
The headline is a correctness bug, not a performance one. Sync route handlers
run in FastAPI's threadpool and call rebuild_levels, which reaches
asyncio.Queue.put_nowait on every subscriber. asyncio.Queue is not thread-safe:
it wakes a consumer by resolving a Future, which has to happen on the loop
thread. A dropped wakeup means a drawing made in one browser does not reach
another until the next tick — invisible today only because the stream ticks
about once a second and covers it.
Below that: level rebuilding is CPU-bound on the loop and is the whole of the 82
second startup, and disarming an alert writes to disk from a coroutine.
Also states what not to do, since the obvious reading of "make it async" is
wrong here. Sync routes stay sync — FastAPI's threadpool is what keeps their
work off the loop, and converting them would drag the rebuild cost onto it.
ManualLineStore's threading lock stays, because both the loop and threadpool
threads reach that store.
Keeping it that way is three layers: a short async section in AGENTS.md, which
is the only file both agents load every session; comments on the lines someone
would actually edit, starting with the worker count in Procfile; and a loop-lag
probe on /api/status so a stall reports itself as a number rather than as "the
chart feels laggy". The risk register gains a row per finding pointing here.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
