Two changes, and the second was found by verifying the first.
## Watching a camera from the app, in another building
Snapshots answer "is that camera working". They do not answer "what is
happening in my shop right now", which is what somebody who opens the app away
from the counter is asking. Head office's browser already had that answer -
LiveHub plus cameras.Live, where the shop PC asks outbound whether anybody is
watching and pushes JPEG frames for as long as somebody is - and the app could
not reach it.
cloud.CameraLive opens that feed and the app's own loopback relay re-emits it
as multipart MJPEG. That is the trick: frames arrive base64 over SSE, an <img>
cannot render that, and an <img> renders MJPEG natively - so a tile is an
ordinary <img> pointed at loopback whether the camera is in this room or
another city.
- Reconnecting happens in the relay, not the page. The server caps one push at
five minutes, so doing it here means the <img> never sees the stream end.
- The headers are flushed before the first frame. Go writes them on the first
body write, so without that the whole response waits for the shop PC to
start pushing. Measured against production: 30 seconds and not even a
Content-Type, which surfaces as the request timing out.
- One camera at a time. Watching makes a shop PC upload, so a grid that went
live at once would put an estate's worth of cameras on the wire because
somebody opened a page.
- live.mjpeg is behind the same per-run token as the engine routes, and a
wrong token is a 404 that never reaches head office at all.
- CameraLive uses its own HTTP client: the shared one's 30s timeout covers the
whole response and would sever a working view every thirty seconds - the
trap that made the server set WriteTimeout to zero for its own SSE endpoint.
## A camera read "Connected" for 34 minutes after the shop PC went blind
Which is why the verification above looked like a failure: head office
registered the viewer and no frame ever came.
reportWith returns early when the engine is unreachable - correctly, it has
nothing to say - so the last state it sent stays in the database looking
current. Measured live: cam2 and entrance both reading Connected, in green,
with last_seen_at 34 minutes old, while the heartbeat from the same PC said
cameras_up 0 of 0. Two surfaces reading two stored fields and disagreeing.
false could not be the answer. It means "this camera is not connecting", which
sends an installer to check cabling on a camera that was working perfectly the
last time anybody could ask it. So there are four states and one function:
connected reported recently, and working
not_connecting reported recently, and the stream will not open
waiting no shop PC has ever reported this camera
stale reported once, and not lately
- Connected is CLEARED when stale or waiting. A stale true left in place stays
available to every client reading the field directly, and leaves two fields
on one object disagreeing - how the shops screen once came out labelled
Working, in green, above "2 of 3 cameras not connecting".
- Computed in scanCamera, so every camera anybody reads passes through it. A
state computed per handler is one a handler forgets, and this had already
reached three screens.
- CameraStaleAfter is 5 minutes: five missed reports, not one. Same reasoning
as three missed heartbeats - an indicator that cries wolf gets ignored.
- An unparseable last_seen_at is stale. It should be impossible, which is why
it must not fall through to the state that says everything is fine.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGcjxF1cNLcuwc3DAPcnfj
Suriyakumarvijayanayagam
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