ADR-0034: Presence Is Heard, Not Remembered: Everything Says It Is There Every Thirty Seconds

Context

Every part of EdS says it is there the same way: on connecting, it publishes online, retained, to its status topic, and leaves the broker a last will of offline, retained. The broker sends the will when it sees the connection die.

On 2026-10-14 the broker went away at about the time lp-stand-01 was unplugged. The broker never saw the stand’s connection die, so it sent no will. It keeps retained messages on disk, and every restart since, including the whole mobile kit’s, has brought back the stand’s online. On 2026-10-16 the console still showed the stand green, two days after it last said anything.

ADR-0023 accepted this for the beacon, where it is worse: a beacon left online holds the turntable, silent, until it is next switched on and off.

EdS speaks MQTT 3.1.1 everywhere: paho.mqtt.golang in the services, and ESP-MQTT without its MQTT 5 option in the firmware. A retained message in 3.1.1 carries no time. What a subscriber can tell is whether a message came to it as a stored copy, sent because it subscribed, or live, because it was just published: the retain flag on delivery says which.

Options considered

A time in the message. {"online":true,"at":…}. Every device needs a clock it can trust (SNTP), and every reader of the plain words online and offline changes. It answers “when” and still needs a rule for “too long ago”.

MQTT 5 message expiry. The broker deletes a retained online nobody has renewed. The cleanest on paper. It means a different Go library in three services, MQTT 5 in three firmwares, and Deevnet IoTaaS’s broker agreeing to it; and an expired message tells nobody it expired, so readers still need a timer.

Not keeping presence across a broker restart. Mosquitto persists retained messages or doesn’t, and the stand’s scene must survive a restart. It would also be Deevnet IoTaaS’s broker setting, not EdS’s.

Say it again, and believe only what is heard.

Decision

Whatever publishes a status topic republishes its word, retained, every thirty seconds while it is connected. Whoever reads one believes an online for ninety seconds after hearing it live; a stored online is only a claim, and lapses in ninety seconds unless a live one follows.

• The words do not change. online and offline on the same topics, retained, with the same wills. The repeat is the only thing added on the publishing side: the stand, the cover screen and the beacon in their event loops; lightd, nowplayd, the console and each of npagent’s drivers on a ticker. A driver repeats whichever word it last said.
• offline counts at once, stored or live. A will is still how a part that dies with the broker up is known to be gone, in a few seconds rather than ninety.
• Ninety seconds is three repeats missed. A part is not grayed by one lost message, and not believed for long after it is gone.
• The console draws a claim gray. Green is a part heard from in the last ninety seconds. Gray is a claim not yet heard, or a part not heard for ninety seconds. Red is a part that said offline. After a broker restart every live part is green within thirty seconds, because the repeat arrives live.
• nowplayd keeps a claim, for ninety seconds. On its own restart it must not drop a source that is playing, or dim the room mid-record, while it waits for the repeat. A source’s claim that lapses is offline; a beacon’s is unknown, which ADR-0023 already treats as “nothing changes”.
• The console’s header separates the kinds of part, on its own lines: services, sources, devices and power switches. A source’s pill is its driver’s presence, and the turntable’s driver is labeled “Song detector”, because it says that the sound card is being heard and not that the turntable is on. The source is still turntable everywhere else.

Consequences

• A part that is up shows gray for up to thirty seconds after the console or nowplayd starts. That is the price of not showing a part that is down as green.
• Old publishers look dead to new readers. Firmware and services that do not repeat go gray after ninety seconds, and a beacon on old firmware lapses to unknown. The devices are flashed and the publishers deployed before the readers.
• About one message every thirty seconds per part, retained, to the broker. There are ten or so parts.
• The broker being down is now visible. Every pill goes gray while it is, rather than staying as it last was.
• A dev broker needs repeating too. dev/feed.sh republishes its status topics on its loop, or the dev console grays them.

What would settle it

• The stand unplugged, the broker restarted, and the console showing it gray, never green, afterward.
• A record playing through a nowplayd restart without the room dimming.
• The turntable’s beacon unplugged while the broker is down: after ninety seconds the turntable no longer holds the stand.