ADR-0009: lightd Takes Covers From a Topic as a Second Input

Context

nowplayd publishes the current cover as image bytes on eds/nowplaying/current/art. lightd turns image bytes into a scene, and today takes them only over HTTP, at POST /v1/cover. Something has to carry the bytes from the topic to lightd.

lightd’s boundary is that it knows about color and hardware and nothing about music (ADR-0002).

Options considered

A third service that subscribes to the topic and POSTs to lightd. The boundary is untouched, but the service’s whole job is to move bytes from one transport to another, and it is one more thing to deploy and to fail.

nowplayd calls lightd. One fewer process, but nowplayd would then know about lightd, and the bus would stop being the thing subscribers share.

lightd subscribes. lightd already holds a broker connection and an account. It gains a second input that delivers exactly what the first does.

Decision

lightd gains LIGHTD_COVER_TOPIC, unset by default. When set, lightd subscribes to it and feeds each message through the same path as POST /v1/cover.

Consequences

• The boundary holds: lightd still receives image bytes and nothing else. It does not learn what a track is, and the topic is a setting, not knowledge.
• POST /v1/cover stays, and remains the way to drive a stand by hand.
• lightd subscribes for the first time. Subscriptions are made in the on-connect handler so they survive a reconnect, and its broker account gains one subscribe grant.
• Two inputs can now set the scene, and the last one wins. That is the existing rule, and it is acceptable while there is one stand.
• A retained cover is delivered again whenever lightd reconnects, so it republishes a scene the broker already holds. Harmless, and it means a restarted lightd puts the room right by itself.
• A cover that palette cannot use is logged and ignored, as a bad upload is.