ADR-0006: Now Playing Is a Bus of Small Drivers Behind One Contract

Context

The stand lights from a cover image, but nothing tells EdS what is playing. lightd has carried POST /v1/cover since the first vertical as “the interface the future album-cover resolver will call”; this is what sits upstream of it.

The players are Spotify, TIDAL, MusicBee and iTunes, all desktop clients on Windows 11. Subscribers need at least the track title, the album title and the cover art. More than one thing will want to react to a track change, and some deployments will be small: something whose only job is to follow iTunes.

Nothing can dial the EdS workload (ADR-0003), and the workload cannot reach a home LAN.

Options considered

One Windows agent on the system media controls. Windows exposes what players report through one API (GlobalSystemMediaTransportControlsSessionManager), and it looked like a universal driver. It is not, for these four:

It also cannot be read from a background service on Windows 11, only from the logged-in session, and its first-class bindings are .NET. Sources: Music Presence player notes, dotnet/runtime#84293, mb_MediaControl issues, iTunes-SMTC.

Cloud only. Ask each service what the account is playing. Only Spotify offers this to a third party. MusicBee and iTunes play local files and have no cloud at all.

Scrobbling as the bus. Have every player scrobble to Last.fm or ListenBrainz and read “now playing” from there. It reaches TIDAL, but it is 15–30 seconds late, has no pause state, carries no usable art, and iTunes needs a helper whose state on Windows 11 is doubtful. The best-known aggregator, multi-scrobbler, has no MQTT output and no source for TIDAL, MusicBee, iTunes or the Windows media controls. Source: multi-scrobbler sources.

A plugin inside each player. Exact, but a different toolchain per player, .NET for MusicBee, and nothing at all for Spotify or TIDAL.

Small drivers behind one contract. Each driver knows one player and reaches it however that player is best reached. All of them publish the same message to the bus.

Decision

Now playing is a bus of small drivers behind one versioned contract.

• A driver knows one player. It publishes that player’s state, retained, to eds/nowplaying/source/<id>/state, with presence on …/status and the cover, when the player supplies one, as image bytes on …/art.
• A deployment runs whichever drivers suit where it sits: one, or several.
• nowplayd reads every source and publishes the one current track (ADR-0008).
• Subscribers read eds/nowplaying/current and do their own thing. The first lights the stand (ADR-0009).

The contract is stamped "v": 1, and a mismatched version is refused, as with the scene descriptor.

Consequences

• A player is added, replaced or dropped without touching anything else. A mechanism that stops working — an API withdrawn, a pairing refused — costs one driver and one superseded record.
• Each player gets the mechanism that suits it, at the price of several mechanisms to maintain instead of one.
• Drivers differ in what they know: one supplies art, another has no album. The contract allows a source to be incomplete, and nowplayd makes the guarantee to subscribers.
• Art travels as its own retained message, so the JSON stays readable in mosquitto_sub. A hash in the state ties the two together.
• Every driver deployment needs a broker account granted its own source topics, and an ungranted publish fails silently.
• This is a new source of truth for “what is playing” beside the vinyl the product was first described around. It is an additional source, not a replacement: a turntable that identifies its record would be one more driver on the same bus.