ADR-0040: A Request Can Name an Album to Play, and iTunes Plays It From Its Own Library

Context

Every command so far is a button. It acts on whatever is playing, so a controller never needs to say where it is sent, and ADR-0020 made that a rule: a controller never names a source. A request to play an album is not a button. Nothing may be playing at all, and if something is, it may be the turntable, which takes no command.

The phone remote that iTunes pairs with (ADR-0011) can do this: it queues an album and plays it. The pairing EdS already holds is the same credential, so nothing new is granted by the player. What the phone also does, browsing the library, iTunes for Windows refuses this remote: databases and update answer 403 on a session whose buttons work.

Options considered

Leave the five commands as they are and send the request as a search (type the words into the player’s search). iTunes has no such request over DACP, and a search answers with tracks, not an album in its order.

Play from an outside catalog, such as Spotify’s, which can play any album by name. It is not the owner’s library, it needs the authorization widened again (ADR-0032 asked for no more than control), and Spotify is switched off (CHG-0024).

A sixth command that names an album, carried out by iTunes from its own library. The player is asked to play the songs whose artist and album have those names, from the first track, in the album’s order. Looking the album up first, and playing it by its id, was the plan until the player refused the lookup.

Decision

play_album names an album, and the iTunes driver plays it from the library it is paired with. It is the one command that is not sent to the current source.

• The command is {"v":1,"command":"play_album","artist":"Van Halen","album":"Van Halen","from":"…"} on nowplaying/command, never retained, as the others. It names the album in words. Which album that is in the library is the driver’s to find, because only the driver can ask the library.
• It goes to the player that has a library, NP_ALBUM_SOURCE (default itunes), whatever is playing. This is the exception to “a controller never names a source”: it still names none, and nowplayd chooses the one source that can carry it out.
• The player matches the names, and what it then plays is the proof. The library cannot be asked first, and the player answers a request that matched nothing as it answers one that matched. So the driver asks what is playing afterwards, and a request whose artist and album are not what plays is logged as not found. It never guesses a nearer name, so a misheard request is not answered with the wrong record.
• Playing replaces what plays. The queue is emptied and the album queued from its first track, sorted as the album is. This is what the phone does.
• Which album “the first album” is, and turning words into the command, is not the driver’s and is not decided here.

The client sends two requests it has not sent before, as the phone remote sends them: ctrl-int/1/cue?command=clear, then cue?command=play&query='daap.songartist:<artist>'+'daap.songalbum:<album>'&index=0&sort=album. A name’s apostrophes and plus signs are escaped inside its quotes. npagent play-album ARTIST ALBUM sends them by hand.

Consequences

• EdS can choose what plays, where it could only press buttons on what was already playing. The pairing allowed this all along; now it is used.
• A request reaches iTunes while a record is on the turntable. iTunes then plays over it. Whether a request should wait while the turntable’s beacon is on is left to whatever sends it.
• Names have to match the library’s, and EdS cannot read the library to find out what they are. “Van Halen” finds Van Halen; “Van Halen I” does not. Making a person’s words into the library’s names is the sender’s job, from what it knows elsewhere (a catalog, the Discogs collection), and a miss is only learned by trying.
• The match is on the track’s artist. An album whose tracks name guests (“Santana feat. …”) is found only in part, or not at all.
• The broker account of whatever sends play_album needs nowplaying/command, as the cover screen has. The agent’s already reads its source’s command topic.

What would settle it

Tested against a stand-in for the player: the two requests as the remote sends them, a name’s apostrophe or plus sign kept inside its quotes, and nothing sent without both names.

Shown on 2026-10-23, from the relay Pi against the real iTunes for Windows:

• databases and update answer 403 to this remote, with or without Viewer-Only-Client; without it, the buttons fail too (501). The library cannot be listed.
• npagent play-album "Van Halen" "Van Halen" starts Van Halen (1978) at “Runnin’ With The Devil”, and the player says so.

Not yet shown:

• What the player does when nothing matches: whether it stops, or plays on from where it was.
• A name with an apostrophe, and a title the library capitalizes differently: Appetite for Destruction by Guns N’ Roses played as the library’s Appetite For Destruction (2026-10-24).
• An album whose tracks name guests.
• play_album on the bus, carried out by the driver (CHG-0033): Aerosmith by Aerosmith, from “Make It”, within a second.
• play_album on nowplaying/command, passed on by nowplayd from a sender with an account of its own: voiced’s, on 2026-10-24 (CHG-0034).