Change Notifications

When something in your library changes, connected apps should notice without you pressing refresh — and without asking UMS every few seconds whether anything happened. UPnP solves this with subscriptions: an app registers its interest once, and UMS tells it when there is news.

This works in both directions, and the two are controlled separately.

What UMS announces about your library

UMS publishes two pieces of information whenever the library changes:

• a counter that goes up on any change, which tells a subscriber that something happened
• a list of the containers that changed, which tells it what

The second one is what keeps things quick. An app that learns only "something changed" has to reload; one that learns "this folder changed" reloads that folder and leaves the rest alone.

Both are announced at most a few times per second. A scan that imports a thousand files therefore does not produce a thousand notifications — the changes are collected and reported as a handful of messages.

Typical moments when this happens:

• a file appears in, or disappears from, a shared folder
• you edit a playlist from a control point
• you rate something, because that changes what the ratings folders contain
• artwork is added for a station or a folder

Whether your app acts on the news is up to the app. Most refresh the view you are looking at; some only use the information when you open a folder again. If a change does not appear, it is worth opening the folder anew before assuming it was not delivered.

What UMS listens to on your players

The other direction is UMS subscribing to your players, so it can show what they are doing — what is playing, whether they are paused, how loud they are — and offer controls for them.

# Follow the players on the network. On by default.
upnp_subscribe_services = TRUE

Leave this on when UMS is the place you control playback from.

Turn it off when playback is driven by a separate control point. In that arrangement UMS has no business following and claiming control of the players: the control point is doing that, and two parties steering the same device is a good way to get confusing results. Switching this off makes UMS keep its hands off and simply serve the media.

See Two-box and three-box setups for what that arrangement looks like.