I've started to prototype and lay the foundations for the indicators to use the Upstart User Sessions. It's an exciting change to our desktop foundations, and while it's still very fresh, I think it's important to start understanding what it can do for us. For right now you're going to need a patch to Unity and a patch to indicator-network to even get anything working, not recommended for trying at home.

Previously for indicators the way that they've worked is that a small loadable module was loaded by the panel service that had indicator specific UI in it. That plugin also took care of the responsibility to restart the indicator backend, respawning it if it crashed. While this works and it has created a robust desktop (most people don't notice when their indicator backends crash) it has had some downsides. For one, it makes it difficult to build and test new backends as you pretty much have to restart Unity to stop the previous service from getting respawned. Also all the debugging messages end up coming under the DBus process in ~/.xsession-errors because we were using DBus activation to start them.

With upstart user sessions we're now getting a lot more power and flexibility in managing the jobs in the user session, it makes sense that indicators would start to use it to control the backend services. This comes with a set of advantages.

The first one is that there is better developer control of the state of the process. It's really easy to start and stop the service:

$ stop indicator-network
$ start indicator-network
and the ever exciting:
$ restart indicator-network
All of these ensure that the same commands are run each time in a recreatable way. Plus give the user and/or developer full control.

Upstart also takes the output of each process and puts it into its own log file. So for our example here there is a ~/.cache/upstart/indicator-network.log that contains all of the junk that the backend spits out. While this is nice to just make xsession-errors cleaner, it also means that we can have a really nice apport-hook to pick up that file. Hopefully this will lead to easier debugging of every indicator backend bug because they'll have more focused information on what the issue is. You can also file general bugs with ubuntu-bug indicator-network and get that same information attached.

In the future we'll also be able to do fine tuned job control using external events. So we could have the indicator network backend not start if you don't have any networking devices, but startup as soon as you plug in that USB cellular modem. We're not there yet, but I'm excited that we'll be able to reduce the memory and CPU footprint on devices that don't have all the features of higher end devices, scaling as the features are required.

Those that know me know that I love diagrams and visualizations, and so I'll have to real quickly say that I'm excited about being able to map our desktop startup using intlctl2dot. This gives a Graphviz visualization of startup and how things will interact. I expect this to be a critical debugging tool in the future.

What's next? Getting all the indicators over to the branch new Saucy world. We also want to get application indicators using a similar scheme and get a fast responsive desktop. Hope to have a blog post or two on that in the near future.

posted Apr 25, 2013 | permanent link