When we started to look at how to confine applications enough to allow an appstore that allows for anyone to upload applications we knew that Apparmor could do the filesystem and IPC restrictions, but we needed something to manage the processes. There are kernel features that work well for this, but we didn't want to reinvent the management of them, and we realized that Upstart already did this for other services in the system. That drove us to decide to use Upstart for managing application processes as well. In order to have a higher level management and abstraction interface we started a small library called upstart-app-launch and we were off. Times change and so do init daemons, so we renamed the project ubuntu-app-launch expecting to move it to systemd eventually.
For the most part, no one should notice anything different. Applications will start and stop in the same way. Even users of ubuntu-app-launch shouldn't notice a large difference in how the library works. But for people tinkering with the system they will notice a few things. Probably the most obvious is that application log files are no longer in
~/.cache/upstart. Now the log files for applications are managed by journald, which as we get all the desktop services ported to use systemd, will mean that you can see integrated events from multiple processes. So if Unity8 is rejecting your connection you'll be able to see that next to the error from your application. This should make debugging your applications easier. You'll also be able to redirect messages off a device realtime, which will help debugging your application on a phone or tablet.
For those who are more interested in details we're using systemd's transient unit feature. This allows us to create the unit on the fly with multiple instances of each application. Under Upstart we used a job with instances for each application, but now that we're taking on more typical desktop style applications we needed to be able to support multi-instance applications, which would have been hard to manage with that approach. We're generating the service name using this pattern:
ubuntu-app-launch--$(application type)--$(application id)--$(time stamp).service
The time stamp is used to make a unique name for applications that are multi-instance. For applications that ask us to maintain a single instance for them the time stamp is not included.
Hopefully that's enough information to get you started playing around with applications running under systemd. And if you don't care to, you shouldn't even notice this transition.
posted Mar 23, 2017 | permanent link