Sessions¶
A session is the time from when Firefox starts until it shut down. A session can be very long-running. E.g. for Mac users that are used to always put their laptops into sleep-mode, Firefox may run for weeks. We slice the sessions into smaller logical units called subsessions.
Subsessions¶
A subsessions data consists of:
- general information: the date the subsession started, how long it lasted, etc.
- specific measurements: histogram & scalar data, etc.
This has some advantages:
- Latency - Sending a ping with all the data of a subsession immediately after it ends means we get the data from installs faster. For
mainpings, we aim to send a ping at least daily by starting a new subsession at local midnight. - Correlation - By starting new subsessions when fundamental settings change (i.e. changes to the environment), we can correlate a subsessions data better to those settings.
Subsession splits¶
The first subsession starts when the browser starts. After that, we split the subsession for different reasons:
daily, when crossing local midnight. This keeps latency acceptable by triggering a ping at least daily for most active users.environment-change, when a change to the environment happens. This happens for important changes to the Firefox settings and when addons activate or deactivate.
On a subsession split, a main ping with that reason will be submitted. We store the reason in the pings payload, to see what triggered it.
A session always ends with a subsession with one of two reason:
shutdown, when the browser was cleanly shut down. To avoid delaying shutdown, we only save this ping to disk and send it at the next opportunity (typically the next browsing session).aborted-session, when the browser crashed. While Firefox is active, we write the currentmainping data to disk every 5 minutes. If the browser crashes, we find this data on disk on the next start and send it with this reason.
