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# Sessions
A *session* is the time from when Firefox starts until it shuts down.
A session can be very long-running. E.g. for users that always put their computers into sleep-mode, Firefox may run for weeks.
We slice the sessions into smaller logical units called *subsessions*.
## Subsessions
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 add-ons activate or deactivate.
On a subsession split, a {doc}`main ping <../data/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 current `main` ping 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.
```{image} subsession_triggers.png
```
## Subsession data
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 `main` pings, 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 better correlate a subsession's data to those settings.