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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.