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[[session-configuration-sessioncache]]
=== The L1 Session Cache

==== The DefaultSessionCache

In the absence of any explicit configuration, Jetty will instantiate an instance of the `DefaultSessionCache` per context.
If you wish to change any of the default values, you need to enable the `session-cache-hash` link:#startup-modules[module].

Once the `session-cache-hash` module has been enabled, you can view a list of all the configurable values by opening `start.d/session-cache-hash.ini`:

[source, screen, subs="{sub-order}"]
----
--module=session-cache-hash

#jetty.session.evictionPolicy=-1
#jetty.session.saveOnInactiveEvict=false
#jetty.session.saveOnCreate=false
#jetty.session.removeUnloadableSessions=false
#jetty.session.flushOnResponseCommit=false
----

jetty.session.evictionPolicy::
Integer.
Controls whether session objects that are held in memory are subject to eviction from the memory cache.
Evicting sessions can reduce the memory footprint of the cache.
Eviction is usually used in conjunction with a `SessionDataStore` that persists sessions.
Values are:
* -1 : sessions are never evicted from the cache
*  0 : sessions are evicted from the cache as soon as the last active request for it finishes
*  >= 1 : any positive number is the time in seconds after which a session that is in the cache but has not experienced any activity will be evicted

____
[NOTE]
If you are not using a `SessionDataStore` that persists sessions, be aware that evicted sessions will be lost.
____

jetty.session.saveOnInactiveEvict::
Boolean, default `false`.
Controls whether a session will be saved to the `SessionDataStore` just prior to its eviction.

jetty.session.saveOnCreate::
Boolean, default `false`.
Controls whether a session that is newly created will be immediately saved to the `SessionDataStore` or lazily saved as the last request for the session exits.

jetty.session.removeUnloadableSessions::
Boolean, default `false`.
Controls whether a session that cannot be restored - for example because it is corrupted - from the `SessionDataStore` is deleted by the `SessionDataStore`.

jetty.session.flushOnResponseCommit::
Boolean, default `false`.
If true, if a session is "dirty" - ie its attributes have changed - it will be written to the backing store as the response is about to commit.
This ensures that all subsequent requests whether to the same or different node will see the updated session data.
If false, a dirty session will only be written to the backing store when the last simultaneous request for it leaves the session.

For more general information on the uses of these configuration properties, see link:#sessions-details[Session Components].


==== The NullSessionCache

The `NullSessionCache` is a trivial implementation of the `SessionCache` that does not cache any session information.
You may need to use it if your clustering setup does not have a sticky load balancer, or if you want absolutely minimal support for sessions.
If you use this in conjunction with the `NullSessionDataStore`, then sessions will neither be retained in memory nor persisted.

To enable the `NullSessionCache`, enable the `sesssion-cache-null` link:#startup-modules[module]. 
Configuration options are:

jetty.session.saveOnCreate::
Boolean, default `false`.
Controls whether a session that is newly created will be immediately saved to the `SessionDataStore` or lazily saved as the last request for the session exits.

jetty.session.removeUnloadableSessions::
Boolean, default `false`.
Controls whether a session that cannot be restored - for example because it is corrupted - from the `SessionDataStore` is deleted by the `SessionDataStore`.

jetty.session.flushOnResponseCommit::
Boolean, default `false`.
If true, if a session is "dirty" - ie its attributes have changed - it will be written to the backing store as the response is about to commit.
This ensures that all subsequent requests whether to the same or different node will see the updated session data.
If false, a dirty session will only be written to the backing store when the last simultaneous request for it leaves the session.

For more general information on the uses of these configuration properties, see link:#sessions-details[Session Components].
