Each document stored in the database has an optional expiration value (TTL, time to live). The default is for there to be no expiration, i.e. the information will be stored indefinitely. The expiration can be used for data that naturally has a limited life that you want to be automatically deleted from the entire database.
The expiration value is user-specified on a document basis at the point when the data is stored. The expiration can also be updated when the data is updated, or explicitly changed through the Couchbase protocol. The expiration time can either be specified as a relative time (for example, in 60 seconds), or absolute time (31st December 2012, 12:00pm).
Typical uses for an expiration value include web session data, where you want the actively stored information to be removed from the system if the user activity has stopped and not been explicitly deleted. The data will time out and be removed from the system, freeing up RAM and disk for more active data.