04.markdown

Redis can be told that a key should only exist for a certain length of time. This is accomplished with the EXPIRE and TTL commands, and by the similar PEXPIRE and PTTL commands that operate using time in milliseconds instead of seconds.

    SET resource:lock "Redis Demo"
    EXPIRE resource:lock 120

This causes the key resource:lock to be deleted in 120 seconds. You can test how long a key will exist with the TTL command. It returns the number of seconds until it will be deleted.

    TTL resource:lock => 113
    // after 113s
    TTL resource:lock => -2

The -2 for the TTL of the key means that the key does not exist (anymore). A -1 for the TTL of the key means that it will never expire. Note that if you SET a key, its TTL will be reset.

    SET resource:lock "Redis Demo 1"
    EXPIRE resource:lock 120
    TTL resource:lock => 119
    SET resource:lock "Redis Demo 2"
    TTL resource:lock => -1

The SET command is actually able to accept further arguments in order to directly set a time to live (TTL) to a key, so you can alter the value of a key and set its TTL at the same time in a single atomic operation:

    SET resource:lock "Redis Demo 3" EX 5
    TTL resource:lock => 5

It is also possible to cancel the time to live of a key removing the expire and making the key permanent again.

    SET resource:lock "Redis Demo 3" EX 5
    PERSIST resource:lock
    TTL resource:lock => -1