CQL for Cassandra 1.2

Expiring columns

Data in a column can have an optional expiration date called TTL (time to live). Whenever a column is inserted, the client request can specify an optional TTL value, defined in seconds, for the data in the column. TTL columns are marked as having the data deleted (with a tombstone) after the requested amount of time has expired. After columns are marked with a tombstone, they are automatically removed during the normal compaction (defined by the gc_gracegc_grace_seconds) and repair processes.

Use CQL to set the TTL for a column.

If you want to change the TTL of an expiring column, you have to re-insert the column with a new TTL. In Cassandra, the insertion of a column is actually an insertion or update operation, depending on whether or not a previous version of the column exists. This means that to update the TTL for a column with an unknown value, you have to read the column and then re-insert it with the new TTL value.

TTL columns have a precision of one second, as calculated on the server. Therefore, a very small TTL probably does not make much sense. Moreover, the clocks on the servers should be synchronized; otherwise reduced precision could be observed because the expiration time is computed on the primary host that receives the initial insertion but is then interpreted by other hosts on the cluster.

An expiring column has an additional overhead of 8 bytes in memory and on disk (to record the TTL and expiration time) compared to standard columns.

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