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You are viewing the archived documentation of TiDB, which no longer receives updates. It is recommended that you use the latest LTS version of the TiDB database.

Locking Functions

TiDB supports most of the user-level locking functions available in MySQL 5.7.

Supported functions

Name	Description
`GET_LOCK(lockName, timeout)`	Acquires an advisory lock. The `lockName` parameter must be NO longer than 64 characters. Waits maximum `timeout` seconds before timing out and returns a failure.
`IS_FREE_LOCK(lockName)`	Checks if a lock is free.
`IS_USED_LOCK(lockName)`	Checks if a lock is in use. If true, it returns the corresponding connection ID.
`RELEASE_ALL_LOCKS()`	Releases all locks held by the current session.
`RELEASE_LOCK(lockName)`	Releases a previously acquired lock. The `lockName` parameter must be NO longer than 64 characters.

MySQL compatibility

The minimum timeout permitted by TiDB is 1 second, and the maximum timeout is 1 hour (3600 seconds). This differs from MySQL, where both 0 second and unlimited timeouts (timeout=-1) are permitted. TiDB will automatically convert out-of-range values to the nearest permitted value and convert timeout=-1 to 3600 seconds.
TiDB does not automatically detect deadlocks caused by user-level locks. Deadlocked sessions will timeout after a maximum of 1 hour, but can also be manually resolved by using KILL on one of the affected sessions. You can also prevent deadlocks by always acquiring user-level locks in the same order.
Locks take effect on all TiDB servers in the cluster. This differs from MySQL Cluster and Group Replication where locks are local to a single server.
IS_USED_LOCK() returns 1 if it is called from another session and is unable to return the ID of the process that is holding the lock.

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