Locking Functions

TiDB supports most of the user-level locking functions available in MySQL 8.0.

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.

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 time out 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.