Places a lock on an application resource.
sp_getapplock [ @Resource = ] 'resource_name', [ @LockMode = ] 'lock_mode' [ , [ @LockOwner = ] 'lock_owner' ] [ , [ @LockTimeout = ] 'value' ] [ , [ @DbPrincipal = ] 'database_principal' ] [ ; ]
[ @Resource = ] 'resource_name'
Is a string specifying a name that identifies the lock resource. The application must ensure that the resource name is unique. The specified name is hashed internally into a value that can be stored in the SQL Server lock manager. resource_name is nvarchar(255) with no default. If a resource string is longer than nvarchar(255), it will be truncated to nvarchar(255).
resource_name is binary compared, and thus is case-sensitive regardless of the collation settings of the current database.
After an application lock has been acquired, only the first 32 characters can be retrieved in plain text; the remainder will be hashed.
- [ @LockMode = ] 'lock_mode'
Is the lock mode to be obtained for a particular resource. lock_mode is nvarchar(32) and has no default value. The value can be any of the following: Shared, Update, IntentShared, IntentExclusive, or Exclusive.
- [ @LockOwner = ] 'lock_owner'
Is the owner of the lock, which is the lock_owner value when the lock was requested. lock_owner is nvarchar(32). The value can be Transaction (the default) or Session. When the lock_owner value is Transaction, by default or specified explicitly, sp_getapplock must be executed from within a transaction.
- [ @LockTimeout = ] 'value'
Is a lock time-out value in milliseconds. The default value is the same as the value returned by @@LOCK_TIMEOUT. To indicate that a lock request should return an error instead of wait for the lock when the request cannot be granted immediately, specify 0.
- [ @DbPrincipal = ] 'database_principal'
Is the user, role, or application role that has permissions to an object in a database. The caller of the function must be a member of database_principal, dbo, or the db_owner fixed database role to call the function successfully. The default is public.
Return Code Values
>= 0 (success), or < 0 (failure)
The lock was successfully granted synchronously.
The lock was granted successfully after waiting for other incompatible locks to be released.
The lock request timed out.
The lock request was canceled.
The lock request was chosen as a deadlock victim.
Indicates a parameter validation or other call error.
Locks placed on a resource are associated with either the current transaction or the current session. Locks associated with the current transaction are released when the transaction commits or rolls back. Locks associated with the session are released when the session is logged out. When the server shuts down for any reason, all locks are released.
The lock resource created by sp_getapplock is created in the current database for the session. Each lock resource is identified by the combined values of:
- The database ID of the database containing the lock resource.
- The database principle specified in the @DbPrincipal parameter.
- The lock name specified in the @Resource parameter.
Only a member of the database principal specified in the @DbPrincipal parameter can acquire application locks that specify that principal. Members of the dbo and db_owner roles are implicitly considered members of all roles.
Locks can be explicitly released with sp_releaseapplock. When an application calls sp_getapplock multiple times for the same lock resource, sp_releaseapplock must be called the same number of times to release the lock.
If sp_getapplock is called multiple times for the same lock resource, but the lock mode that is specified in any of the requests is not the same as the existing mode, the effect on the resource is a union of the two lock modes. In most cases, this means the lock mode is promoted to the stronger of the lock modes, the existing mode, or the newly requested mode. This stronger lock mode is held until the lock is ultimately released even if lock release calls have occurred before that time. For example, in the following sequence of calls, the resource is held in
Exclusive mode instead of in
USE AdventureWorks; GO BEGIN TRANSACTION; DECLARE @result int; EXEC @result = sp_getapplock @Resource = 'Form1', @LockMode = 'Shared'; EXEC @result = sp_getapplock @Resource = 'Form1', @LockMode = 'Exclusive'; EXEC @result = sp_releaseapplock @Resource = 'Form1'; COMMIT TRANSACTION; GO
A deadlock with an application lock does not roll back the transaction that requested the application lock. Any rollback that might be required as a result of the return value must be done manually. Consequently, we recommend that error checking be included in the code so that if certain values are returned (for example, -3), a ROLLBACK TRANSACTION or alternative action is initiated.
Here is an example:
USE AdventureWorks; GO BEGIN TRANSACTION; DECLARE @result int; EXEC @result = sp_getapplock @Resource = 'Form1', @LockMode = 'Exclusive'; IF @result = -3 BEGIN ROLLBACK TRANSACTION; END ELSE BEGIN EXEC @result = sp_releaseapplock @Resource = 'Form1'; COMMIT TRANSACTION; END; GO
SQL Server uses the current database ID to qualify the resource. Therefore, if sp_getapplock is executed, even with identical parameter values on different databases, the result is separate locks on separate resources.
Use the sys.dm_tran_locks dynamic management view or the sp_lock system stored procedure to examine lock information, or use SQL Server Profiler to monitor locks.
Requires membership in the public role.
The following example places a shared lock, which is associated with the current transaction, on the resource
Form1 in the
USE AdventureWorks; GO BEGIN TRAN; DECLARE @result int; EXEC @result = sp_getapplock @Resource = 'Form1', @LockMode = 'Shared'; COMMIT TRAN; GO
The following example specifies
dbo as the database principal.
EXEC sp_getapplock @DbPrincipal = 'dbo', @Resource = 'AdventureWorks', @LockMode = 'Shared'; GO