DBMS_LOCK

95.1 DBMS_LOCK Overview

The DBMS_LOCK package has many beneficial uses.

These uses include the following:

Providing exclusive access to a device, such as a terminal
Providing application-level enforcement of read locks
Detecting when a lock is released and cleanup after the application
Synchronizing applications and enforcing sequential processing

95.2 DBMS_LOCK Security Model

There might be operating system-specific limits on the maximum number of total locks available. This must be considered when using locks or making this package available to other users. Consider granting the EXECUTE privilege only to specific users or roles.

A better alternative would be to create a cover package limiting the number of locks used and grant EXECUTE privilege to specific users. An example of a cover package is documented in the DBMS_LOCK.SQL package specification file. The abbreviations for these locks as they appear in Enterprise Manager monitors are in parentheses.

95.3 DBMS_LOCK Constants

The DBMS_LOCK package includes several constants to use when specifying parameter values.

These constants are shown in the following table.

Table 95-1 DBMS_LOCK Constants

Name	Alternate Name(s)	Type	Value	OEM Abbreviation	Description
`NL_MODE`	`NuL1`	`INTEGER`	`1`	-	-
`SS_MODE`	`Sub Shared`	`INTEGER`	`2`	`ULRS`	This can be used on an aggregate object to indicate that share locks are being acquired on subparts of the object.
`SX_MODE`	`Sub eXclusive` `Row Exclusive Mode`	`INTEGER`	`3`	`ULRX`	This can be used on an aggregate object to indicate that exclusive locks are being acquired on sub-parts of the object.
`S_MODE`	`Shared` `Row Exclusive Mode` `Intended Exclusive`	`INTEGER`	`4`	`ULRSX`	-
`SSX_MODE`	`Shared` `Sub eXclusive` `Share Row Exclusive Mode`	`INTEGER`	`5`	-	This indicates that the entire aggregate object has a share lock, but some of the sub-parts may additionally have exclusive locks.
`X_MODE`	`Exclusive`	`INTEGER`	`6`	`ULX`	-

These are the various lock modes (nl -> "NuLl", ss -> "Sub Shared", sx -> "Sub eXclusive", s -> "Shared", ssx -> "Shared Sub eXclusive", x -> "eXclusive").

95.4 DBMS_LOCK. Rules and Limits

When another process holds "held", an attempt to get "get" succeeds or fails, based on the held mode and type of get.

The following table describes the results:

Table 95-2 Lock Compatibility

HELD MODE	GET NL	GET SS	GET SX	GET S	GET SSX	GET X
`NL`	`Success`	`Success`	`Success`	`Success`	`Success`	`Success`
`SS`	`Success`	`Success`	`Success`	`Success`	`Success`	`Fail`
`SX`	`Success`	`Success`	`Success`	`Fail`	`Fail`	`Fail`
`S`	`Success`	`Success`	`Fail`	`Success`	`Fail`	`Fail`
`SSX`	`Success`	`Success`	`Fail`	`Fail`	`Fail`	`Fail`
`X`	`Success`	`Fail`	`Fail`	`Fail`	`Fail`	`Fail`

maxwait  constant integer := 32767;

The constant maxwait waits forever.

95.5 DBMS_LOCK Operational Notes

User locks never conflict with Oracle locks because they are identified with the prefix "UL". You can view these locks using the Enterprise Manager lock monitor screen or the appropriate fixed views.

User locks are automatically released when a session terminates. The lock identifier is a number in the range of 0 to 1073741823.

Because a reserved user lock is the same as an Oracle lock, it has all the functionality of an Oracle lock, such as deadlock detection. Be certain that any user locks used in distributed transactions are released upon COMMIT, or an undetected deadlock may occur.

DBMS_LOCK is most efficient with a limit of a few hundred locks for each session. Oracle strongly recommends that you develop a standard convention for using these locks in order to avoid conflicts among procedures trying to use the same locks. For example, include your company name as part of your lock names.

95.6 Summary of DBMS_LOCK Subprograms

This table lists the DBMS_LOCK subprograms and briefly describes them.

Table 95-3 DBMS_LOCK Package Subprograms

Subprogram	Description
ALLOCATE_UNIQUE Procedure	Allocates a unique lock ID to a named lock
CONVERT Function	Converts a lock from one mode to another
RELEASE Function	Releases a lock
REQUEST Function	Requests a lock of a specific mode.

95.6.1 ALLOCATE_UNIQUE Procedure

This procedure allocates a unique lock identifier (in the range of 1073741824 to 1999999999) a specified lock name. Lock identifiers are used to enable applications to coordinate their use of locks. This is provided because it may be easier for applications to coordinate their use of locks based on lock names rather than lock numbers.

Syntax

DBMS_LOCK.ALLOCATE_UNIQUE (
   lockname         IN  VARCHAR2,
   lockhandle       OUT VARCHAR2,
   expiration_secs  IN  INTEGER   DEFAULT 864000);

Parameters

Table 95-4 ALLOCATE_UNIQUE Procedure Parameters

Parameter Description

Parameter	Description
`lockname`	Name of the lock for which you want to generate a unique ID. Do not use lock names beginning with `ORA$`; these are reserved for products supplied by Oracle.
`lockhandle`	Returns the handle to the lock ID generated by `ALLOCATE_UNIQUE`. You can use this handle in subsequent calls to `REQUEST`, `CONVERT`, and `RELEASE`. A handle is returned instead of the actual lock ID to reduce the chance that a programming error accidentally creates an incorrect, but valid, lock ID. This provides better isolation between different applications that are using this package. `LOCKHANDLE` can be up to `VARCHAR2` (128). All sessions using a lock handle returned by `ALLOCATE_UNIQUE` with the same lock name are referring to the same lock. Therefore, do not pass lock handles from one session to another.
`expiration_specs`	Number of seconds to wait after the last `ALLOCATE_UNIQUE` has been performed on a specified lock, before permitting that lock to be deleted from the `DBMS_LOCK_ALLOCATED` table. The default waiting period is 10 days. You should not delete locks from this table. Subsequent calls to `ALLOCATE_UNIQUE` may delete expired locks to recover space.

lockname

Name of the lock for which you want to generate a unique ID.

Do not use lock names beginning with ORA$; these are reserved for products supplied by Oracle.

lockhandle

Returns the handle to the lock ID generated by ALLOCATE_UNIQUE.

You can use this handle in subsequent calls to REQUEST, CONVERT, and RELEASE.

A handle is returned instead of the actual lock ID to reduce the chance that a programming error accidentally creates an incorrect, but valid, lock ID. This provides better isolation between different applications that are using this package.

LOCKHANDLE can be up to VARCHAR2 (128).

All sessions using a lock handle returned by ALLOCATE_UNIQUE with the same lock name are referring to the same lock. Therefore, do not pass lock handles from one session to another.

expiration_specs

Number of seconds to wait after the last ALLOCATE_UNIQUE has been performed on a specified lock, before permitting that lock to be deleted from the DBMS_LOCK_ALLOCATED table.

The default waiting period is 10 days. You should not delete locks from this table. Subsequent calls to ALLOCATE_UNIQUE may delete expired locks to recover space.

Usage Notes

If you choose to identify locks by name, you can use ALLOCATE_UNIQUE to generate a unique lock identification number for these named locks.

The first session to call ALLOCATE_UNIQUE with a new lock name causes a unique lock ID to be generated and stored in the dbms_lock_allocated table. Subsequent calls (usually by other sessions) return the lock ID previously generated.

A lock name is associated with the returned lock ID for at least expiration_secs (defaults to 10 days) past the last call to ALLOCATE_UNIQUE with the specified lock name. After this time, the row in the dbms_lock_allocated table for this lock name may be deleted in order to recover space. ALLOCATE_UNIQUE performs a commit.

WARNING:

Named user locks may be less efficient, because Oracle uses SQL to determine the lock associated with a specified name.

Exceptions

ORA-20000, ORU-10003: Unable to find or insert lock <lockname> into catalog dbms_lock_allocated.

95.6.2 CONVERT Function

This function converts a lock from one mode to another. CONVERT is an overloaded function that accepts either a user-defined lock identifier, or the lock handle returned by the ALLOCATE_UNIQUE procedure.

Syntax

DBMS_LOCK.CONVERT(
   id         IN INTEGER || 
   lockhandle IN VARCHAR2,
   lockmode   IN INTEGER,
   timeout    IN NUMBER DEFAULT MAXWAIT)
  RETURN INTEGER;

Parameters

Table 95-5 CONVERT Function Parameters

Parameter Description

id or lockhandle

User assigned lock identifier, from 0 to 1073741823, or the lock handle, returned by ALLOCATE_UNIQUE, of the lock mode you want to change

lockmode

New mode that you want to assign to the specified lock.

For the available modes and their associated integer identifiers, see Constants.

timeout

Number of seconds to continue trying to change the lock mode.

If the lock cannot be converted within this time period, then the call returns a value of 1 (timeout).

Return Values

Table 95-6 CONVERT Function Return Values

Return Value	Description
`0`	Success
`1`	Timeout
`2`	Deadlock
`3`	Parameter error
`4`	Don't own lock specified by `id` or `lockhandle`
`5`	Illegal lock handle

95.6.3 RELEASE Function

This function explicitly releases a lock previously acquired using the REQUEST function.

Locks are automatically released at the end of a session. RELEASE is an overloaded function that accepts either a user-defined lock identifier, or the lock handle returned by the ALLOCATE_UNIQUE procedure.

Syntax

DBMS_LOCK.RELEASE (
   id         IN INTEGER)
  RETURN INTEGER;

DBMS_LOCK.RELEASE (
   lockhandle IN VARCHAR2)
  RETURN INTEGER;

Parameters

Table 95-7 RELEASE Function Parameter

Parameter	Description
`id` or `lockhandle`	User assigned lock identifier, from 0 to 1073741823, or the lock handle, returned by `ALLOCATE_UNIQUE`, of the lock mode you want to change

Return Values

Table 95-8 RELEASE Function Return Values

Return Value	Description
`0`	Success
`3`	Parameter error
`4`	Do not own lock specified by `id` or `lockhandle`
`5`	Illegal lock handle

95.6.4 REQUEST Function

This function requests a lock with a specified mode.

REQUEST is an overloaded function that accepts either a user-defined lock identifier, or the lock handle returned by the ALLOCATE_UNIQUE procedure.

Syntax

DBMS_LOCK.REQUEST(
   id                 IN  INTEGER ||
   lockhandle         IN  VARCHAR2,
   lockmode           IN  INTEGER DEFAULT X_MODE,
   timeout            IN  INTEGER DEFAULT MAXWAIT,
   release_on_commit  IN  BOOLEAN DEFAULT FALSE)
  RETURN INTEGER;

The current default values, such as X_MODE and MAXWAIT, are defined in the DBMS_LOCK package specification.

Parameters

Table 95-9 REQUEST Function Parameters

Parameter	Description
`id` or `lockhandle`	User assigned lock identifier, from 0 to 1073741823, or the lock handle, returned by `ALLOCATE_UNIQUE`, of the lock mode you want to change
`lockmode`	Mode that you are requesting for the lock. For the available modes and their associated integer identifiers, see Constants.
`timeout`	Number of seconds to continue trying to grant the lock. If the lock cannot be granted within this time period, then the call returns a value of 1 (`timeout`).
`release_on_commit`	Set this parameter to `TRUE` to release the lock on commit or roll-back. Otherwise, the lock is held until it is explicitly released or until the end of the session.

Return Values

Table 95-10 REQUEST Function Return Values

Return Value	Description
`0`	Success
`1`	Timeout
`2`	Deadlock
`3`	Parameter error
`4`	Already own lock specified by `id` or `lockhandle`
`5`	Illegal lock handle