46
DBMS_LOCK

The DBMS_LOCK package provides an interface to Oracle Lock Management services. You can request a lock of a specific mode, give it a unique name recognizable in another procedure in the same or another instance, change the lock mode, and release it.

This chapter contains the following topics:

• Using DBMS_LOCK
  • Overview
  • Security Model
  • Constants
  • Rules and Limits
  • Operational Notes
• Summary of DBMS_LOCK Subprograms

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Using DBMS_LOCK

• Overview
• Security Model
• Constants
• Rules and Limits
• Operational Notes

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Overview

Some uses of user locks:

• 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

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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.

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Constants

Table 46-1 Constants - Names and Oracle Enterprise Manager Abbreviations

Constant	Name	Alternate Name(s)	Oracle Enterprise Manager Abbreviation
nl_mode (1)	NuLl
ss_mode (2)	Sub Shared		ULRS
sx_mode (3)	Sub eXclusive	Row Exclusive Mode,	ULRX
s_mode (4)	Sub eXclusive	Row Exclusive Mode, Intended Exclusive	ULRSX
ssx_mode (5	Shared	Share Row Exclusive Mode
x_mode (6)	eXclusive		ULX

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

A sub-share lock can be used on an aggregate object to indicate that share locks are being acquired on subparts of the object. Similarly, a sub-exclusive lock can be used on an aggregate object to indicate that exclusive locks are being acquired on sub-parts of the object. A share-sub-exclusive lock indicates that the entire aggregate object has a share lock, but some of the sub-parts may additionally have exclusive locks.

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Rules and Limits

When another process holds "held", an attempt to get "get" does the following:

Table 46-2 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.

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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.

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Summary of DBMS_LOCK Subprograms

Table 46-3 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.
SLEEP Procedure	Puts a procedure to sleep for a specific time.

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ALLOCATE_UNIQUE Procedure

This procedure allocates a unique lock identifier (in the range of 1073741824 to 1999999999) given a 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 46-4 ALLOCATE_UNIQUE Procedure Parameters

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 given 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 given 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.

Exceptions

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

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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 46-5 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 given 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 46-6 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

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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 46-7 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 46-8 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

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REQUEST Function

This function requests a lock with a given 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 46-9 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 46-10 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

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SLEEP Procedure

This procedure suspends the session for a given period of time.

Syntax

DBMS_LOCK.SLEEP (
   seconds  IN NUMBER);

Parameters

Table 46-11 SLEEP Procedure Parameters

Parameter	Description
seconds	Amount of time, in seconds, to suspend the session. The smallest increment can be entered in hundredths of a second; for example, 1.95 is a legal time value.