19
DBMS_LOCK

Oracle Lock Management services for your applications are available through procedures in the DBMS_LOCK package. 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.

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.

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.

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

Requirements

DBMS_LOCK is most efficient with a limit of a few hundred locks per 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.

Security

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 DBMSLOCK.SQL package specification file.

Viewing and Monitoring Locks

Oracle provides two facilities to display locking information for ongoing transactions within an instance:

Enterprise Manager Monitors (Lock and Latch Monitors)

The Monitor feature of Oracle Enterprise Manager provides two monitors for displaying lock information of an instance. Refer to the Oracle Enterprise Manager documentation for complete information about the Enterprise Manager monitors.

UTLLOCKT.SQL

The UTLLOCKT.SQL script displays a simple character lock wait-for graph in tree structured fashion. Using any ad hoc SQL tool (such as SQL*Plus) to execute the script, it prints the sessions in the system that are waiting for locks and the corresponding blocking locks. The location of this script file is operating system dependent. (You must have run the CATBLOCK.SQL script before using UTLLOCKT.SQL.)

Constants

nl_mode  constant integer := 1;
ss_mode  constant integer := 2;       -- Also called 'Intended Share'
sx_mode  constant integer := 3;       -- Also called 'Intended Exclusive'
s_mode   constant integer := 4;
ssx_mode constant integer := 5;
x_mode   constant integer := 6;
  

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 aquired on sub-parts of the object. Similarly, a sub-exclusive lock can be used on an aggregate object to indicate that exclusive locks are being aquired 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.

Lock Compatibility Rules

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

Table 19-1 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.