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When you want to share data in a multi-user environment, you need to prevent more than one user from changing the same data at the same time. Locking files and records enables programs in a multi-user environment to share common files while maintaining data integrity.
The sharing mode indicates whether a file is to participate in file sharing and record locking.
Three sharing modes are available:
Do not allow sharing with any other user of this file.
Allow sharing with other users who have the file opened for input.
Allow sharing with all other users.
For a full table showing the results when an open is attempted in the various sharing modes, see the table Opening available shared files that are currently open by another file connector in the chapter Procedure Division - MERGE - OPEN in your Language Reference .
To specify one of these sharing modes, use the SHARING phrase either on an OPEN statement or as part of the SELECT statement. The SHARING phrase in the OPEN statement overrides that in the SELECT statement.
If you do not include the SHARING phrase in either the SELECT or the OPEN statement, the sharing mode is determined when the first of the following conditions is satisfied:
If file sharing is in effect, you can set locks on individual records. In this way, you can prevent other users from accessing the locked records whilst enabling these users to access other unlocked records.
Note: You cannot lock records within a line sequential file or a file that you have opened for input.
To lock a record, use either manual or automatic record locking.
If you are using manual record locking, use the READ statement to acquire a record lock.
To specify manual locking, use:
For example:
select fd-name assign to "muser.dat" lock mode is manual ... read fd-name with lock
If you are using automatic record locking, the READ statement acquires a record lock automatically. To avoid locking a record, use the WITH NO LOCK clause in the READ statement.
To specify automatic locking, use the LOCK MODE IS AUTOMATIC clause in the SELECT statement for the file:
select fd-name assign to "muser.dat" lock mode is automatic ... read fd-name
You can lock either a single record or multiple records.
If you are using single record locking, you can hold one record lock only at any one time. Each new I/O operation on the file releases any previous lock.
To specify single record locking, do not use the WITH LOCK ON MULTIPLE RECORDS clause after either the LOCK MODE IS MANUAL clause or the LOCK MODE IS AUTOMATIC clause.
Multiple record locking permits many record locks to be held at once.
To specify multiple record locking, follow either the LOCK MODE IS MANUAL clause or the LOCK MODE IS AUTOMATIC clause with the WITH LOCK ON MULTIPLE RECORDS clause. For example:
select fd-name assign to "muser.dat" lock mode is automatic with lock on multiple records ...
Notes:
By default, when a read operation encounters a locked record, the operation returns a locked record status. However, the operation does update the record area to show the contents of the record. After this operation, the current record pointer remains associated with the locked record, so that a following READ NEXT operation continues to attempt to access the same record.
The following configuration options affect this behaviour:
When set to ON, specifies that record locks are to be ignored when reading files open for input.
When set to ON, specifies that operations that attempt to access locked records should be retried until the operation is successful. Can also be set to an integer, which specifies the maximum number of retries before the operation is aborted.
When set to 1, record locks are placed on the record itself, which means that a read operation that meets a locked record cannot return the data record. In this case the contents of the record area following the read are undefined.
When set to ON specifies that, following a read encountering a locked record, a READ NEXT operation returns the following record.
Note:
To bypass the locked record, you can also use the START…KEY IS GREATER THAN.. statement.
When the program that acquired the lock does one of the following, the action releases all record locks:
Additionally, when the program that acquired the lock accesses any other record in the file with any file operation, except START, this action releases single record locks.
If your program runs in a multi-user environment, you should always check for the following file status codes:
| 9/065 | This status code is returned by an OPEN statement and indicates that the file is locked. |
| 9/068 | This status code is returned by a READ, DELETE or REWRITE statement and indicates that the record is locked. |
| 9/213 | This status code indicates that the maximum number of permitted locks has been reached. The maximum number of permitted locks depends on your network and/or operating system. |
An example file and record locking application is provided. It consists of a main program LOCKING.CBL and other programs called by this main program. It can be found using Infomgr.
When running this application, use option 4 initially to create a file. To view the affects of record locking, use options 2 and/or 3 in multiple sessions.
Copyright © 2000 MERANT International Limited. All rights reserved.
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| File Status | File Handler Configuration | |