Reserving segments for the exclusive use of your program

You may want to reserve a segment and prohibit other programs from updating the segment while you are using it. To some extent, IMS does this for you through resource lock management. The Q command code lets you reserve segments in a different way.

Resource lock management and the Q command code both reserve segments for your program's use, but they work differently and are independent of each other. To understand how and when to use the Q command code and the DEQ call, you must understand resource lock management.

The function of resource lock management is to prevent one program from accessing data that another program has altered until the altering program reaches a commit point. Therefore, you know that if you have altered a segment, no other program (except those using the GO processing option) can access that segment until your program reaches a commit point. For database organizations that support the Q command code, if the PCB processing option allows updates and the PCB holds position in a database record, no other program can access the database record.

The Q command code allows you to prevent other programs from updating a segment that you have accessed, even when the PCB that accessed the segment moves to another database record.

Related reading: For more information on the Q command code, see the topic "Q command code" in IMS Version 15.3 Application Programming APIs.