During PCB generation, you can use five options of the PROCOPT parameter to indicate to IMS DB whether your program can read segments in the hierarchy, or whether it can also update segments.
From most restrictive to least restrictive below are the Options
- Your program can read segments.
- Your program can read and replace segments.
- Your program can insert segments.
- Your program can read and delete segments.
- Your program needs all the processing options. It is equivalent to specifying G, R, I, and D.
Processing options provide data security because they limit what a program can do to the hierarchy or to a particular segment.
Specifying only the processing options the program requires ensures that the program cannot update any data it is not supposed to.
For example, if a program does not need to delete segments from a database, the D option need not be specified.
When an application program retrieves a segment and has any of the just-described processing options, IMS locks the database record for that application.
If PROCOPT=G is specified, other programs with the option can concurrently access the database record.
If an update processing option (R, I, D, or A) is specified, no other program can concurrently access the same database record.
If no updates are performed, the lock is released when the application moves to another database record or, in the case of HDAM, to another anchor point.
The following locking protocol allows IMS to make this determination. If the root segment is updated, the root lock is held at update level until commit.
If a dependent segment is updated, it is locked at update level. When exiting the database record, the root segment is demoted to read level.
When a program enters the database record and obtains the lock at either read or update level, the lock manager provides feedback indicating whether or not another program has the lock at read level.
This determines if dependent segments will be locked when they are accessed. For HISAM, the primary logical record is treated as the root, and the overflow logical records are treated as dependent segments.
When using block-level or database-level data sharing for online and batch programs, you can use additional processing options.
Click here for more info GOx and data integrity
With the E option, your program has exclusive access to the hierarchy or to the segment you use it with.
The E option is used in conjunction with the options G, I, D, R, and A. While the E program is running, other programs cannot access that data, but may be able to access segments that are not in the E program’s PCB.
No dynamic enqueue by program isolation is done, but dynamic logging of database updates will be done.
When your program retrieves a segment with the GO option, IMS does not lock the segment.
While the read without integrity program reads the segment, it remains available to other programs.
This is because your program can only read the data (termed read-only); it is not allowed to update the database.
No dynamic enqueue is done by program isolation for calls against this database.
Serialization between the program with PROCOPT=GO and any other update program does not occur; updates to the same data occur simultaneously.
If a segment has been deleted and another segment of the same type has been inserted in the same location, the segment data and all subsequent data that is returned to the application may be from a different database record.
A read-without-integrity program can also retrieve a segment even if another program is updating the segment.
This means that the program need not wait for segments that other programs are accessing.
If a read-without-integrity program reads data that is being updated by another program, and that program terminates abnormally before reaching the next commit point, the updated segments might contain invalid pointers.
If an invalid pointer is detected, the read-without-integrity program terminates abnormally, unless the N or T options were specified with GO. Pointers are updated during insert, delete and backout functions.
When you use the N option with GO to access a full-function database or a DEDB, and the segment you are retrieving contains an invalid pointer, IMS returns a GG status code to your program.
Your program can then terminate processing, continue processing by reading a different segment, or access the data using a different path.
The N option must be specified as PROCOPT=GON, GON, or GONP.
When you use the T option with GO and the segment you are retrieving contains an invalid pointer, the response from an application program depends on whether the program is accessing a full-function or Fast Path database.
For calls to full-function databases, the T option causes DL/I to automatically retry the operation.
You can retrieve the updated segment, but only if the updating program has reached a commit point or has had its updates backed out since you last tried to retrieve the segment.
If the retry fails, a GG status code is returned to your program.
For calls to Fast Path DEDBs, option T does not cause DL/I to retry the operation. A GG status code is returned.
The T option must be specified as PROCOPT=GOT, GOT, or GOTP.
GOx and data integrity
For a very small set of applications and data, PROCOPT=GOx offers some performance and parallelism benefits.
However, it does not offer application data integrity. For example, using PROCOPT=GOT in an online environment on a full-function database can cause performance degradation.
The T option forces a re-read from DASD, negating the advantage of very large buffer pools and VSAM hiperspace for all currently running applications and shared data.