There is always a chance that various generations of a file generation group are processed, created or deleted simultaneously by parallel jobs. Such uncontrolled simultaneous access can very easily lead to inconsistencies within the sequence of generations or to data inconsistencies if the file generations result from a series of processing steps, each of which is based on the previous one.
One way of avoiding the problems inherent in concurrent access is to reserve the file generation group by means of the SECURE-RESOURCE-ALLOCATION command (operand FILE). This does not lock the volumes, but simply prevents access to the group entry and the file generations. This reservation option should be used whenever multiple concurrent access to an FGG cannot be ruled out and would result in inconsistencies. It is not necessary, for example, if only read access to the generations is permitted.
The following example illustrates potential problems in the case of simultaneous access from different jobs without reservation.
Example: parallel access to an FGG
Initial situation: the FGG GROUP1 consists of up to three generations, currently generations 4, 5 and 6; generation 5 is defined as the base for relative indexing (BASE-NUM=5). Jobs 0001 and 0002 are started at the same time and initiate the following activities:
Job 0001:
/CREATE-FILE-GEN GEN-NAME=GROUP1(*7)
/MODIFY-FILE-GROUP-ATTRIBUTES GROUP-NAME=GROUP1,
GENERATION-PARAMETER=*GENERATION-PARAMETER(BASE-
NUMBER=ABSOLUTE(NUMBER=0))
Job 0002:
/ADD-FILE-LINK LINK-NAME=EINGABE,FILE-NAME=GROUP1(-1)
If the ADD-FILE-LINK command of the second job is entered after execution of the CREATE-FILE-GENERATION command, the user will receive the error message “File not found”. Even if the absolute generation number (*4) had been entered in the ADD-FILE-LINK command, the same error message would have been issued.
If the MODIFY-FILE-GROUP-ATTRIBUTES command is also completed before the user issues the ADD-FILE-LINK command, DMS will find a file generation GROUP1(-1). However, this is not the generation job 0002 expects (namely *4), but generation (*6). The subsequent processing of the file will lead to incorrect results.