When any step of a job requests exclusive control of a data set, with an exception of when the job is allowed to downgrade ENQs via the DSENQSHR specification, the system converts all requests for shared control of that data set within that job (DISP=SHR) to requests for exclusive control. One of two methods can be used to request exclusive control:
- DISP=NEW, DISP=MOD, or DISP=OLD on a JCL request.
- DISP=NEW, DISP=MOD, or DISP=OLD on a dynamic allocation request,
including dynamic allocation requests that result from the use of certain utility control statements. For example, utility control statements that delete/scratch a data set will result in exclusive use of that data set.
If a dynamic allocation requests exclusive control of a data set then all subsequent DISP=SHR JCL references to that data set within that job will be upgraded to exclusive control. The job will retain exclusive control of that data set until the end of the last step of that job which references that data set in its JCL. For example:
//STEP1 EXEC PGM=anypgm1
//DD1 DD DSN=A.B.C,DISP=SHR
//STEP2 EXEC PGM=IDCAMS
DELETE A.B.C
DEFINE A.B.C
//STEP3 EXEC PGM=anypgm3
//DD3 DD DSN=A.B.C,DISP=SHR
//STEP4 EXEC PGM=anypgm4
Before the start of STEP1, the job will request shared control of data set A.B.C. In STEP2, the DELETE/DEFINE of data set A.B.C will cause the shared control from STEP1 to be upgraded to exclusive control. In STEP3, control of data set A.B.C will remain exclusive, since it is not possible to downgrade an ENQ from EXCL to SHR in the case that a dynamic allocation upgrades the ENQ. At the end of STEP3, control of data set A.B.C will be released. The ENQ on data set A.B.C will therefore not be held at all during STEP4. If, however, the job also contained a STEP5 which requested use of data set A.B.C, then exclusive control of that data set would continue to be held by the job all the way through both STEP4 and STEP5.
DISP and ENQ: Before starting the first step of a job, the initiator requests control of all of the data sets in that job by issuing an ENQ for each of them, using the value specified for DISP to determine the kind of ENQ issued. The initiator issues the ENQ for each data set at the highest level required for that data set by any step of the job. For example, if all steps of the job request shared control of a specific data set (DISP=SHR) then the ENQ for that data set is requested as SHR. If, on the other hand, any step of the job requests exclusive control of a specific data set (DISP=NEW, DISP=MOD, or DISP=OLD), then the ENQ for that data set is requested EXCL.
If the job is allowed to downgrade ENQs, then ENQs will be downgraded from exclusive control to shared control at the last step for which the data set is referenced EXCL. The downgrade will not occur if the ENQ has been upgraded by a dynamic allocation request in the current or a previous step. If the job is not allowed to downgrade ENQs, then ENQs cannot be downgraded from EXCL to SHR. If one step needs the ENQ EXCL and a following step only needs it SHR, the ENQ is still held as EXCL.
In both cases, the ENQ is held until the end of the last step which references that data set, at which point the ENQ is released entirely.
DISP and ENQ for generation data sets: The way the initator issues an ENQ to control generation data sets can be different than with other data sets. The initiator only issues the ENQ for the GDG base name for a generation data set that is referenced by either:
- Their relative GDG names (for example, DSN=TEST.GDG.DATASET(0)).
- As GDG ALLs (for example, DSN=TEST.GDG.DATASET)
For example, the initiator issues the ENQ for the GDG base name, TEST.GDG.DATASET for the generation data sets shown in the previous list. Generation data sets referenced by either their relative GDG names or as GDG ALLs are processed this way because the initiator does not know which specific absolute generation names will be required. This is because the conversion from relative generation name to absolute generation name is done during the allocation for the step referencing the data set. The ENQ for the specific, absolute, generation (G0000V00) data set name or names is issued at the start of the step requesting the relative GDG or GDG ALL.
Note that the initiator does not issue an ENQ for the GDG base name for a generation data set that is referenced by its absolute GDG name. Instead it issues an ENQ for that specific G0000V00 data set name.
