Cross-Region Share Options

The level of cross-region sharing permitted by VSAM is established (when the data set is defined) with the SHAREOPTIONS value:

Cross-region SHAREOPTIONS 1: The data set can be shared by any number of VSAM control blocks for read processing, or the data set can be accessed by only one VSAM control block for read and write (OUTPUT) processing. With this option, VSAM ensures complete data integrity for the data set. This setting does not permit any type of non-RLS access when the data set is already open for RLS processing.
Cross-region SHAREOPTIONS 2: If the data set has not already been opened for record-level sharing (RLS) processing, the data set can be accessed by any number of non-RLS users for read processing and it can also be accessed by one non-RLS user for write processing. With this option, VSAM ensures write integrity by obtaining exclusive control for a control interval when it is to be updated.
If the data set has already been opened for RLS processing, non-RLS accesses for read are allowed. VSAM provides full read and write integrity to its RLS users, but it is the non-RLS user's responsibility to ensure read integrity.

If you require read integrity, it is your responsibility to use the ENQ and DEQ macros appropriately to provide read integrity for the data the program obtains. For information about using ENQ and DEQ see z/OS MVS Programming: Authorized Assembler Services Reference ALE-DYN and z/OS MVS Programming: Authorized Assembler Services Reference EDT-IXG.
Cross-region SHAREOPTIONS 3: The data set can be fully shared by any number of users. With this option, each user is responsible for maintaining both read and write integrity for the data the program accesses. This setting does not allow any type of non-RLS access when the data set is already open for RLS processing.
This option requires that the user's program use ENQ/DEQ to maintain data integrity while sharing the data set, including the OPEN and CLOSE processing. User programs that ignore the write integrity guidelines can cause VSAM program checks, lost or inaccessible records, uncorrectable data set failures, and other unpredictable results. This option places responsibility on each user sharing the data set.
Cross-region SHAREOPTIONS 4: The data set can be fully shared by any number of users, and buffers used for direct processing are refreshed for each request. This setting does not allow any type of non-RLS access when the data set is already open for RLS processing. With this option, as in SHAREOPTIONS 3, each user is responsible for maintaining both read and write integrity for the data the program accesses. See the description of SHAREOPTIONS 3 for ENQ/DEQ and warning information that applies equally to SHAREOPTIONS 4.

With options 3 and 4 you are responsible for maintaining both read and write integrity for the data the program accesses. These options require your program to use ENQ/DEQ to maintain data integrity while sharing the data set, including the OPEN and CLOSE processing. User programs that ignore the write integrity guidelines can cause VSAM program checks, lost or inaccessible records, uncorrectable data set failures, and other unpredictable results. These options place heavy responsibility on each user sharing the data set.

When your program requires that no updating from another control block structure occur before it completes processing of the requested data record, your program can issue an ENQ to obtain exclusive use of the VSAM data set. If your program completes processing, it can relinquish control of the data set with a DEQ. If your program is only reading data and not updating, it is probably a good practice to serialize the updates and have the readers wait while the update is occurring. If your program is updating, after the update has completed the ENQ/DEQ bracket, the reader must determine the required operations for control block refresh and buffer invalidation based on a communication mechanism or assume that everything is down-level and refresh each request.

The extent of cross-region sharing is affected by using DISP=SHR or DISP=OLD in the DD statement. If the data set's DD statement specifies DISP=OLD, only the dsname associated with the DD statement is exclusively controlled. In this case, only the cluster name is reserved for the OPEN routine's exclusive use. You can include DD statements with DISP=OLD for each of the cluster's components to reserve them as well. Doing this ensures that all resources needed to open the data set will be exclusively reserved before your task is initiated.

Protecting the cluster name with DISP processing and the components by VSAM OPEN SHAREOPTIONS is the normally accepted procedure. When a shared data set is opened with DISP=OLD, or is opened for reset processing (IDCAMS REUSE command), or is empty, the data set is processed using SHAREOPTIONS 1 rules.

Scheduler disposition processing is the same for VSAM and non-VSAM data sets. This is the first level of share protection.