Differences in CSD management between RLS and non-RLS access

Although a CICS® system definition data set (CSD) accessed in RLS mode is protected by VSAM RLS locking, this operates at the CICS file control level. It does not change the way the CEDA and CEDB transactions manage the integrity of CSD groups.

The CEDx transactions protect resource definitions in the same way for RLS mode and non-RLS mode CSDs. They protect individual resource definitions against concurrent updates by a series of internal locks on the CSD. The RDO transactions apply these locks at the group level. While RDO transactions are executing a command that updates any element in a group, they use the internal lock to prevent other RDO transactions within a CICS region from updating the same group. The locks are freed only when the updating command completes execution. Operations on lists are protected in the same way. However, in an RLS environment, these internal locks affect all CICS regions that open the CSD in RLS mode. In the non-RLS case they apply only to the CICS region that has the data set open for update (which can only be a single region).

The use of a single buffer pool by the SMSVSAM server removes some of the problems of sharing data that you get with a non-RLS CSD.

Some other points to note are:
  • If a CSD is defined with CSDACC=READWRITE and CSDRLS=YES, more than one CICS region can open the CSD concurrently. However, file control closes the CSD opened in RLS mode at termination of each CEDx transaction, in the same way as for a non-RLS CSD. CSDACC=READONLY is not necessary for a CSD accessed in RLS mode.
  • The number of concurrent requests that can be processed against the CSD, is always 1024 for RLS. Diagnostic messages about CSDSTRNO value do not occur for RLS-mode CSDs.
  • The VSAM cluster definition SHAREOPTIONS parameter is ignored by SMSVSAM when an application, such as CICS, opens a data set in RLS mode.
  • A CSD accessed in RLS mode could give rise to RDO transaction failures that do not occur for a non-RLS mode CSD: For example:
    • An RDO transaction could be holding an RLS exclusive lock while it updates a record, which causes another RDO transaction to time out.
    • If the CSD is recoverable and CICS or MVS™ fails, update locks of failed-inflight RDO transactions are converted into retained locks. This could result an RDO transaction receiving a LOCKED response from VSAM which, in turn, would cause the RDO transaction to fail.