Global ENQ/DEQ extends the CICS/ESA application programming interface to provide an enqueue mechanism that serializes access to a named resource across a specified set of CICS® regions contained within a sysplex. Because Global ENQ/DEQ eliminates the most significant remaining cause of inter-transaction affinity, it enables better exploitation of parallel sysplex. It also reduces the need to provide intertransaction affinity rules to dynamic routing mechanisms such as CICSPlex/SM, thus reducing the system management cost of exploiting parallel sysplex.
Global ENQ/DEQ uses z/OS® global resource serialization (GRS) services to achieve locking that is unique across multiple MVS™ images in a sysplex. GRS can be configured as either GRS=STAR or GRS=RING.
When GRS is initialized as a star configuration, all the information about resource serialization is held in the ISGLOCK coupling facility structure. GRS accesses the coupling facility when a requestor issues an ENQ or DEQ on a global names resource.
GRS=RING, however, should be used with extreme caution, as this configuration can result in serious performance constraints.
The performance impact can be for a many reasons, but primarily it is due to the delay in having the request complete the ring. The larger the number of MVS images in the ring combined with a large value for RESMIL will cause delays in the request completing the ring. The ENQ request cannot be granted until the request returns to the originating MVS image. The value specified for RESMIL (in the GRSCNF member of SYS1.Parmlib) should be no greater than 1, preferably 0. For performance reasons, in a sysplex of greater than 2 MVS images, a GRS STAR configuration should be used.