Efficient sequential data set access

CICS provides a number of different sequential processing options. Each has different performance characteristics.

Temporary storage and intrapartition transient data queues (already discussed in Temporary storage queues and in Intrapartition transient data ) are the most efficient to use, but they must be created and processed entirely within CICS.

Extrapartition transient data is the CICS way of handling standard sequential (QSAM/BSAM) data sets. It is the least efficient of the three forms of sequential support listed, because CICS has to issue operating system waits to process the data sets, as it does when handling BDAM. Moreover, extrapartition transient data sets are not recoverable. VSAM ESDSs, on the other hand, are recoverable within limitations, and processing is more efficient. The recovery limitation is that records added to an ESDS during an uncompleted UOW cannot be removed physically during the backout process, because of VSAM restrictions. They can, however, be flagged as deleted by a user exit routine.

CICS journals provide another good alternative to extrapartition transient data, although only for output data sets. Journals are managed by the MVS™ system logger, but flexible processing options permit very efficient processing. Each journal command specifies operation characteristics, for example, synchronous or asynchronous, whereas extrapartition operations are governed entirely by the parameters in the transient data queue definition.

Transactions should journal asynchronously, if possible, to minimize task waits in connection with journaling. However, if integrity considerations require that the journal records be physically written before end of task, you must use a synchronous write. If there are several journal writes, the transaction should use asynchronous writes for all but the last logical record, so that the logical records for the task are written with a minimum number of physical I/Os and only one wait.