Mainframe, MVS and zOS Discussion

Q: Why can't we use sequential files in CICS?

Answer: In the form of TDQ we can use it but not apart from that.

That was one of the question answer in one interview. But I think it's not that complete. Can someone of you plese tell me why can't we use a sequentail file in a CICS region?

Is it like CICS does not support QSAM access method? Please help with an answer.

CICS is a transactional system: it was designed for someone to pull up a screen of data (often representing a single record), possibly make changes, then apply those changes to the data set / database. The concept of a sequential data set applies to batch systems: a set of records is processed as a group through the system, applying changes and possibly generating reports along the way. There are times when sequential data is handy, hence the TDQ facility of CICS -- but note that TDQ is a limited sequential processing facility. CICS terminal response time targets are often under 1 second, and you simply cannot process a lot of sequential data and maintain those response times.

Thanks Robert. So what is the answer for the question of CICS not allowed to use sequential files? That, CICS won't be able to maintain the response time with in limits?

Part of the answer is that sequential data sets aren't very compatible with CICS. Also, consider locking - writing to a sequential data set requires locking at least at the record level and more likely at the data set level to prevent corruption of data. Since you can have thousands of CICS users in a single region, using a sequential data set VASTLY increases the chances of data corruption unless there are stringent locking rules applied.

Thank you Robert, this is really helpful.

I thought sequential files can be used with CICS even though that's very rare, is not that true?

Transient data queues are used to access sequential data, and they're not really that rare. However, otherwise using a sequential data set in CICS is something I haven't heard of in 46+ years of working with mainframes and CICS.

Transient data queues are used to access sequential data, and they're not really that rare.

They are not sequential files?

There are multiple types of TDQs: extrapartition point to sequential files (data sets); intrapartition are only defined and used in the CICS region (there are other types, too). So a TDQ may (or may not) be pointing to a sequential file.

Thanks Robert.