IBM SC34-7012-01 manual Mechanisms for passing data between transactions, Main storage areas

back out only the updates made during that individual step; the application is responsible for restarting at the appropriate point in the conversation, which might involve recreating a screen format.

However, other tasks might try to update the database between the time when update information is accepted and the time when it is applied to the database. Design your application to ensure that no other application can update the database at a time when it would corrupt the updating by your own application.

Mechanisms for passing data between transactions

In those applications where one transaction needs to access working data created by a previous transaction, you must decide which mechanism will pass the data between the transactions.

You have two options for passing data between transactions:

vMain storage areas

vCICS recoverable resources

Main storage areas

The advantages of main storage areas are realized only where recovery is not important, or when passing data between programs servicing the same task.

Main storage areas that you can use to pass data between transactions include:

vThe communication area (COMMAREA)

vThe common work area (CWA)

vTemporary storage (main)

vThe terminal control table user area (TCTUA)

CICS does not log changes to these areas (except as noted later in this section). Therefore, in the event of an uncontrolled shutdown, data stored in any of these areas is lost, which makes them unsuitable for applications needing to retain data between transactions across an emergency restart. Also, some of these storage areas can cause inter-transaction affinities, which are a hindrance to dynamic transaction routing. To avoid inter-transaction affinities, use either a COMMAREA or the TCTUA. For information about intertransaction affinities, see the CICS Application Programming Guide.

Design programs so that they do not rely on the presence or absence of data in the COMMAREA to indicate whether or not control has been passed to the program for the first time (for example, by testing for a data length of zero). Consider the abend of a transaction where dynamic transaction backout and automatic restart are specified. After the abend, a COMMAREA could be passed to the next transaction from the terminal, even though the new transaction is unrelated. Similar considerations apply to the terminal control table user area (TCTUA).

CICS recoverable resources

Resources recoverable by backout that can be used for communication between transactions include:

vTemporary storage (auxiliary)

vTransient data queues

vUser files and DL/I and DB2 databases

Chapter 13. Programming for recovery 145