Function shipping examples

Figure 4 and Figure 5 give examples to illustrate the lifetime of the mirror transaction.

Figure 4. Function shipping--simple inquiry. Here, no resource is being changed; the session is freed and the mirror task is terminated immediately.
 The picture consists of three columns. The left column represents System A, the right System B, and the middle column what flows between the two systems.  An application on System A issues an EXEC CICS READ FILE request. A 'READ' request is sent to System B. System B attaches the mirror transaction, which performs the READ and sends a reply to System A. Because no resources are being changed, the mirror frees the session and terminates. System A frees the session and passes B's reply to the application, which continues processing.
Figure 5. Function shipping--update. Because the mirror must wait for the REWRITE, it does not terminate until SYNCPOINT is received. Note that the enqueue on the updated record would not be held beyond the REWRITE command if the file was not recoverable.
 The picture consists of three columns. The left column represents System A, the right System B, and the middle column what flows between the two systems.  An application on System A issues an EXEC CICS READ UPDATE FILE request. A 'READ UPDATE' request is sent to System B. System B attaches the mirror transaction, which performs the READ UPDATE and sends a reply to System A. The mirror then waits for the REWRITE, so becoming long-running. System A passes B's reply to the application, which issues an EXEC CICS REWRITE FILE request. The REWRITE request is sent to System B. On System B, the mirror performs the REWRITE, sends a reply to System A, and waits, still holding the enqueue on the updated record. System A passes B's reply to the application, which issues an EXEC CICS SYNCPOINT request which is sent to System B. On System B, the mirror takes the syncpoint, releases the enqueue, frees the session, and terminates.  A positive response is sent to System A, On System A, the syncpoint is completed and the application continues.

In Figure 5 the mirror is long-running.

Related concepts
Introduction to function shipping
Transparency to application
Remote resources that can be accessed
How function shipping works
Synchronization
Related reference
CICS product communication support
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