Pseudo-conversational programming

Between sending a screen and getting a reply, a CICS program does not wait — it ends. When the user presses Enter, a new task starts and picks up where the last one left off. This is the central idea in CICS application design.

Why it works this way

A conversational program would sit in storage holding its region resources while a human reads the screen. With a thousand users, that is a thousand idle tasks holding storage and locks. Ending the task instead means the region holds nothing between interactions — which is how the same hardware serves far more users.

Conversational vs pseudo-conversational

Conversational

Send screenWait for the userTask alive the whole timeResources heldDoes not scale

Pseudo-conversational

Send screenTask endsNothing heldNew task on replyScales to thousands

Carrying state across the gap: COMMAREA

The standard shape of a pseudo-conversational program

       WORKING-STORAGE SECTION.
       01  WS-COMMAREA.
           05  WS-NEXT-STEP     PIC X(01).
           05  WS-ACCT-ID       PIC X(10).
           05  WS-SAVED-DATA    PIC X(200).
       LINKAGE SECTION.
       01  DFHCOMMAREA          PIC X(211).

       PROCEDURE DIVISION.
           IF EIBCALEN = ZERO
               * first time in — no commarea passed
               PERFORM 1000-SEND-FIRST-MAP
           ELSE
               MOVE DFHCOMMAREA TO WS-COMMAREA
               EVALUATE WS-NEXT-STEP
                   WHEN '1' PERFORM 2000-HANDLE-ENQUIRY
                   WHEN '2' PERFORM 3000-HANDLE-UPDATE
                   WHEN OTHER PERFORM 9000-INVALID
               END-EVALUATE
           END-IF

           EXEC CICS RETURN
               TRANSID('PAY1')
               COMMAREA(WS-COMMAREA)
               LENGTH(LENGTH OF WS-COMMAREA)
           END-EXEC.

EIBCALEN is the length of the commarea passed in. Zero means this is the first time — the standard test for 'is this a fresh start?'.
RETURN TRANSID(…) COMMAREA(…) ends the task, tells CICS which transaction to run next time this terminal sends data, and hands forward the state.
DFHCOMMAREA in the LINKAGE SECTION is how the incoming state arrives.
Modern alternatives — a channel and containers — carry more than the traditional 32KB commarea limit and are typed. New work should prefer them where the site allows.

The EIB: what CICS tells your program

Field	Contains
EIBCALEN	Length of the commarea passed in
EIBAID	Which key the user pressed — Enter, PF3, PF12, Clear
EIBTRNID	The transaction ID that started this task
EIBTRMID	The terminal ID
EIBDATE / EIBTIME	When the task started
EIBRESP / EIBRESP2	The outcome of the last CICS command

Handling a not-found instead of abending

           EXEC CICS READ
               FILE('ACCTFILE')
               INTO(WS-ACCT-RECORD)
               RIDFLD(WS-ACCT-ID)
               RESP(WS-RESP)
           END-EXEC

           EVALUATE WS-RESP
               WHEN DFHRESP(NORMAL)
                   PERFORM 2200-SHOW-ACCOUNT
               WHEN DFHRESP(NOTFND)
                   MOVE 'ACCOUNT NOT FOUND' TO MSGO
               WHEN OTHER
                   PERFORM 9000-CICS-ERROR
           END-EVALUATE

Example, line by line

COBOL + CICSWhat it means

IF EIBCALEN = 0

First time in: no COMMAREA was passed, so there is no saved state.

PERFORM 1000-SEND-FIRST-SCREEN

Show the empty screen.

ELSE

Not the first time: the user has typed something.

MOVE DFHCOMMAREA TO WS-STATE

Restore the state saved at the end of the previous task.

PERFORM 2000-PROCESS-INPUT

Read the screen and do the work.

END-IF

EXEC CICS RETURN TRANSID('ACCT')

End this task now, and start ACCT again when the user next presses a key…

COMMAREA(WS-STATE)

…passing the saved state to that next task.

END-EXEC

Try it yourself

TRY IT YOURSELF

Which EIB field tells the program whether a COMMAREA was passed in?

Show a hint

It holds the COMMAREA length.

Show the solution

EIBCALEN — zero means no COMMAREA, so this is the first time in.

Why it works this way

Carrying state across the gap: COMMAREA

The EIB: what CICS tells your program

Example, line by line

Try it yourself

Common mistakes

What you will see at work

Key terms