CICS Transaction Manager

The transaction manager is the Task Control Program, DFHKCP. It creates one task for every execution of a transaction and manages all the tasks running in the region.

What is the transaction manager

• The transaction manager is the Task Control Program, DFHKCP. It is part of the CICS nucleus.
• A transaction is work defined in the PCT by a TRANSID, for example ORD1.
• A task is one execution of a transaction. The same transaction can run as many tasks at once - one per user.
• Task Control attaches a task when work arrives and detaches it when the work finishes.

How tasks are started

• Terminal input: the user types a TRANSID or presses a PF key. Terminal Control passes it to Task Control, which attaches the task.
• ATI (Automatic Transaction Initiation): an intrapartition transient data queue reaches its trigger level and CICS starts the named transaction by itself.
• EXEC CICS START: one program starts another transaction, either at once or at a future time.
• Each task gets a task number (EIBTASKN) that is unique inside the region while the task runs.

Multitasking, multithreading and quasi-reentrancy

• Multitasking: CICS runs many tasks at the same time. Under the covers it is one MVS task switching rapidly between CICS tasks.
• Multithreading: one copy of a program in storage can be used by many tasks at once, each with its own working storage.
• Quasi-reentrant: a CICS program must be re-enterable between CICS commands. Between two EXEC CICS commands, another task may run the same program code.
• Because of this, never keep task data in storage shared by all tasks. Keep it in the COMMAREA, the TWA, or GETMAIN storage.

Task lifecycle

• Attach: Task Control creates the task control block and assigns a task number.
• Dispatch: the CICS dispatcher gives the task CPU time to run.
• Suspend: the task waits - for example for a file read or terminal input. Other tasks run meanwhile.
• Resume: the wait is over and the task continues where it left off.
• Terminate: the program issues EXEC CICS RETURN or abends. Task storage is freed automatically.

Example - reading task information in COBOL

• EIBTRNID holds the transaction id and EIBTASKN holds the task number. Both come free with every task.
• WORKING-STORAGE SECTION. 01 WS-TRAN-ID PIC X(4). 01 WS-TASK-NUM PIC 9(7). PROCEDURE DIVISION. MOVE EIBTRNID TO WS-TRAN-ID MOVE EIBTASKN TO WS-TASK-NUM EXEC CICS SEND TEXT FROM('TRANSACTION STARTED') END-EXEC EXEC CICS RETURN END-EXEC.
• Logging the TRANSID and task number helps you match program messages to CICS dumps and traces later.