Task vs Transaction

Beginners often mix up task and transaction. They are related, but they are not the same thing.

What is a transaction

• A transaction is the logical unit of work — what the user wants done, like “add a customer”.
• It is identified by its 1-to-4 character transaction id.
• From the user’s point of view the transaction is one continuous job, even across many screens.
• In a pseudo-conversational design one transaction lives across several separate tasks.

What is a task

• A task is one execution of a program under CICS — the runtime instance doing the work.
• CICS creates a task every time a transaction (or part of it) starts running.
• Each task gets its own working storage, its own EIB, and its own task control area (TCA).
• Many tasks for the same transaction id can run at the same time — one per user.
• A task ends when the program issues RETURN; the transaction may or may not end with it.

Key differences

• Meaning: transaction = what the user wants done; task = one run of the program that does part of it.
• Count: one transaction can need many tasks (pseudo-conversational); one task always belongs to one transaction.
• Lifetime: a task ends at every RETURN; the transaction ends only at the final RETURN without TRANSID.
• Scheduling: CICS schedules and dispatches tasks, not transactions.
• Isolation: tasks are isolated from each other — one task’s abend does not kill other tasks.
• A task can find out which transaction started it:-
  
  *--- A task can find out which transaction started it ---* WORKING-STORAGE SECTION. 01 WS-TRAN-ID PIC X(4). PROCEDURE DIVISION. EXEC CICS ASSIGN TRANID(WS-TRAN-ID) END-EXEC. * WS-TRAN-ID now holds the transaction id, for example 'MNT1'