How tasks are started

Work is started in CICS® - that is, tasks are initiated - from unsolicited input, or by automatic task initiation (ATI).

Automatic task initiation occurs when:

An existing task asks CICS to create another one. The START command, the IMMEDIATE option on a RETURN command (discussed in RETURN IMMEDIATE), and the SEND PAGE command (in The SEND PAGE command) all do this.
CICS creates a task to process a transient data queue (see Automatic transaction initiation (ATI)).
CICS creates a task to deliver a message sent by a BMS ROUTE request (see Message routing). The CSPG tasks you see after using the CICS-supplied transaction CMSG are an example of this. CMSG uses a ROUTE command which creates a CSPG transaction for each target terminal in your destination list.

The primary mechanism for initiating tasks, however, is unsolicited input. When a user transmits input from a terminal which is not the principal facility of an existing task, CICS creates a task to process it. The terminal that sent the input becomes the principal facility of the new task.

Principal facility: CICS allows a task to communicate directly with only one terminal, namely its principal facility. CICS assigns the principal facility when it initiates the task, and the task owns the facility for its duration. No other task can use that terminal until the owning task ends. If a task needs to communicate with a terminal other than its principal facility, it must do so indirectly, by creating another task that has the terminal as its principal facility. This requirement arises most commonly in connection with printing, and how you can create such a task is explained in Using CICS printers.

Unsolicited inputs from other systems are handled in the same way: CICS creates a task to process the input, and assigns the conversation over which the input arrived as the principal facility. (Thus a conversation with another system may be either a principal or alternate facility. In the case where a task in one CICS region initiates a conversation with another CICS region, the conversation is an alternate facility of the initiating task, but the principal facility of the partner task created by the receiving system. By contrast, a terminal is always the principal facility.)

Alternate facility: Although a task may communicate directly with only one terminal, it can also establish communications with one or more remote systems. It does this by asking CICS to assign a conversation with that system to it as an alternate facility. The task owns its alternate facilities in the same way that it owns its principal facility. Ownership lasts from the point of assignment until task end or until the task releases the facility.

Not all tasks have a principal facility. Tasks that result from unsolicited input always do, by definition, but a task that comes about from automatic task initiation may or may not need one. When it does, CICS waits to initiate the task until the requested facility is available for assignment to the task.

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Last updated: Thursday, 27 June 2019