Work is started in CICS®--that is, tasks are initiated--in
one of two ways:
- From unsolicited input
- 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.
Notes:
- You can specify a terminal destination other than your principal
facility in a SEND command if the destination is under TCAM control,
an apparent exception to this rule. This is possible because communications
with TCAM terminals are always queued. Thus your task does not write
directly to the destination terminal, but instead writes to a queue
that will be delivered to it subsequently by TCAM (see Using TCAM) . BMS routing, described in Message routing,
is another form of indirect access to other terminals by queues.
In CICS TS 3.1, local TCAM terminals are
not supported. The only TCAM terminals supported are remote terminals
connected to a pre-CICS TS 3.1 terminal-owning region by the DCB (not
ACB) interface of TCAM.
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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