Asynchronous processing methods

In CICS®, asynchronous processing can be done in either of two ways:

  1. By using the interval control commands START and RETRIEVE.

    You can use the START command to schedule a transaction in a remote system in much the same way as you would in a single CICS system. This type of asynchronous processing is in effect a form of CICS function shipping, and as such, it is transparent to the application. The systems programmer determines whether the attached transaction is local or remote.

    If you use the START command for asynchronous processing, you can communicate only with systems that support the special protocol needed for function shipping; that is, CICS itself and IMS™.

    A CICS transaction that is initiated by a remotely-issued start request can use the RETRIEVE command to retrieve any data associated with the request. Data transfer is restricted to a single record passing from the initiating transaction to the transaction initiated.

  2. By using distributed transaction processing (DTP).

    This is a cross-system method and has no single-system equivalent. You can use it to initiate a transaction in a remote system that supports one of the DTP protocols.

    When you use DTP to attach a remote transaction, you also allocate a session and start a conversation. This permits you to send data directly and, if you want, to receive data from the remote transaction. Your transaction design determines the format and volume of the data you exchange. For example, you can use repeated SEND commands to pass multirecord files.

    When you have exchanged data, you terminate the conversation and quit the local transaction, leaving the remote transaction to run on independently.

    The procedure to be followed by the two transactions during the time that they are working together is determined by the application programming interface (API) for the protocol you are using. APPC is the preferred one, although you must use LUTYPE6.1 if you want to communicate with IMS. You may want to take advantage of the flexible data exchange facilities by employing this method across MRO links too.

    Whatever protocol you decide to use, you must observe the rules it imposes. However short the conversation, during the time it is in progress, the processing is synchronous. In terms of command sequencing, error recovery, and syncpointing, it is full DTP.

In both forms of asynchronous processing (and also in synchronous processing), a CICS transaction can use the EXEC CICS ASSIGN STARTCODE command to determine how it was initiated.

CICS-to-IMS communication includes a special case of the DTP method described above. Because it restricts data communication to one SEND LAST command answered by a single RECEIVE, this book refers to it elsewhere as the SEND/RECEIVE interface. The circumstances under which it is used are described in CICS-to-IMS applications.

The remainder of this section is devoted to asynchronous processing using START and RETRIEVE commands. Distributed transaction processing is described in Distributed transaction processing.

Related concepts
Overview of asynchronous processing
System programming considerations
Asynchronous processing--examples
Related tasks
Asynchronous processing using START and RETRIEVE commands
Intercommunication facilities
Application programming for asynchronous processing
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