Overview of CICS synchronous and asynchronous processing for ISC

CICS® implements ISC using its command level (EXEC) application programming interfaces (APIs). The CICS APIs can be synchronous or asynchronous.

The synchronous API uses the SEND and RECEIVE EXEC commands. In synchronous mode in an ISC session, the session is held between the moment a request is entered and the moment a reply is returned. Thus, a direct correlation can be made between the input request and the output reply.

ISC TCP/IP connections do not support the SEND and RECEIVE EXEC commands.

The asynchronous API uses the START and RETRIEVE EXEC commands. In asynchronous mode in an ISC session, no correlation can be made between an input request and an output reply. No assumptions can be made as to the timing of the output reply or the availability of any other output for that session.

CICS supports ISC TCP/IP sessions with only the START and RETRIEVE EXEC commands.

When used within the context of a CICS application program, the EXEC commands create the data flow control protocols for synchronous or asynchronous processing that are associated with messages on the session.

The CICS implementation is quite different from IMS, in which the data flow control protocols sent with a message are created by the system and based on:
  • Whether the attributes (such as recoverability and segmentation) are defined for the message on the TRANSACT, TERMINAL, and SUBPOOL macro statements during IMS system definition or on an ETO logon descriptor
  • For VTAM® connections, whether the characteristics are defined in the bind parameters
  • For TCP/IP connections, whether the characteristics are defined in the CICS IPCONN and TCPIPSERVICE resource definitions
  • Whether the message is a reply that is returned on the same session as an associated request or on a different session than the associated request
  • Whether the message is unsolicited asynchronous output

When an asynchronous message is sent from CICS to IMS, the sending transaction might terminate. The returned reply can be processed by a newly initiated CICS transaction according to parameters in the function management headers returned on the message by IMS.

The following figure illustrates the concept of synchronous SEND and RECEIVE processing.

Figure 1. Synchronous processing with SEND and RECEIVE
Begin figure. The same application in CICS that sends the message to an IMS application program receives the reply message. End figure.

The following figure illustrates the concept of asynchronous START and RETRIEVE processing.

Figure 2. Asynchronous processing with START and RETRIEVE
Begin figure. A different application than the application in CICS that sends the message to an IMS application program receives the reply message. End figure.