CICS versus IMS conversation mode

In IMS, conversation mode is an attribute of a transaction in which the conversation is carried on through the transfer of a scratchpad area (SPA) and an associated message between the terminal and transaction message queues.

Each transfer of information to the transaction message queue and back to the terminal queue results in a sync point, and causes the commitment of resources and the release of locked resources. Also, all of the steps of an IMS conversation must occur entirely within a bracket. EB cannot be sent at any intermediate point in the conversation, because the receipt of EB causes IMS to discard the output message, terminate the conversation, and invoke the conversational abnormal termination exit. During a conversation, if a primary resource name (PRN) parameter is supplied in the input function management header, it is ignored, because the transaction code is carried in the SPA.

In CICS®, a conversation is a series of interactions within a bracket between a terminal and an application. End-bracket occurs upon termination of the application. CICS also permits a pseudo-conversation, which is a sequence of transactions between a terminal and an application. Each transaction has one terminal input and one terminal reply within its own bracket. Either of these types of conversations can interact with IMS on the ISC session between the requests and replies by using the EXEC CONVERSE command. A message with end-bracket being sent to the terminal by CICS also causes an end-bracket message to be returned on the ISC session to IMS, if IMS has not previously sent an end-bracket status on the reply.

The differences in the way in which conversations are defined in CICS and IMS have the following implications for ISC:

A terminal connected to CICS can only be held synchronously with the ISC session and IMS transaction processing if the CICS transaction is conversational in the CICS sense.
If more than one input and output is to occur on the ISC session within a single bracket, the CICS transaction can interact with an IMS transaction that is defined as conversational. However, in the event of a session failure, the conversation might not be restartable.
If only one input and output is to occur on the ISC session within a single bracket, the CICS transaction can interact with an IMS transaction defined as response mode. A series of transactions that make up a CICS pseudo-conversation can interact on an ISC session with a series of IMS transactions defined as response mode.

IMS conversational mode requires that IMS terminate the conversation by sending the last reply with RQD2,EB. Intermediate conversational messages are sent RQE2,CD. The CICS application can decide to terminate the conversation normally by issuing SYNCPOINT or RETURN. CICS can then terminate the conversation by sending LUSTATUS RQD2,EB in response to the last conversational output from IMS. In this case, the LUSTATUS acts as the acknowledgment for the RQE2,CD sent by IMS and closes that logical unit of work. The RQD2,EB causes IMS to respond with DR2 to close the bracket, dequeue the output message, and notify the Conversational Abnormal Termination exit routine that the conversation has been terminated normally by the remote program.