Application-related concepts

This topic describes some application-related concepts to establish a common frame of reference for both CICS® and IMS users.

Subsystem design: direct-control versus queued
CICS is a direct-control subsystem, while IMS is a queued subsystem. That is, CICS accepts data entered from a terminal, and as a result, invokes the appropriate application program to process that data.
Synchronous and asynchronous processing on ISC VTAM links
Message transmission can be synchronous or asynchronous between the terminal entering the message and the receiving subsystem. Messages can also be either synchronous or asynchronous with respect to the ISC session.
Principal and alternate facility
The functions of CICS synchronous EXEC terminal control commands (such as SEND, RECEIVE, CONVERSE) are exercised against a principal or an alternate facility.
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.
Sending IMS commands from CICS
IMS commands can be entered from any terminal authorized to use them.
Sync points
ISC session protocols define RQ*2 requests and their associated responses (DR2 or exception DR2) as sync point requests and responses.

Parent topic: How IMS and CICS use the ISC interface