Connecting ISC sessions from CICS to IMS

You can connect CICS® ISC sessions to IMS in two distinct ways. You can initiate the session explicitly or the CICS operator can initiate the session through a command.

You can connect CICS ISC sessions to IMS in one of the following ways:

  • You can initiate the session explicitly by specifying AUTOCONNECT YES on the DEFINE CONNECTION command in the CICS system definition (CSD) utility, or by specifying CONNECT=AUTO in the DFHTCT TYPE=TERMINAL program. When CICS is started, it attempts to establish all sessions for which AUTOCONNECT is specified.

    In order for session to be initiated, IMS must be active when CICS is started.

  • The CICS operator can initiate a session by entering the following command:

    CEMT SET TERMINAL(termid) [ACQUIRED | COLDACQ | RELEASED]

    where termid is the four-character session name (defined on the DEFINE SESSIONS command in the CSD utility), or the TRMIDNT in the DFHTCT TYPE=TERMINAL program. The following describes the effects of the keywords for the CEMT SET TERMINAL command:

    ACQUIRED
    Specifies normal resynchronization with the IMS half-session. CICS is in session with the logical unit represented by the terminal.
    COLDACQ
    Specifies that no resynchronization is done.

    COLDACQ is a special form of ACQUIRED, in which no resynchronization is required. If the previous session ended abnormally, using COLDACQ overrides CICS integrity control, which can lead to integrity problems. Also, check the CSMT log for an activity keypoint (similar to an IMS checkpoint) after the restart of a session following a CICS failure. If there is no activity keypoint, reissue the CEMT SET TERMINAL(termid) COLDACQ command after the next emergency restart.

    RELEASED
    Specifies that CICS is not in session with the logical unit represented by the terminal.

    Specifying RELEASED for a terminal that has an active session causes the session to be terminated. Running transactions are allowed to finish unless you also specify PURGE or FORCEPURGE.

    The display area of the CEMT [INQUIRE | SET] TERMINAL command shows the status of the CICS half-session. If the session initiates successfully, the status changes from REL (released) to ACQ (acquired). If the session does not initiate successfully, CICS writes an error message to the transient data destination CSMT.

  • An application program can initiate a session implicitly by using the ALLOCATE command. A program can use the ALLOCATE command only for the SEND/RECEIVE interface.
    Recommendation: Use the ALLOCATE SYSID form of the command because it allows CICS to select an available session.

    If a session is not immediately available, control is returned to the application program if either of the following occur:

    • The program issues a HANDLE CONDITION command.
    • The program specifies NOQUEUE on the ALLOCATE command.
    Otherwise, the command is queued until a session becomes available.

    Each of the following conditions causes a session to be not immediately available.

    • All sessions to the specified system (or the specified session) are in use.
    • The only available sessions are not bound.
    • The only available sessions are contention losers.
  • CICS automatic task initiation (ATI) can initiate a session.

To determine the primary and secondary RU sizes, CICS uses the values for the RECEIVESIZE and SENDSIZE parameters on the DEFINE CONNECTION | SESSION command in the CSD utility or the values for the RUSIZE and BUFFER parameters on the DFHTCT TYPE=TERMINAL | SYSTEM program.