Defining multiple links to an IMS system

You can define more than one intersystem link between a CICS® and an IMS system by defining two or more connections (systems), with their associated session definitions, having the same NETNAME but different SYSIDs.

Although all the system definitions resolve to the same NETNAME, and therefore to the same IMS system, using a SYSID name in CICS causes CICS to allocate a session from the link with the specified SYSIDNT.

Recommendation: Define up to three links (that is, groups of sessions) between a CICS and an IMS system, depending upon the application requirements of your installation:

A group of sessions for CICS-initiated distributed transaction processing (synchronous processing).
CICS applications that use the SEND/RECEIVE interface can use the SYSIDNT of this group to allocate a session to the remote system. The session is held (“busy”) until the conversation is terminated.
A group of sessions for CICS-initiated asynchronous processing.
CICS applications that use the START command can name the SYSIDNT of this group. CICS uses the first “nonbusy” session to ship the start request.
IMS sends a positive response to CICS as soon as it has queued the start request, so that the session is in use for a relatively short period. Consequently, the first session in the group shows the heaviest usage, and the frequency of usage decreases towards the last session in the group.
A group of sessions for IMS-initiated asynchronous processing.
This group is also useful as part of the solution to a performance problem that can arise with CICS-initiated asynchronous processing. An IMS transaction that is initiated as a result of a START command shipped on a particular session uses the same session to ship its “reply” START command to CICS. For the reasons given in (2), the CICS START command is probably shipped on the busiest session, and, because CICS is the contention winner, the replies from IMS can create a backlog while waiting for a chance to use the session.
However, facilities exist in IMS for a transaction to alter its default output session, and a switch to a session in this third group can reduce backlog problems.

The following series of tables provide an example of defining three groups of sessions on an IMS node. The tables contain examples of defining the session groups in both CICS Resource Definition Online (RDO) format and CICS macro format.

Table 1. CICS system initialization parameters
RDO definition	Macro-level definition
`DFHSIT TYPE=CSECT ,SYSIDNT=CICL ,APPLID=SYSCICS`	`DFHSIT TYPE=CSECT ,SYSIDNT=CICL ,APPLID=SYSCICS`

RDO definition Macro-level definition

Table 2. CICS-initiated distributed transaction processing
RDO definition	Macro-level definition
`DEFINE CONNECTION(IMSA) ACCESSMETHOD(VTAM) NETNAME(SYSIMS) DEFINE SESSIONS(csdname) PROTOCOL(LU61) SESSNAME(IMS1) CONNECTION(IMSA) NETNAMEQ(DTP1) DEFINE SESSIONS(csdname) ⋮`	`DFHTCT TYPE=SYSTEM ,ACCMETH=VTAM ,SYSIDNT=IMSA ,NETNAME=SYSIMS DFHTCT TYPE=TERMINAL ,TRMTYPE=LUTYPE6 ,TRMIDNT=IMSA ,SYSIDNT=IMSA ,NETNAMQ=DTP1 DFHTCT TYPE=TERMINAL ⋮`

DEFINE CONNECTION(IMSA)
        ACCESSMETHOD(VTAM)
        NETNAME(SYSIMS)


DEFINE SESSIONS(csdname)
        PROTOCOL(LU61)
        SESSNAME(IMS1)
        CONNECTION(IMSA)
        NETNAMEQ(DTP1)

DEFINE SESSIONS(csdname)
   ⋮

DFHTCT TYPE=SYSTEM
       ,ACCMETH=VTAM
       ,SYSIDNT=IMSA
       ,NETNAME=SYSIMS

DFHTCT TYPE=TERMINAL
       ,TRMTYPE=LUTYPE6
       ,TRMIDNT=IMSA
       ,SYSIDNT=IMSA
       ,NETNAMQ=DTP1

DFHTCT TYPE=TERMINAL
   ⋮

RDO definition Macro-level definition

Table 3. CICS-initiated asynchronous processing
RDO definition	Macro-level definition
`DEFINE CONNECTION(IMSB) ACCESSMETHOD(VTAM) NETNAME(SYSIMS) DEFINE SESSIONS(csdname) PROTOCOL(LU61) SESSNAME(IMS1) CONNECTION(IMSB) NETNAMEQ(ASP1) DEFINE SESSIONS(csdname) ⋮`	`DFHTCT TYPE=SYSTEM ,ACCMETH=VTAM ,SYSIDNT=IMSB ,NETNAME=SYSIMS DFHTCT TYPE=TERMINAL ,TRMTYPE=LUTYPE6 ,TRMIDNT=IMSA ,SYSIDNT=IMSA ,NETNAMQ=DTP1 DFHTCT TYPE=TERMINAL ⋮`

DEFINE CONNECTION(IMSB)
        ACCESSMETHOD(VTAM)
        NETNAME(SYSIMS)


DEFINE SESSIONS(csdname)
        PROTOCOL(LU61)
        SESSNAME(IMS1)
        CONNECTION(IMSB)
        NETNAMEQ(ASP1)

DEFINE SESSIONS(csdname)
   ⋮

DFHTCT TYPE=SYSTEM
       ,ACCMETH=VTAM
       ,SYSIDNT=IMSB
       ,NETNAME=SYSIMS

DFHTCT TYPE=TERMINAL
       ,TRMTYPE=LUTYPE6
       ,TRMIDNT=IMSA
       ,SYSIDNT=IMSA
       ,NETNAMQ=DTP1

DFHTCT TYPE=TERMINAL
   ⋮

RDO definition Macro-level definition

Table 4. IMS-initiated asynchronous processing
RDO definition	Macro-level definition
`DEFINE CONNECTION(IMSC) ACCESSMETHOD(VTAM) NETNAME(SYSIMS) DEFINE SESSIONS(csdname) PROTOCOL(LU61) SESSNAME(IMS1) CONNECTION(IMSC) NETNAMEQ(IST1) DEFINE SESSIONS(csdname) ⋮`	`DFHTCT TYPE=SYSTEM ,ACCMETH=VTAM ,SYSIDNT=IMSC ,NETNAME=SYSIMS DFHTCT TYPE=TERMINAL ,TRMTYPE=LUTYPE6 ,TRMIDNT=IMS1 ,SYSIDNT=IMSC ,NETNAMQ=IST1 DFHTCT TYPE=TERMINAL ⋮`

DEFINE CONNECTION(IMSC)
        ACCESSMETHOD(VTAM)
        NETNAME(SYSIMS)


DEFINE SESSIONS(csdname)
        PROTOCOL(LU61)
        SESSNAME(IMS1)
        CONNECTION(IMSC)
        NETNAMEQ(IST1)

DEFINE SESSIONS(csdname)
   ⋮

DFHTCT TYPE=SYSTEM
       ,ACCMETH=VTAM
       ,SYSIDNT=IMSC
       ,NETNAME=SYSIMS

DFHTCT TYPE=TERMINAL
       ,TRMTYPE=LUTYPE6
       ,TRMIDNT=IMS1
       ,SYSIDNT=IMSC
       ,NETNAMQ=IST1

DFHTCT TYPE=TERMINAL
   ⋮