Defining links for multiregion operation

This section describes how to define an interregion communication connection between the local CICS® system and another CICS region in the same operating system.

Note:
The external CICS interface (EXCI) uses a specialized form of MRO link, that is described in topic Defining links for use by the external CICS interface. This present section describes MRO links between CICS systems. However, most of its contents apply also to EXCI links, except where noted otherwise in topic Defining links for use by the external CICS interface.

From the point of view of the local CICS system, each session on the link is characterized as either a SEND session or a RECEIVE session. SEND sessions are used to carry an initial request from the local to the remote system and to carry any subsequent data flows associated with the initial request. Similarly, RECEIVE sessions are used to receive initial requests from the remote system.

Defining an MRO link

The definition for an MRO link is shown in Figure 38.

Note:
For reasons of clarity and conciseness, inapplicable and inessential options have been omitted from Figure 38, and from all the example definitions in this section, and no attempt has been made to mimic the layout of the CEDA DEFINE panels. For details of all RDO options, refer to the the CICS Resource Definition Guide.

You define the connection and the associated group of sessions separately. The two definitions are individual "objects" on the CICS system definition file (CSD), and they are not associated with each other until the group is installed. The following rules apply for MRO links:

As explained earlier in this section, the sysidnt is the local name for the CICS system to which the link is being defined. The netname must be the name with which the remote system logs on to the interregion SVC; that is, its applid. If you do not specify a netname, then sysidnt must satisfy these requirements.

Figure 38. Defining an MRO link
 
DEFINE
  CONNECTION(sysidnt)
  GROUP(groupname)
  NETNAME(name)
  ACCESSMETHOD(IRC|XM)
  QUEUELIMIT(NO|0-9999)
  MAXQTIME(NO|0-9999)
  INSERVICE(YES)
  ATTACHSEC(LOCAL|IDENTIFY)
  USEDFLTUSER(NO|YES)
DEFINE
  SESSIONS(csdname)
  GROUP(groupname)
  CONNECTION(sysidnt)
  PROTOCOL(LU61)
  RECEIVEPFX(prefix1)
  RECEIVECOUNT(number1)
  SENDPFX(prefix2)
  SENDCOUNT(number2)
  SESSPRIORITY(number)
  IOAREALEN(value)

On the CONNECTION definition, the QUEUELIMIT option specifies the maximum number of requests permitted to queue for free sessions to the remote system. The MAXQTIME option specifies the maximum time between a queue becoming full and it being purged because the remote system is unresponsive. Further information is given in Intersystem session queue management.

For information about the ATTACHSEC and USEDFLTUSER security options see the CICS RACF® Security Guide.

On the SESSIONS definition, you must specify the number of SEND and RECEIVE sessions that are required (at least one of each). Initial requests can never be sent on a RECEIVE session. Bear this in mind when deciding how many SEND and RECEIVE sessions you need.

You can also specify the prefixes which allow the sessions to be named. A prefix is a one-character or two-character string that is used to generate session identifiers (TRMIDNTs). If you do not specify prefixes, they default to '>' (for SEND) and '<' (for RECEIVE). It is recommended that you allow the prefixes to default, because:

For an explanation of how CICS generates names for MRO sessions, see the CICS Resource Definition Guide.

Choosing the access method for MRO

You can specify ACCESSMETHOD(XM) to select MVS™ cross-memory services for an MRO link. Cross-memory services are used only if the other end of the link also specifies cross-memory. To select the CICS Type 3 SVC for interregion communication, use ACCESSMETHOD(IRC).

The use of MVS cross-memory services reduces the number of instructions necessary to transmit messages between regions. Also, less virtual storage is required in the MVS common service area. However, cross-memory services may be less attractive from the security point of view (see the CICS RACF Security Guide).

Cross-memory services also require CICS address spaces to be nonswappable. For low-activity systems that would otherwise be eligible for address space swapping, you may prefer to accept the greater path length of the CICS interregion SVC rather than the greater real storage requirements of nonswappable address spaces.

Note:
If you are using cross-system multiregion operation (XCF/MRO), CICS selects the XCF access method dynamically--overriding the CONNECTION definition, which can specify either XM or IRC.

Figure 39 shows a typical definition for an MRO link.

Figure 39. Example of MRO link definition
DEFINE
  CONNECTION(CICB)     local name for remote system
  GROUP(groupname)     groupname of related definitions
  NETNAME(CICSB)       applid of remote system
  ACCESSMETHOD(XM)     cross-memory services
  QUEUELIMIT(NO)       if no free sessions, queue all requests
  INSERVICE(YES)
  ATTACHSEC(LOCAL)     use security of the link only
  USEDFLTUSER(NO)
DEFINE
  SESSIONS(csdname)    unique csd name
  GROUP(groupname)     same group as the connection
  CONNECTION(CICB)     related connection
  PROTOCOL(LU61)
  RECEIVEPFX(<)
  RECEIVECOUNT(5)      5 receive sessions
  SENDPFX(>)
  SENDCOUNT(3)         3 send sessions
  SESSPRIORITY(100)
  IOAREALEN(300)       minimum TIOA size for sessions

Defining compatible MRO nodes

An MRO link must be defined in both of the systems that it connects. You must ensure that the two definitions are compatible with each other. For example, if one definition specifies six sending sessions, the other definition requires six receiving sessions.

The compatibility requirements are shown in Figure 40.

Figure 40. Defining compatible MRO nodes
       CICSA                                  CICSB

DFHSIT  TYPE=CSECT                          DFHSIT  TYPE=CSECT
       ,APPLID=CICSA       1            4          ,APPLID=CICSB

DEFINE CONNECTION(CICB)    2            8   DEFINE CONNECTION(CICA)
    GROUP(PRODSYS)         3            9       GROUP(TESTSYS)
    NETNAME(CICSB)         4            1       NETNAME(CICSA)
    ACCESSMETHOD(IRC)                           ACCESSMEHOD(IRC)
    QUEUELIMIT(500)                             QUEUELIMIT(NO)
    MAXQTIME(500)                                 
    INSERVICE(YES)                              INSERVICE(YES)
                                                ATTACHSEC(LOCAL)

DEFINE SESSIONS(SESS01)                     DEFINE SESSIONS(SESS02)
    GROUP(PRODSYS)         3            9       GROUP(TESTSYS)
    CONNECTION(CICB)       2            8       CONNECTION(CICA)
    PROTOCOL(LU61)         5            5       PROTOCOL(LU61)
    RECEIVEPFX(<)                               RECEIVEPFX(<)
    RECEIVECOUNT(8)        6            7       RECEIVECOUNT(10)
    SENDPFX(>)                                  SENDPFX(>)
    SENDCOUNT(10)          7            6       SENDCOUNT(8)              

In Figure 40, related options are shown by identical numbers.

Related concepts
Introduction to link definition
Multiregion operation
Installation considerations for multiregion operation
Generic and specific applids for XRF
Related tasks
Identifying remote systems
Defining links for use by the external CICS interface
Defining APPC links
Defining logical unit type 6.1 links
Defining CICS-to-IMS LUTYPE6.1 links
Defining indirect links for transaction routing
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