Requirements for XCF/MRO

Communication across MVS™ images using XCF/MRO requires the MVS images to be joined in a sysplex.

A sysplex consists of multiple MVS images, coupled together by hardware elements and software services. In a sysplex, MVS images provide a platform of basic services that multisystem applications like CICS® can exploit. As an installation’s workload grows, additional MVS images can be added to the sysplex to enable the installation to meet the needs of the greater workload.

Usually, a specific function (one or more modules/routines) of the MVS application subsystem (such as CICS) is joined as a member (a member resides on one MVS image in the sysplex), and a set of related members is the group (a group can span one or more of the MVS images in the sysplex). A group is a complete logical entity in the sysplex. To use XCF to communicate in a sysplex, each participating CICS region joins an XCF group as a member, using services provided by DFHIRP.

Sysplex hardware and software requirements

For definitive information about installing and managing MVS systems in a sysplex, see the MVS/ESA Setting Up a Sysplex manual, GC28-1449.

Generating XCF/MRO support

  1. Depending on the versions of CICS installed in the MVS images participating in XCF/MRO, the versions of DFHIRP installed in the images can be different. For all the MVS images containing CICS systems to be linked, ensure that the version of DFHIRP in the extended link pack area (ELPA) is at the required level. The DFHIRP module should be that from the most current CICS release in the image, or higher.
  2. Ensure that each CICS APPLID is unique within the sysplex. You must do this even if the level of MVS/ESA in some MVS images is earlier than 5.1, which is the minimum level for XCF/MRO support. This is because CICS regions always issue the IXCJOIN macro to join the CICS XCF group when IRC is opened, regardless of the level of XCF in the MVS image.
  3. Ensure that the value of the MAXMEMBER MVS parameter, used to define the XCF couple datasets, is high enough to allow all your CICS regions to join the CICS XCF group. The maximum size of any XCF group within a sysplex is limited by this value. The theoretical maximum size of any XCF group is Start of change2047 membersEnd of change, which is therefore also the maximum number of CICS regions that can participate in XCF/MRO in a single sysplex.

    External CICS interface (EXCI) users that use an XCF/MRO link will also join the CICS XCF group. You should therefore set the value of MAXMEMBER high enough to allow all CICS regions (with IRC support) and EXCI XCF/MRO users to join the CICS XCF group concurrently.

    To list the CICS regions and EXCI users in the CICS XCF group, use the MVS DISPLAY command. The name of the CICS group is always DFHIR000, so you could use the command:

    DISPLAY XCF,GROUP,DFHIR000,ALL
    Attention:

    Do not rely on the default value of MAXMEMBER, which may be too low to allow all your CICS regions and EXCI users to join the CICS XCF group.

    Likewise, do not set a value much larger than you need, because this will result in large couple data sets for XCF. The larger the data set, the longer it will take to locate entries.

    We suggest that you make the value of MAXMEMBER 10-15 greater than the combined number of CICS regions and EXCI users.

Each CICS region joins the CICS XCF group when it logs on to DFHIRP. Its member name is its APPLID (NETNAME) used for MRO partners. The group name for CICS is always DFHIR000.

At connect time, CICS invokes the IXCQUERY macro to determine whether the CICS region being connected to resides in the same MVS image. If it does, CICS uses IRC or XM as the MRO access method, as defined in the connection definition. If the partner resides in a different MVS image, and XCF is at the MVS/ESA 5.1 level or later, CICS uses XCF as the access method, regardless of the access method defined in the connection definition.

Related concepts
Installation considerations for intersystem communication
Related tasks
Steps to install MRO
Further steps
Defining links for multiregion operation
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