If you are running CICS® with XRF, both the active and alternate CICS regions must refer to the same CICS system definition data set (that is, the CSD must be passively shared). The active and alternate CICS regions can share the same CSD even if they are running on different MVS™ images. A CICS region running with XRF=YES may also share the CSD with other CICS regions within the same MVS image. (For a definition of passively and actively shared data sets in an XRF environment, see topic Actively and passively shared data sets.)
The CSD is allocated by MVS when the CICS job step is initiated. This means the DD statements in the CICS startup job streams defining the CSD for the active and alternate CICS regions must specify DISP=SHR.
The alternate CICS region does not open the CSD during initialization, or before takeover occurs. The alternate CICS region does not even open the CSD during takeover, if the CSD was not changed at any time by the active CICS region. (For example, the CSD might have been used only to install a group list at CICS startup, and subsequently by read-only operations.) However, if you use the CEDA transaction in an active CICS region to alter resource definitions, the CSD might be opened at takeover, to perform any file backout that is necessary. To enable file backout to occur, you must define the CSD as a recoverable resource by the system initialization parameter CSDRECOV; see Defining CSD attributes.
For more information about using the CSD as a recoverable file, see Planning for backup and recovery.
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