CDS selection function (function code X'03')

The CDS selection function is invoked at a network node before it refers a Locate search request to a central directory server (CDS). The exit receives an ordered list of all known CDSs in the network. The first entry in the list is the CDS that VTAM® uses for the first search request, followed by the remaining CDSs within the range of the CDSREFER start option, followed by the remaining CDSs in the network. The exit has the following options:

Approve the CDS list as received
Reorder the CDS list

For any given Locate search, VTAM uses the first CDS specified in the list. If the request to the selected CDS fails, VTAM tries the request again by using the second CDS returned in the list. The remaining CDSs are not used for that request.

VTAM ensures that at least one entry is returned by the exit. The following conditions in the returned CDS list are not valid and result in VTAM issuing a FFST™ probe, deactivating the function, and continuing with the original list:

No entries are returned in the list.
Entries are added that are not in the original list.
Duplicate entries are found in the list.
Entries are found that contain corrupted data.

The user-written exit returns to VTAM with a return code:

Register 15:

Return code

X'00' (0): The CDS list has been modified.
X'04' (4): The CDS list has not been modified.

An unexpected return code is treated as return code X'04', but message IST1183I is issued to record the unexpected return code.