Should we code CA TPX exits to automatically make virtual terminals (VTs) unavailable when SENSE CODEs 0857 or 0821 are encountered?

Document ID : KB000003938
Last Modified Date : 14/02/2018
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Issue:

I experienced a problem in TPX when selecting the a CICS region. Appears to be related to a previously occurring 0857 sense code error for another user on the GRP VT. My sessions were failing with 0821 errors.
We can get around the problem by manually making the VT unavailable in TPXOPER. 
Can we automatically mark VTs unavailable for 0857 and 0821 sense code errors as well?

Environment:
CA TPX for z/OS
Cause:

CA TPX will select the next available VT when attempting to start a session.

When VTAM returns a SENSE code indicating the session was not established, TPX leaves the VT marked as available for selection to start the next session request.  In some cases this can inhibit the start of new sessions if the SENSE code is not resolved for the application and the bind is consistently rejected.

Resolution:

While it is possible to use the TPXUERR1 exit to mark the VT unavailable on either 0857 or 0821 sense codes, we do not recommend this. 

These sense codes can be quite common and you may be making a much larger number of virtual terminals unavailable than is really necessary. 

When this is impacting other users, we recommend using TPXOPER to mark the VT that had the 0857 and 0821 sense codes unavailable.