CASA_File_Set is an internal service that is invoked by the "casfile" command line utility. The "casfile" utility allows you to issue commands from the command line to manipulate files owned by CICS (for example, opening and closing files).
A long-running CASA_File_Set may indicate that a "casfile" command had been issued asking that a particular CICS file be closed, a file which cannot be closed because it is still held open by another SEP (for example, the file may be in use by another CICS transaction). If the CASA_File_Set process is hung, other transactions can also then be on hold.
The CICS file in question can possibly be identified by looking at the FCT page in ESMAC (go to ESMAC, then select "Active" under Resources, then select FCT) and its ability to display file state. That is, you will see statuses such as Enabled, Opened and Closing for the files.
If the CASA_File_Set SEP is killed ((using ESMAC or if that fails, Task Manager on Windows) to clear it, you would see other transactions that were waiting on the file suddenly begin to run.
The best option is to investigate whether and why "casfile" commands are being used to close CICS files which are still open in another SEP.
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