I have had two conversations recently about what happens if you have only a single PSAPPSRV process in a domain. One of which was on the DBA Forum.
Basically, you should always have at least two instances of any server process that has a non-zero recycle count.
It is rare to see only one PSAPPSRV process in Application Server domains that support the PIA, but customers who use the Integration Broker often have separate Application Server domains for the publication and subscription servers. These domains are often not heavily used, in which case they have been configured with just one of each server process.
This advice applies to the PSAPPSRV, PSQRYSRV, PSBRKHND, PSSUBHND, PSANALYTICSRV servers
The exceptions are
The problem occurs when the server process recycles. This occurs when the number of services handled reaches the recycle count. When the only remaining server process on a shared queue shuts down the queue is also deleted, and the advertised services are removed from the Tuxedo Bulletin Board. If a service requests arrives in the application server domain before the new server process has started, and updated the bulletin board with advertised processes, the Jolt handler (JSH) will determine that the service request is not advertised and will raise an error.
It is quite simple to demonstrate this in PeopleSoft. In my demo system, I set the recycle count on PSAPPSRV to just 10 and the minimum number of servers to 1.
It is not long until the PSAPPSRV process recycles, and you get this message in the application server log.
You can also see in the shutdown message in the TUXLOG file.
The last line is the error message from the JSH process that cannot enqueue the service request because the Application Server is down. If you suspect that you have been getting this problem look for that error message.
Hence, you should always have at least two PSAPPSRVs process, so that queue is not removed, and the other server(s) can handle requests. Of course there is a small chance that two servers could recycle at the same time, but that is very unlikely.