Mercury seriously needs a feature that will allow limiting it to only 1 instance running at a time.
It may need some adjustable options, though, to suit different users' preferences. For example, I would prefer than in addition to it trying to determine if other instances are currently executing, it would also create a "flag" file somewhere on the mailbox directory path, which indicates that somewhere an instance of Mercury is running. The error message would be different depending on whether it found actual executing instances, or only found the file. (I.e. telling the user s/he cannot run more than one instance, versus saying that multiple instances might be running or perhaps Mercury was shut down improperly the last time.)
Personally, I would prefer to have to manually delete the file and try the startup again, instead of Mercury offering me the option to override and start up anyway. The warning dialog should refer me to the online help... and the online help should remind me how to find the file so I can delete it.
(An experienced system administrator doesn't need this... but junior admins or assistants do. Because they have to do an extra step or two, they'll have time to think about whether they know what is actually happening and what to do about it. They will think it through or will ask someone, instead of just "clicking through" to get the job started.)
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This request is borne out of two recent experiences!
One: my son recently accidentally started 2 copies of Mercury without it being noticed for about a half day. The contention between the two instances ended up clobbering some messages, hopefully none of them important.
Two, a little while back, a colleague had a similar problem at work (but with an entirely different program, not Mercury or Pegasus). That other program used a "flag" file as I suggested above, but his junior co-worker just ignored a warning message and told the program to start up anyway. The problem was, the program was already running, but on a different application server (both app servers used the same file server). It was instructive to both of us that the junior person simply read the dialog and thought to himself "well of course I want the program to start" and clicked the override. It wasn't until half an hour later that his brain engaged and he asked his supervisor.