Make a process singleton

Question

I have this case where the process must be fired when a specific CDT is created but only once. 
 Is there an easy way to make sure of that? 
 I though of setting up through the start node some kind of expression to be fired when a certain query returns a count higher than 0 or anything alike. That doesn't seem possible and the other problem is it seems I can't enforce the creation of the process only once at all. Lets say in my case two CDT are created at "the same time" I'll end up for sure with two process threads? 
 Or perhaps I'm approaching my problem the wrong way. 
 If you have any ideas please share, thanks!

Josh · Accepted Answer

Why is it a requirement to only have one process instance? Why not just launch the process, check for the lock in the database, continue with the process if there is no lock, but terminate the process instance if there is a lock?

sergeiz · Answer

Appian processes are not alike to operation system threads. There is no synchronization mechanism and you have to do it manually on external object like CDT. You can put a check query at your start form to prevent starting a new process every time. But the result for two simultaneous calls can be the same because no transaction yet updated the process. In this case you should have an action that updates the database (@Version annotation), so one update will pass, and others will fail. 
 
Another way I can think of is to use a process with a timer. Any user can click "start" which will update boolean flag in a CDT and this timer can run every few seconds starting another process if the flag is on and using own flag (process variable) to prevent duplication. From user perspective there will be some pause between "start" and process start.