I want to know about to show confirmation form on any error or completion of a particular activity.
Suppose, in write to data store entity, i want to show the alert box on the success updation/insertion of the data.
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The simplest way to do exactly what you specify is to populate PV's using the onSuccess output and onError outputs that come pre-defined in the Write to Datastore node. Then use an XOR gateway on the process model to send flow to one of two User Input tasks. Make your forms to show the error or success, plus any data you got from Write to Datastore you want to show. That and a big friendly submit button the user can use to close the form when they've looked it over enough.
The onSuccess message probably just closes the process when they submit that form. You are free to do whatever you want to handle the error when they submit that form, or just end the process too. Another helpful thing is a timer exception on the confirmation screens, so that it eventually submits on behalf of the user, should they navigate away or do something else like that.
OK then scratch a lot of what I said.
You can either have a confirmation screen and utilize the savedValues output to show info to the users, or if it does break, the process will be paused by exception. You can have a parent process that only waits so long before it does something else and terminates killing the stalled process. That something else can include a "Something went horribly wrong" message to the user.