[rules-users] Flows, Subflows and fault.

[Swindells, Thomas]

I've just started to use Drools Flow in anger and I'm really struggling to understand the correct way to use it and think I am missing something.

This is what I want to achieve:
The app code executes a StartProcess command and waits for the process to return (1 main process per session, 1 session per thread).
The process either succeeds, or it fails with a result.
A process could be complicated so I want to split it up into a parent process calling subprocesses.
I want to minimize boilerplate code (particularly in the ruleflow files) and keep the workflows as simple and maintainable as possible.

It sounds simple enough and doesn't seem to be particularly unusual.

Issue 1: startProcess is semi-synchronous. It starts the process, runs through as many nodes as it can until it gets to a non-synchronous task (rules, long lived customer work item etc) at which point it returns - or it returns when the entire process has been completed.
Challenge 1a: How to detect when the process has finished and use it in a synchronous manner.
Solution: A ProcessEventListener which completes a countdown latch when teh process has completed.
Challenge 1b: Subprocesses also trigger process events
Solution: Have the latch track the first process that started (the parent) and only return when that process completes.

Issue 2: Drools Flow is littered with error checking code which just prints to System.err and either carrys on or just stop. Eg if a work item handler isn't registered the process just stalls with a message.
Solution: Make sure code is perfectly written and doesn't have any bugs... hope that it doesn't happen in production ... complain on mailing list and add TODO to raise issue.

Issue 3: Fault nodes seem to be the correct way to say that something has gone wrong and the process should terminate.
Challenge: How does the calling code actually work out what the fault was and get the variable? The guide talks very very briefly about exception handlers but from what I can understand this is a way for the process to define what to do in this conditions. I want the process to be aborted and the calling application to be informed of this and the details.
Solution: Add an event listener which detects when a fault node is being entered and extracts the details - seems like a big hack surely there is a better way.

Issue 4: If a child process executes a fault node it is aborted. The parent process is set to wait for completion and has the child node as not being independent so I would expect that the parent process should also be marked as aborted.
Challenge: The parent process isn't marked as complete - it still has a status of Executing. However the process is stalled. Looking at subprocessNodeInstance internalTrigger (line 117) if the node returns a state of completed then trigger completed is called, otherwise it just adds a process listener - which will never be called as the process has already aborted. The parent process just stalls left in the state of active forever.
Sollution: ?? Hack my fault listener to work round the issue and update the parent process, make the processEventListener assert that if a child process is aborted then we should return? Ask on the mailing list to get me out of a blind alley and point me to the correct drools way of doing it...

Thomas

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