topic Re: Question about parallel gateway in Alfresco Archive

Question about parallel gateway

vladi — Mon, 23 Aug 2010 09:33:40 GMT

Hi,i have tryed a very simple process with parallel gateway and two user task an each path:<process id="concurency"> <startEvent id="start" /> <parallelGateway id="fork" /> <userTask name="task_1" id="task_1" /> <parallelGateway id="join" /> <

Re: Question about parallel gateway

tombaeyens — Tue, 24 Aug 2010 06:44:59 GMT

process concurrency != multithreaded program execution

as you state correctly, activiti engine uses the db for synchronization. a process is executed in transactional steps. each step is calculated with a single thread.

because this question is asked frequently, i wrote the answer in a faq: http://activiti.org/faq.html#WhatIsTheDifferenceBetweenProcessConcurrencyAndJavaConcurrency

let me know if this helps you further.

Re: Question about parallel gateway

vladi — Tue, 24 Aug 2010 07:54:12 GMT

Thanks for this link, but it unfortunately does not clarify the situation. The described use case is exactly what i want to do - i have a process that will be driven by asynchronous events.

If i working with human user tasks the synchronisation over the database is certainly good enough. But in my case the events will be produced by a measuring system and probability for such clash is significantly higher.

The problem i see is that the database can not afford the synchronisation in real time in principle… What is needed is activiti engine synchronize the joinig parallel paths over an java monitor object or a semaphore. Is it realistic in any way? Can i as user of api influence it?

Regards, Vladimir

Re: Question about parallel gateway

tombaeyens — Tue, 24 Aug 2010 10:08:25 GMT

What is needed is activiti engine synchronize the joinig parallel paths over an java monitor object or a semaphore. Is it realistic in any way?

Nope. That doesn't work in a cluster. Databases have decades research on optimisations to handle the concurrent access. They are much better then anything we could provide in that respect.

Your process and software design can also greatly reduce the number of potential optimistic locking failures. I don't see yet how leveraging the DB for purposes that it is good at (isolation and concurrency) can be bad.

Re: Question about parallel gateway

ruediger_hain — Tue, 24 Aug 2010 12:09:05 GMT

The test case above shows the process execution is stuck at the merging gateway when both tasks are completed at the same time.
It seems there is no exception thrown so we can't detect there is a problem with process execution.
There is no rollback done on one of the task completions either, otherwise we would still have one of the user tasks in the list, right?
@Tom: do you think this is supposed to work with the current version of Activiti (and the test case is wrong or we are having a configuration issue etc.) ?

Re: Question about parallel gateway

tombaeyens — Wed, 25 Aug 2010 15:40:39 GMT

Try remodelling your test code without the workers and make it run with only 1 single test thread. Then you should be able get more insight or otherwise better explain your problem here.

Re: Question about parallel gateway

vladi — Thu, 26 Aug 2010 13:11:44 GMT

Imagine, you have two usertasks are completed by two different users at the same time. The testcase above simulate this situation. Or two asynchronous messages arrives at the same. You can't running it in your application in a single thread, it can be two different web sessions or the like.

I would expect a proper Exception signaling a conflict will be thrown by activiti, in order to i can retry the operation.