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From "Mike Sparr -" <>
Subject RE: [SCXML] Concurrent users
Date Tue, 13 Dec 2005 23:34:32 GMT
Hi Rahul,

I suspect the null context value is our implementation but will look
into that tonight.  If we cannot figure out, I do appreciate your
offering another set of eyes.  My 'helper' had originally stored the
executor per client type and not per client, hence the concurrency
issue... I think we got it sorted:

	private SCXMLExecutor getExec(String clientIdentifier){
		    try {
	    	exec = new SCXMLExecutor(engine,new SimpleDispatcher(),
new SimpleErrorReporter());
	        }catch (ModelException me){
		return exec;

Thanks and I will look at those use cases.  I had figured you must've
designed for more than a single threaded use.. After all, what if I want
to control all 10 of my microwaves at once!  :-)


-----Original Message-----
From: Rahul Akolkar [] 
Sent: Tuesday, December 13, 2005 12:39 PM
To: Jakarta Commons Users List;
Subject: Re: [SCXML] Concurrent users

On 12/12/05, Mike Sparr - <> wrote:
> Rahul,
> Your code worked (from your personal snapshot) fine - thanks.

OK, thanks for the update :-)

> One issue
> we're now facing is handling concurrent users from multiple clients.  
> My goal is to use one scxml doc to manage app flow for all users.

Indeed, there will usually be only one SCXML document for a particular
application task (or flow, or dialog) -- which could be of arbitrary
granularity -- yet the per user (or per session) "instances" of the
state machine must be able to execute independently.

> It looks
> like there is one context and it's shared, so if we trigger an event, 
> it will affect any user.

Each user should have a separate Commons SCXML engine (or more
precisely, an instance of the org.apache.commons.scxml.SCXMLExecutor
class) assigned, and the events must be channeled to the appropriate
engine for the same user/session.

>  We have created a HashMap (in memory) to store
> key/handle to client for sending response, but are having troubles 
> dealing with concurrent users and their state.
> For example:
> Web request (state 1, event 3)
> IM request (state 2, event 1)
> Can the engine handle that or do you know how best to implement?  My 
> thought was to store state and last event per user, then navigate to 
> that somehow for their requests.  I assume there must be a better way?

We have a couple of published Commons SCXML usecases here:

Both the usecases are examples within a servlet container environment
(so similar to what you're attempting), and the SCXMLExecutor instance
gets created when needed, then cached and retrieved from the user's

To summarize:

 * A SCXML document describes the state machine for a task/flow/dialog
in an application.

 * The org.apache.commons.scxml.model.SCXML class is the representation
of the document using the Commons SCXML Java object model.

 * An appropriately configured SCXMLExecutor instance creates a "live"
state machine (or engine) upon which external events may be fired.

 * SCXMLExecutor instances must be created per user/session and cached
throughout the life of the task/flow/dialog.


> Thanks,
> Mike

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