[Springframework-developer] new feature proposal for SWF: asynchronous flows

[Steven D. <ste...@in...>]

Guys,

I've skyped with Keith today about a new feature I would like to see  
added to SWF: asynchronous flows (useful with AJAX technology).

Asynchronous flows are normal SWF flows that are launched  
asynchronously from a view state, typically thru AJAX calls, separate  
browser windows or frames.

An asynchronous flow is thus a flow definition that is started  
through an event id that's defined in a view state:

<view-state id="myViewState" view="myView">
     <transition on="submit" to="myActionState"/>
     <async-flow on="myAsyncEvent" flow="myAsyncFlow"/>
</view-state>

When an asynchronous flow is launched the parent flow execution is  
not transitioned. Asynchronous events are accessible through the flow  
execution id of the parent. When an asynchronous flow has been  
started a new flow execution is created and that flow execution is  
referenced to by a new flow execution id. This allows for multiple  
invocations of the same asynchronous flow by the same client. When an  
asynchronous flow execution ends it can trigger the transition of the  
parent view state (more on that later).

Otherwise asynchronous flows have the exact same behavior of normal  
SWF flows. They can have action states, view states, subflow states  
and decision states. When the view state of the parent transitions  
the flow executions of the asynchronous flows are terminated. The  
FlowAttributeMapper can be used in the same way as with sub states to  
map scope attributes between a parent flow and asynchronous flows.

A typical use case for an asynchronous flow is a mini-wizard  
contained as a widget in a view. The asynchronous flow is launched by  
an AJAX client-side HTTP component that replaces the inner html of a  
div tag with the html send back by the view states of the  
asynchronous flow (a normal JSP views for example). We probably need  
a JSP tag that renders the client-side JavaScript:

<ajax:sendRequest eventId="myMiniEvent" flowExecutionId="$ 
{flowExecutionId}"/>

This JSP tag can than be used in the views of asynchronous flows to  
mimic browser behaviour. This feature could make creating mini- 
wizards child's play. It should be noted that when the view is shown  
by the browser and the JavaScript code of the JSP tag kicks it will  
immediately launch a call to the asynchronous event. A optimization  
could be to let this JSP tag execute the call to asynchronous flow on  
the server if that's feasible.

An example of a AJAX mini-wizard is a section on the site of a travel  
booking agent where:

  - your can enter a departure and destination city on the first page
  - next a page to confirm the names of the cities
  - then a page to select the dates of departure and return and
  - then a page with a list of available flights

In this scenario it may be desirable to allow an end state of an  
asynchronous flow to trigger the transition of the view state of the  
parent, for example when a flight in the mini-wizard is clicked on.  
Since the flow execution of the mini-flow only knows its own flow  
execution id the id of the parent cannot be used. For this scenario  
we could use the name of the asynchronous flow + the name of the end  
state as the event name of a transition on the parent view state:

<view state ...>
    <async-flow on="myAsyncEvent" flow="myAsyncFlow"/>
    <transition on="myAsyncFlow.myEndState" to="myNextState"/>
</view-state>

Mini-wizards can also be displayed in a pop up browser window when  
selecting items to complete a form, for example to select products in  
an order form. These mini-wizards obviously also can be implemented  
as AJAX mini-wizards.

Any thoughts?

Steven Devijver
Senior consultant
@ Interface21
ste...@in...