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Re: [Springframework-developer] new feature proposal for SWF: asynchronous flows
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From: Steven D. <ste...@in...> - 2005-09-21 10:13:42
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Erwin,
On 21 Sep 2005, at 10:58, Erwin Vervaet wrote:
> We already have asynchronous flows: you can start any number of top-
> level flow executions concurrently.
>
> So what exactly is the benefit of the additional support you
> describe? Why can't you just use a subflow or a second independent
> top-level flow for those 'mini wizards' you describe? I'm not sure
> all the added complexity really pays off...
>
Subflows are not the same thing since they require a transition of
the parent flow execution. Either each step in a mini-wizard would
then require a separate view template for the entire page, not just
the section. Or javascript code makes the call and triggers the
transition of the parent flow execution. This would mean all other
possible transitions in other sections of the page become unavailable.
Top-level flows that are executed via javascript solve this problem
since the parent or original flow execution is not transitioned.
However, no scope data can be copied between the original flow and
request scope and the new flow scope. Making the transition on the
parent could be achieved through javascript but that's leaves the
developer with boilerplate javascript code.
> Stuff like "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." and "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" is a bit strange IMHO: if you want that
> kind of stuff, just use subflows and a flow execution listener.
>
Again, I don't agree. To me these semantics make sense from an AJAX
perspective. Subflows are great to synchronously execute page flows.
AJAX however is about asynchronous execution.
> Also, it seems that terminating all started asynchronous flows when
> the 'parent' transitions could lead to a number of issues. What if
> a client spawned multiple ansynchronous flows and one triggers a
> transition in the parent. As a result all those 'mini wizards'
> would become unusable...
On the other hand, if you call top-level flows asynchronously thru
AJAX they become stale once the original flow execution transitions.
About the issues that could arise by terminating asynchronous flows:
they are real and in my opinion unavoidable but should be dealt with
by the developer. Once you start using AJAX on your website thru the
functionality I propose you may want to use it in multiple levels
meaning other asynchronous flow executions will not be terminated
just like that. It's a design choice and AJAX is a very popular one.
I think it's up to the developers to decide how they want to use SWF.
>
> Erwin
Steven
>
>
>> 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?
>>
>
>
>
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