Hi Jeff,
> Interesting - sounds like its modeled after Group of 4, Decorator
> Pattern... These event listeners - are they passive or do they muck
> with the result stream?
Well, it doesn't correspond directly with the Decorator pattern (although
there's certainly some similarity). It's really modeled much more closely
after event models like you'd find in Swing, although specifically tailored
for the req/resp paradigm of Http.
Event listeners can be whatever you want them to be. They just get notified
that something happened; its up to the implementater to determine how to
respond. Generally, a ControlEvent handler will muck with the model and then
fire a ViewEvent, which actually renders the view.
I didn't mention that all control events that come through the
ApplicationGateway servlet (that's the HTTP entry point) _must_ have a
ViewEvent generated at some point in the cycle (after all, HTTP dicates that
for every request there must be a response, right?).
Just as the ControlEvents implement Polymorphic interface, the ViewEvents
implement an interface called Exceptional. When a view event is fired, (and
if there is no view event fired it will use a default view event) it tells
the dispatcher that someone _must_ handle it; if not, this represents an
exceptional circumstance, and as such the event dispatcher should fire the
parent event, just like with exceptions. This causes view events to fire up
the chain and guarantees that eventually someone will handle the request to
generate a view. This ensures that a response always gets sent to the client
and handles situations where a programmer might have forgotten to handle
certain view events, etc.
For more details, you really should look at the Barracuda site (I don't want
to getting any further off topic here, since this is the Struts list after
all). The gist of both of my emails is basically that:
a) an event model might provide a good foundation for a workflow engine
b) Barracuda has a robust event model implementation
c) it could probably be integrated with Struts
Hope that helps...
Christian
------------------------------------------------
Christian Cryder [[EMAIL PROTECTED]]
Barracuda - MVC Component Framework for Webapps
http://barracuda.enhydra.org
------------------------------------------------
"What a great time to be a Geek"
> -----Original Message-----
> From: Jeff Trent [mailto:[EMAIL PROTECTED]]
> Sent: Wednesday, June 06, 2001 10:09 AM
> To: [EMAIL PROTECTED]
> Subject: Re: Work flow RFC
>
>
> Interesting - sounds like its modeled after Group of 4, Decorator
> Pattern...
> These event listeners - are they passive or do they muck with the result
> stream?
>
> -jeff
>
>
> ----- Original Message -----
> From: "Christian Cryder" <[EMAIL PROTECTED]>
> To: <[EMAIL PROTECTED]>
> Sent: Wednesday, June 06, 2001 11:37 AM
> Subject: RE: Work flow RFC
>
>
> > > The lead on the Barracuda project is a subscriber to the Struts list!
> > > Christian Cryder ([EMAIL PROTECTED]). He was the one who said he
> > > likes to "lurk" and spoke about security
> >
> > Yes, I lurk. Too much to on my plate to be more actively involved ;-)
> >
> > As didge pointed out:
> >
> > > From Barracuda's Scope Summary (see,
> > > http://barracuda.enhydra.org/Barracuda/docs/scope_summary.html#2.4),
> > > Barracuda's scope explicitly excludes application flow control.
> >
> > This is correct. Barracuda does not force any particular workflow on a
> > system. That said, it was designed from the get-go so as not to preclude
> it
> > either...I know we've already had at least one person build an
> XML driven
> > workflow engine on top of Barracuda (I don't think it was
> public though).
> >
> > Basically, what Barracuda gives you is this:
> >
> > 1. Event model - converts HTTP Requests into true, first class event
> > objects, which can then be dispatched to all interested listeners.
> > Additional events can be fired, etc.
> >
> > 2. Component model - a suite of strongly typed, Swing-like MVC
> components
> > that can be used to manipulate DOM objects, instead of using
> the DOM apis.
> >
> > 3. Form mapping & Validation framework - Automatically convert HTTP Req
> > params to first class objects and apply any number of
> validation rules to
> > them.
> >
> > 4. Localization services - automatically localize *ML templates
> (based on
> > property files) and create XMLC objects from them; ability to load the
> > proper template based on a Locale (much like ResourceBundles,
> it finds the
> > closest match)
> >
> > Each of these sub-systems are self contained and can be used
> independently
> > from one another. In the case of work flow, its the event model portion
> that
> > would prove particularly useful. Here's how it could be applied (I think
> > this is described in the Barracuda docs somewhere but I
> couldn't locate it
> > this morning, so here's a brief summary)
> >
> > In Barracuda, there are two types of events: ControlEvents and
> ViewEvents
> > (which nicely map to the Http Req/Resp paradigm). When the user does
> > something on the client that results in an HttpRequest, that
> request will
> > get translated to a ControlEvent, which then gets delivered to
> listeners.
> > However, there's more to the event dispatch process than might
> first meet
> > the eye.
> >
> > Control events implement a nifty little interface called
> Polymorphic. This
> > tells the dispatcher "hey, before you dispatch me, dispatch all
> my parent
> > events too (since I am after all an _instance_ of those events
> as well").
> So
> > if the event hierarchy looks something like this:
> >
> > AuthorizedEvent
> > HREvent
> > FireEmployee
> > HireEmployee
> >
> > If the client generates a HireEmployee event, this system will
> first fire
> an
> > AuthorizedEvent, then an HREvent, then the actual HireEmployee
> event. This
> > allows me to install a listener on AuthorizedEvent or HREvent to first
> > _validate_ that the client actually has the necessary authority
> to invoke
> > the event they're trying to fire. If they don't, I can wipe the event
> queue
> > and completely redirect the application flow simply by firing a
> different
> > event. For instance, if they're not already logged in, I could fire a
> > GetLoginEvent to redirect them to the login screen.
> >
> > This process can be applied to workflow quite easily. Basically all you
> have
> > to do to define a workflow system is create a mechanism for indicating
> which
> > events can be fired for a given user state. This could take a number of
> > different forms: you might have a mechanism that controls event firing
> based
> > on user roles, or a statechart engine, or an XML schema approach...there
> are
> > a tons of different options (which btw, is why we didn't just build a
> > workflow engine directly into Barracuda...there are two many
> ways of doing
> > workflow, so we didn't want to lock anyone into one particular
> approach).
> >
> > At any rate, hopefully this better explains how "No, Barracuda does not
> > provide a workflow engine but Yes, its event model mechanism is
> very well
> > suited to that problem domain".
> >
> > The other question is "Could Barracuda be used to glue a workflow engine
> > into Struts". I think the answer is yes, but I'm not a Struts guru so I
> > won't try and offer any details. Basically, the Barracuda event model
> > provides Model 2 flow control...your event handlers would have
> to be done
> > with Barracuda style code, but I don't see any reason why your
> view event
> > handlers couldn't just redirect to the appropriate JSP page.
> >
> > Ok, I'll shut up now ;-)
> >
> > Christian
> > (See why I just lurk? Its too time consuming when I get involved in
> > discussions! ;-)
> > ------------------------------------------------
> > Christian Cryder [[EMAIL PROTECTED]]
> > Barracuda - MVC Component Framework for Webapps
> > http://barracuda.enhydra.org
> > ------------------------------------------------
> > "What a great time to be a Geek"
> >
> >
> > > -----Original Message-----
> > > From: Jonathan [mailto:[EMAIL PROTECTED]]
> > > Sent: Tuesday, June 05, 2001 11:10 AM
> > > To: [EMAIL PROTECTED]
> > > Subject: Re: Work flow RFC
> > >
> > >
> > > The lead on the Barracuda project is a subscriber to the Struts list!
> > > Christian Cryder ([EMAIL PROTECTED])
> > > He was the one who said he likes to "lurk" and spoke about security on
> > > Monday May 7, "RE: Potential Security Flaw in Struts MVC"
> > >
> > >
> > > ----- Original Message -----
> > > From: "Ted Husted" <[EMAIL PROTECTED]>
> > > To: <[EMAIL PROTECTED]>
> > > Sent: Tuesday, June 05, 2001 12:48 PM
> > > Subject: Re: Work flow RFC
> > >
> > >
> > > > I'm still fuzzy on the mechanism we would use to represent
> and enforce
> > > > the workflow. There has been mention of things like command tokens,
> but
> > > > are any code samples available.
> > > >
> > > > Can anyone explain how Barracuda implements their workflow, and
> whether
> > > > it could be mapped to the Struts Actions/ActionMappings model?
> > > >
> > > > I believe Barracuda and Struts are complementary in most
> ways, and it
> > > > would be in our best interest to adopt and adapt rather than
> > > > roll-our-own, if feasible.
> > > >
> > > > Jonathan wrote:
> > > > >
> > > > > Again, Ive got to say look at the Barracuda project.
> They have one
> of
> > > these
> > > > > gui configurers. Check it out at
> > > > > http://barracuda.enhydra.org/Barracuda/GetBConfig.event
> > > >
> > >
> >
> >
>
