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From "Vadim Gritsenko" <vadim.gritse...@verizon.net>
Subject RE: [RT] Resource Garbage Collection
Date Mon, 29 Oct 2001 21:30:39 GMT
Carsten,

Could the behavior of RequestModelComponent achieved by using object
model:

	if(environment.getObjectModel().get("statistics")==null)
		environment.getObjectModel().put("statistics", new X())

I used (something like) this when implemented profiler...

PS Not that I do not agree with this proposal, I do see necessity for
this interface.

Vadim

> -----Original Message-----
> From: Carsten Ziegeler [mailto:cziegeler@s-und-n.de]
> Sent: Monday, October 29, 2001 10:02 AM
> To: cocoon-dev@xml.apache.org
> Subject: RE: [RT] Resource Garbage Collection
> 
> Yes,
> 
> garbage collection of resources seems to be a must-have.
> Four weeks ago we discussed with Stefano a similar approach
> which would fit in my eyes directly into this concept
> (See http://marc.theaimsgroup.com/?t=100197769000007&w=2&r=1
> for the full thread).
> 
> The concept summarized: A new marker interface named
RequestModelComponent
> (with regards to the SitemapModelComponent, although the name is not
> the point here) which marks an Avalon Component.
> 
>   public interface RequestModelComponent extends Component {
>      void setup(SourceResolver resolver, Map objectModel) throws
>      ProcessingException;
>   }
> 
> The behaviour for this would be:
> - The first time during a request processing a component marked with
>   this interface is looked up, a new instance is created (or got from
>   the pool) and the setup method is called. This object is stored
>   somewhere so that the ComponentManager has access to it for later
>   requests.
> - The second, third.. time during the same request this component is
>   looked up, it is get from the store and the setup() method is
>   not looked up
> - Every time a release is called on this object, nothing really
happens
> - When the request processing is finished, the ComponentManager
>   must really release all stored objects.
> 
> So actually objects marked in this way are more or less ThreadSafe
> but per request exists a different instance.
> Although this is not a "real world example" and might not be
appropriate
> modelled, lets imagine a component for statistics. In order to work
> correctly
> this component needs access to the objectModel (to get the request
object
> and
> from that the current requestURI) and it "logs" how many components
are used
> during the processing of one request.
> Imagine a simple pipeline with a generator, transformer and
serializer.
> The generator looksup the "Statistics" component called SCA, it is
created
> from scratch (or taken from a pool) and the setup() method is called.
> The generator now sends his information to SCA. Now the transformer
> looks up a "Statistics" component to send his information. If now a
new
> instance SCB would be created, this instance would not know about the
> information
> previously send to SCA, so we need the same instance SCA here.
> As this instance is already setup there is no need to call this method
> again.
> 
> If in the same time a different request is processed parallel which
uses
> the same pipeline (or any other, it doesn't really matter), the
generator
> looks up its own "Statistics" component, which of course can not be
SCA as
> it is used in the other pipeline.
> It is a "new" instance which is used exclusivly in this pipeline.
> 
> So summarizing it:
> - a ThreadSafe component is shared by all components  in all requests.
> - a "normal" component is not shared at all, each component gets its
own
>  instance.
> - a RequestModelComponent is shared by all components in the *same*
request.
> 
> Now, implementing this results in a similar behaviour as your
proposal.
> All RequestModelComponents are "collected" during a request by the
> ComponentManager
> and release (= garbage collected) at the end of the request.
> 
> Carsten
> 
> 
> Berin Loritsch wrote:
> >
> > Developing with Cocoon is a fun and rewarding experience,
> > especially because it is hard write non-scalable webapps
> > on the platform.  Alot of this has to do with the limitations
> > on the developer, and forcing them to Do the Right Thing(TM).
> > Most of the enhancments have come out of a real need.  I
> > believe we have one more.
> >
> > The action we have has an interface that is simple, for a
> > reason.  We want to be able to encourage ThreadSafe programming.
> > I don't ever want to lose that.  However, we have to come
> > to grips with the fact that not everything follows this
> > ideal world.
> >
> > One example was that someone wanted to use an Action to obtain
> > a resource from a database (XML:DB to be exact), but needed
> > a way to release the resources at the end of the pipeline.
> > I think this is a reasonable request, but it does raise some
> > questions.
> >
> > * Which pipeline?  One master pipeline that makes use of
> >   aggregation or the "cocoon://" protocol is made up of
> >   several shorter pipelines spliced together.
> >
> > * How do we handle action sets?
> >
> > * Should things like this be handled by a different mechanism?
> >
> > So, how do we come up with a model that works?  The basic
> > problem restated is that we need a way of holding on to
> > resources for the length of the request.  After the request is
> > over, the resource is released.  Logically, this makes sense,
> > but putting something like this together is difficult in practice.
> >
> > The issue actually stems from our greatest asset: components.
> > The very Component Based Design that Cocoon is built with affects
> > how things work together.  The easiest design to implement is
> > one that acts on immediate need.  As an analogy, I will use
> > memory management.
> >
> > In assembly and machine language, the programmer is required to
> > manually set up the work areas, and perform all work within those
> > areas.  This is equivalent to Pre-Cocoon days where your logic,
> > content, and display information were all mixed together in one
> > resource.  It is the way you work with JSP, ASP, and ColdFusion.
> > We all know this is bad, and don't want to go back to those
> > awful working conditions if we can help it.
> >
> > With the advent of higher level languages like C and C++, we
> > were given the blessing and curse of heap managed memory allocation.
> > It was great because now we didn't need to set asside work areas
> > ahead of time.  Sure there was a performance penalty for allowing
> > the OS to set up your memmory allocation and management, but it
> > was worth it.  If you were careful, you would allocate and
deallocate
> > your memmory as you needed to.  The problem came when you had
> > to have some allocated memmory persist between scopes.  It is
> > easy to remember to deallocate memmory when you use it all in the
> > same method.  But what if you needed to hand that memmory reference
> > to another method--one that you may not have written?  If the
> > memmory was deallocated by that method your were SOL (S**t Out of
> > Luck--a military acronym).  This is where Cocoon is now.  If
> > a Component is asked for (looked up), you have to be careful to
> > release it--otherwise it *may* cause resource leaks.
> >
> > Then cam Java and a slew of other languages that favor garbage
> > collection to explicit deallocation.  You have no direct control
> > over when a memmory area is going to be reclaimed, but you do know
> > it won't happen while you are still referencing it.  That means
> > you can pass the memmory reference around your program without
> > fear of it being deallocated.  This is where Cocoon needs to go.
> > In other words, Cocoon needs a mechanism to 1 have a reference
> > to a resource (Component or Object) last for an entire request.
> > At the end of the request, the resources are cleaned up.
> >
> > So how do we design something like this without screwing our heads?
> > The first is to narrowly define the scope and access of these
> > objects.  The second is to mark these objects with an interface
> > so they can be handled properly.  This does require some repurposing
> > of the RequestContext I proposed recently.  While many people are
> > against using the RequestContext as a "communications" area, it is
> > perfectly suitable for passing object references for the life of
> > one request.
> >
> > In order for the RequestContext to behave properly, and enforce the
> > contracts of mapping resouces to specific names, the put() method
> > must be write-once.  That means you can never overwrite a resource.
> > It also means that the developer needs to be smart about the
resource
> > names he uses.  We should not impose a strict naming scheme, but
offer
> > one that should be scalable--especially considering the Cocoon
WebApp
> > proposal by Stefano.
> >
> > One proposed naming scheme would be much like URLs:
> >
> > http://infoplanning.com/schematic-access/user
> >
> > Another would be a simpler form of the above version:
> >
> > infoplanning.com:schematic-access/user
> >
> > Still another solution would be equivalent to the
> > java pakage/classname solution:
> >
> > com.infoplanning.schematic-access.user
> >
> > Using replacement variables, the resources would be named with
> > these variables:
> >
> > ${company.domain}:${project.name}/${object.name}
> >
> > This ensures that the probability of you having a unique name
> > within a system is high.  The developer may find that adding
> > one more level to the heirarchy might make sense (i.e. a
> > ${module.name}), but it isn't necessary for most projects.
> >
> > Next, we need to specify where the resource collection happens.
> > In order to maintain the simplest and most portable solution,
> > I would propose the RequestContext be managed by the Cocoon object.
> > That way, it is handled the same no matter what the environment
> > is.
> >
> > Lastly, we need to provide a marker interface for resources that
> > must be properly released when the request is over.  Objects like
> > Strings, and non-pooled objects can simply have their references
> > nullified.  Other objects like the resources mentioned above
> > need a Reclaimable interface with a single method, reclaim().
> > For instance:
> >
> > public interface Reclaimable {
> >     void reclaim();
> > }
> >
> > The Reclaimable interface tells the resource to clean up after
itself.
> > It is up to the implementor of the Reclaimable interface to decide
on
> > the exact mechanism.  The usual method would be to store a reference
> > to it's pool or component manager and call put() or release() on it.
> > This way, Cocoon is not altered too much architecturally, and the
> > ComponentManager/Selectors are not altered from their purpose.
Finally,
> > it also means that encouraging the use of RequestContext for these
types
> > of things encourages a consistent design and method of cleanup.
> >
> >
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> 
> 
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