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From Stefano Mazzocchi <stef...@apache.org>
Subject Re: [RT] Cocoon subcomponent object model (was: Re: is cocoon symmetry a holy grail?)
Date Tue, 19 Feb 2002 23:49:00 GMT
Sorry, took me a while to respond.

Nicola Ken Barozzi wrote:
> 
> From: "Vadim Gritsenko" <vadim.gritsenko@verizon.net>
> 
> > I have another one, it provides different functionality but it features
> > similar approach. As I don't have a name for this (multiplexer?), here
> > is the diagram:
> >
> >                   - pipeline1 -
> >                  /              \
> > request -> A -> X - pipeline2 - X -> C -> response
> >                  \              /
> >                   - pipelineN -
> >
> > Explanation:
> > 1. Request goes in
> > 2. Pipeline is being constructed from A, X, C
> > 3. SAX events passed from the A to X, where they are dispatched (same as
> > separator) to several other pipelines
> > 4. SAX events passed from these events reassembled into the one SAX
> > stream by the same instance of X component
> > 5. Result passed down the original pipeline to the C
> > 6. C spits out the response
> 
> Oh my, I've seen this in ApacheCon two more that a year ago, we knew it was
> going to come out again! ;-)
> 
> The first comment that come to me is that IMHO, to have better performance
> you need to have good control over what is happening, and that leads to
> KISS. The whole concept of making pipelines split, recombine and branch
> could make it difficult to maintain control.

I agree here.
 
> But the concept is intriguing. IMHO it could be transformed in another
> concept, a sub-component object model.
> 
> We have been seeing the picture from a sitemap POV, but never talked about
> helping the developer in writing the components themselves.
> 
> So, since it's 3:26 and I can't get sleep, here's my first RT.

:)
 
> ===============================
> Cocoon Sub-Component Object Model
> ===============================
> 
> Abstract
> -------------
> 
> This RT describes a finer grained object model for Cocoon that is meant
> to attain a better separation of concerns and usability.
> 
> Description
> -----------------
> 
> Cocoon has a macro object model based on the pipeline metaphore.
> 
> Each Cocoon "object" is a pipeline component and can be of three major
> kinds:
> 1.Generator: initiates the XML pipeline by converting generic data into XML.
> 2.Transformer: filters the XML events.
> 3. Serializer converts the resulting XML into something useful for the
> client.
> 
> The pipelines are defined in a sitemap that specifies order, parameters and
> condition of pipeline components.
> 
> This componentization is useful because it enforces separation of concerns
> between content providers, graphic-layout designers, developers and site
> administrators.
> 
> Cocoon1 made life easy for the first two and quite hard for the last, who
> had the data he is responsible for scattered in all three kinds of
> components. The sitemap of Cocoon2 changed this and put things where they
> belong.
> 
> My opinion is that developers are not yet taken correctly into account.
> While the other three have a componentization which is sufficient for their
> part of work, developers suffer for the lack of it. Usually a developer has
> to write a component, and doesn't have a (sub) component model to deal with.

??? I disagree, you can write avalon-aware sitemap components by hand
(and many people do just as easy as they write servlets... or even
better).
 
> Ok, it's not really true, there are XSPs.
> But in many respects there are not sufficient:
> XSPs are hard to write
> XSPs mix (declarative) XML and (procedural) Java in an unmaintainable and
> undebuggable tangle
> XSPs cannot aid writing transformers
> XSPs must have their main tag
> XSPs do not automagically scale well (no automantic pooling or brokering)
> XSPs have slooow startup and are not good for dynamic pages that change
> often
> XSPs are a nightmare to debug (just try ;-) )
> XSPs have the 64k limit
> XSP taglibs are hard to understand, write and maintain

I agree on all these limitations, but if you don't varying XML content
in your pages (say it pretty much all comes from data stores), I'd
suggest you to write your generators directly, where you have the nice
component model that avalon gives you.
 
> Also, Cocoon components do not have scope and filter all events coming
> in (security: I don't want sensitive tags passing in a transformer that is
> useful but not completely known).

please, let's get real here: I *strongly* doubt you'll ever use a filter
in your pipeline that you don't trust. Security is ok, but at this
granularity becomes a nightmare (and a serious performance limitation)
 
> Cocoon doesn't have context scoping for session or global values.
> 
> As you can see these remarks are not in a small number, but come all from
> simple shortcomings of Cocoon IMHO:
> - The coexistence between Java and XML is a key problem.
> - The current component model is too coarse grained to help pipeline
> component writers.
> 
> A finer grained object model could also have the notion of context
> variables.
> 
> These have nothing to do with and do not endanger necessarily the existence
> of:
> - XSP syntax.
> - Current level of object abstractions for other roles.
> 
> How can we solve this?
> Here are some possibility
> 
> A----------------------------
> First we have to change slightly the notion of cocoon pipeline components
> introducing scope.
> Pipeline components need not access <all> SAX events but only what pertains
> them. This also means that the pipeline coulde be evaluated eventually in
> parallel
> fashion, improving scalability in heavy processor intensive or high latency
> pages.
> 
> For example let's say that we have this XML:
> <page>
> <longquery name="account"/>
> <query name="username"/>
> <page>
> Let's say that in another file (the developer's sitemap) is written that
> query tags must be processed by the foo.sql.QueryTransformer and the
> longquery tags by acme.sql.BankTransformer.
> As SAX events come into action the start page tag is directly sent to the
> serializer.
> Then the acme.sql.BankTransformer is given only the longquery tag and starts
> processing in a non-blocking fashion.
> This means that SAX events can continue and parallely
> foo.sql.QueryTransformer can start processing his tag.
> Now the pipeline has to wait for the first transformer to finish because
> embedded tags link page cannot be processen in non-blocking fashion. When
> they finish their output events are outputted in order and finally the last
> page tag.
> 
> As you can see if there are transformers that take longer to perform (also
> because of latency of DBs and likes) they can be performed this way in
> non-blocking fashion, speeding up total response time.

And this should be KISS?
 
> B----------------------------
> A global context-aware object broker could also be inserted in the scheme.
> This doesn't really change the framework, it's just a useful addition.

Don't forget we can still use Alberto's X:Forge for this.
 
> C----------------------------
> Now let's explain how a finer-grained object model can be devised.
> First of all it must be capable of specifying a pipeline component as a sum
> of smaller components possibly only by writing XML described "glue".
> I's like:
> pipeline component : pipeline = smaller component : pipeline component
> Which basically means that these smaller components are a second level of
> indirection with regerds to the pipeline.
> 
> What guided the specification of the pipeline components?
> The fact that they had to
> - Interface XML with other streams.
> - Transform XML.
> 
> Basically They had to
> - detokenize
> - make-change grammar
> - retokenize
> 
> So it's all about interfacing generic streams to XML so to be able to
> transform them the XML way with Transformers.
> In our case it's about interfacing XML to Java to be able to transform it
> with Java Objects (beans, EJB, etc.).
> This means that we could:
> 1 Change XML tokens with something meaningful to Java: variables and data
> structures.
> 2 Call Java methods on them to have results.
> 3 Retransform Java data structures into XML tokens.
> 
> The great thing is that phase 1 is usually quite long and cumbersome to
> write but is essentially the same code over and over, the usual "if"s in the
> SAX event handlers.
> I think that a basic set of "(De)Tokenizers" can be used in 95% of cases. A
> very used one would for example store a variable with the same name of the
> tag it's in when it has certain parents.
> Phase 2 is where the real "coding" takes place.
> (omissis)
> Phase 3 is easy to write, and it's the only part of XSPs which really works.
> 
> As you can see XSPs don't have phase 1 reusable and are cumbersome with
> phase 2. This is because the mix them into a same phase, putting Java
> code directly on the page with the <xsp:logic> tag.
> 
> Here the separation is done by relegating the interaction of Java and XML to
> the simple and reusable contract of (De)Tokenizers.
> In this way the coding can be done in Java and simply mapped to XML with
> reusable components.
> 
> Seeing this globally the pipeline should work this way:
> - (Generation) Tokenize and make SAX events from streams
> - Filter events and dispatch to Transformers
> - For each new Transformer (in parallel if necessary and requested)
> - de-tokenize events and convert to Java.
> - Call methods
> - Retokenize
> - Serialize SAX events to stream
> 
> With this mail is also an illustrative image.
> 
> Stefano, could you please lend me your asbestos garments, you don't need
> them anymore AFAIK ;-)

What you propose is similar to X:Forge and to DXML that Ricardo was
working on (I say 'was' because I can't reach him anymore :( but was
much simpler:

I'd rather attach X:Forge to Cocoon (at the generation level) than
having to write something so complex at the pipeline level.

I think the tokenize/detokenize part are really far from my view of
keeping it simple and go into a deep mess.

-- 
Stefano Mazzocchi      One must still have chaos in oneself to be
                          able to give birth to a dancing star.
<stefano@apache.org>                             Friedrich Nietzsche
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