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From Reinhard Pötz <reinh...@apache.org>
Subject [C3] Pipeline component event types
Date Tue, 13 Jan 2009 09:38:04 GMT
Grzegorz Kossakowski wrote:
> Jakob Spörk pisze:
>> Hello,
> 
> Hello Jakob,
> 
>> I just want to give my thoughts to unified pipeline and data
>> conversion topic. In my opinion, the pipeline can't do the data
>> conversion, because it has no information about how to do this.
>> Let's take a simple example: We have a pipeline processing XML
>> documents that describe images. The first components process this
>> xml data while the rest of the components do operations on the
>> actual image. Now is the question, who will transform the xml data
>> to image data in the middle of the pipeline?
> 
> I agree with you that pipeline implementation should not handle data
> conversion because there is no generic way to handle it.
> 
> Now I would like to answer your question: it should be another
> /pipeline component/ that handles data conversion.
> 
>> I believe the pipeline cannot do this, because it simply do not
>> know how to transform, because that’s a custom operation. You would
>> need a component that is on the one hand a XML consumer and on the
>> other hand an image producer. Providing some automatic data
>> conversions directly in the pipeline may help developers that need
>> exactly these default cases but I believe it would be harder for
>> people requiring custom data conversions (and that are most of the
>> cases).
>> 
>> The actual architecture allows to fit any components into the
>> pipeline, and only the components itself have to know if they can
>> work with their predecessor or the component following them. That
>> allow most flexibility when thinking about any possible
>> conversions. If a pipeline should do this, you would need
>> "plug-ins" for the pipeline that are registered and allow the 
>> pipeline to do the conversions. But then, it is the responsibility
>> of the developer to register the right conversion plug-ins and you
>> would have get new problems if a pipeline requires two different
>> converters from the same to the same data type because the pipeline
>> cannot have automatically the information which converter to use in
>> which situation.
> 
> I believe that these problems could be addressed by... compiler. In
> my opinion, pipelines should be type-safe which basically means that
> for a given pipeline fragment you know what it expects on the input
> and what kind of output it gives to you. The same goes for
> components. This eliminates "flexibility" of having a component that
> accepts more than one kind of input or more than one kind of output.
> I believe that having more than one output or one input only adds to 
> complexity and does not solve any problem.
> 
> If component was going to accept more than one kind of input how a
> user could know the list of accepted inputs? I guess the only way to
> find out would be checking source and looking for all "instanceof"
> statements in its code.
> 
> I would prefer situation when components have well-defined type of
> input and output and if you one to combine components for which
> input-output pairs do not match you should add converters as
> intermediate components.
> 
> I've been thinking about generic but at the same time type-safe
> pipelines for some time. I've designed them on paper and everything
> looked quite promising. Then moved to implementation of my ideas and
> got rather disappointing result which can be seen here: 
> http://github.com/gkossakowski/cocoonpipelines/tree/master
> 
> The most interesting files are: 
> http://github.com/gkossakowski/cocoonpipelines/tree/master/src/org/apache/cocoon/pipeline/Pipeline.java
> (generic and type-safe pipeline interface)
> 
> http://github.com/gkossakowski/cocoonpipelines/tree/master/src/org/apache/cocoon/pipeline/PipelineComponent.java
>  (generic and type-safe component def.)
> 
> http://github.com/gkossakowski/cocoonpipelines/tree/master/src/org/apache/cocoon/pipeline/demo/RunPipeline.java
>  (shows how to use that thing)
> 
>> The only thing cocoon can help here with is to provide as much
>> "standard" converters for use as possible, but it is still the
>> responsibility of the developer to use the right ones.
> 
> I think Cocoon could define much better, type-safe Pipeline API but
> we are in unfortunate situation that we are using language that makes
> it extremely hard to express this kind of generic solutions.
> 
> Of course, I would like to be proven that I'm wrong and Java is
> powerful enough to let us express our ideas and solve our problems.
> Actually, the whole idea of pipeline is not a rocket science as it's,
> in essence, just ordinary function composition. The only unique
> property of pipelines I can see is that we want to access to
> _partial_ results of pipeline execution so we can make it streamable.
> 
> 
> This become more a brain-dump than a real answer to your e-mail
> Jakob, but I hope you (and others) have got my point.

I don't believe that pipelines should contain components that support
different event types or that we event need components that have
different input and output events.

If you want to mix your components (e.g. using a SAX component in a
pipeline full of StAX components), you should put your 'alien' component
into a wrapper.

-- 
Reinhard Pötz                           Managing Director, {Indoqa} GmbH
                         http://www.indoqa.com/en/people/reinhard.poetz/

Member of the Apache Software Foundation
Apache Cocoon Committer, PMC member                  reinhard@apache.org
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