Stefano Mazzocchi wrote:
Sylvain Wallez wrote:The discussions around Stefano's "Cocoon blocks version 1.1" showed the need for pipelines to provide not only resources, but also services, identified by their URI.
This document defines this concept of "pipeline service", which, as we will see, consists in using pipelines as sitemap components (generator, transformer and serializer). It is separated from the blocks design document since pipeline services can be used without blocks, even if they will be mostly useful in that context.
Thanks for writing this Sylvain, it's very helpful.
[snip]
In short, what you are describing is a model for pipeline extension: one pipeline 'extends' another pipeline by calling it and cocoon transparently adapts it by removing the parts which aren't useful.
I do see the problem of cocoon transparently removing pipeline components, but it feels more like OOP inheritance where the system transparently ignores the overloaded method.
Yes, the parallell is a little streched, but I think that the fact that nobody ever complained about the fact that serializers are ignored on cocoon: subpipeline calls makes me think that the concept is not so unfriendly.
I'm not sure we can say that a pipeline "extends" another one, as this happens at a particular part of the calling pipeline, and we could even have a pipeline composed of several other pipelines. This is more related to defining a function, but even then the parallel is stretched.
This should be more considered as defining composite sitemap components : for example, the "cocoon" (better name than "pipeline") generator is composed of a generator and a set of transformers.
Moreover, the fact of allowing *complete* pipelines is also tremendously helpful for debugging purposes and for block-reuse: instead of having passive pipeline fragments, we'll have the ability to write a pipeline, test it and then extend it with another.
Note that a pipeline might still be required to be called directly.
For example:
<match pattern="*.xml">
<generate type="file" src="{1}.xml"/>
<serialize type="xml"/>
</match>
<match pattern="*.html">
<generate type="pipeline" src="{1}.xml"/>
<transform src="xml2html.xslt"/>
<serialize type="html"/>
</match>
<match pattern="*.pdf">
<generate type="pipeline" src="{1}.xml"/>
<transform src="xml2fo.xslt"/>
<serialize type="fo2pdf"/>
</match>
The nice thing would the above is that if the generation of the *.xml resource is changed, this is automatically inherited by all the pipelines that build on it.
Good example : this shows that a pipeline can be used, depending on the context, either as a complete pipeline, or as a service (a generator in the above sample).
Note that one might also want to connect to "views" of those pipeline... so for example why don't we add the ability to specify which "view" of the pipeline we should start from?
<map:generate type="pipeline" src="/whatever/resource" view="content"/>
What do you think think?
Thinking twice is certainly better than once ;-)
We can already can choose the view for a "cocoon:" resource by means of the standard "cocoon-view" parameter. So I would avoid adding a new attribute and simply write
<map:generate type="cocoon" src="/whatever/resource?cocoon-view=content"/>
Sylvain
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