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From Sylvain Wallez <sylv...@apache.org>
Subject Re: error handling
Date Tue, 08 Jun 2004 14:15:33 GMT
Torsten Curdt wrote:

>> Sorry to jump in late.
>
>
> Thanks for jumping in at all :)
>
>> There's a technical difficulty, however, as internal requests are 
>> handled differently than external ones when it comes to handling 
>> errors occuring during pipeline execution (not during pipeline 
>> building).
>> - pipelines for external requests are executed as soon as the 
>> pipeline is ended, i.e. in the map:serialize statement, hence under 
>> control of the treeprocessor
>> - pipelines for internal requests are executed when getInputStream() 
>> or toSAX() is called on the "cocoon:" source, out of the control of 
>> the treeprocessor.
>
>
> Ok, but isn't the "cocoon:" source going back to treeprocessor after
> all? Or does it just setup the pipelines? I thought every request is
> goint through the TP.
>
> ...or who is passing the request to the pipeline(s)?


You have to consider the two distinct phases that occur when a request 
is handled:
- building the pipeline (executing sitemap statements) : matchers, 
action and flowscript are called, generator, transformers and serializer 
are added to the pipeline.
- processing the pipeline. The generator's generate() method is called, 
which starts the processing chain.

Building the pipeline is done by the TP. Execution of statements stops 
when encountering a "terminal" statement, i.e. a serialize, read, 
redirect or flowscript call.

Processing the pipeline is different for internal and external requests:
- for external requests, the TP starts the processing within the 
terminal statement (e.g. <serialize>). The enclosing <handle-errors> can 
then be handled correctly.
- for internal requests, the TP does *not* start the processing, but 
gives back a filled pipeline object to the SitemapSource. The 
SitemapSource starts the processing when the content of the source is 
needed. This means that pipeline processing occurs out of the 
<handle-errors> enclosing the <serialize>, and that errors occuring in 
that phase cannot be handled (at least with the current architecture).

>> So we can add add handle-errors="always|external|internal" and 
>> "?cocoon:handle-errors=true", but it will handle errors occuring 
>> during the _building_ of the pipeline, and not during its _execution_.
>
>
> That's not what I am after. It does not help for error handling on 
> aggregation. We should aim for the execution time.


It all depends where the errors occur in the aggregated pipelines: is it 
when building the pipeline or when executing it?

>> Handling errors occuring during the execution of internal requests 
>> would require some not so innocent changes in the pipeline machinery 
>> [3].
>
>
> Well, IMHO this is major flaw and should be tackled.
> No matter if we need to change something or not. I think the pipeline 
> machinery is so deep core that probably not to many people would 
> notice anyway.
>
> Don't know... but maybe it would be possible to move the error 
> handling further down to the pipeline level? If we are able to add 
> that to the Abstract... classes even less people would be affected. 
> But I have no clue if that's possible at all.
>
> Especially if we don't want to mix concerns.


A solution that I proposed in [1] is that for internal processing, the 
TP not only builds the pipeline, but also returns a pointer to the 
error-handling statements wrapped in a Processor. That way, the 
SitemapSource can call the appropriate error handling statements.

>> But we can of course go one step at a time and start by catching 
>> pipeline build-time exceptions.
>
>
> Not sure if that's worth the effort.
>
> I'd propose to change what needs to be changed.


Sure :-)

Sylvain

[1] http://marc.theaimsgroup.com/?l=xml-cocoon-dev&m=107876029119774&w=2

-- 
Sylvain Wallez                                  Anyware Technologies
http://www.apache.org/~sylvain           http://www.anyware-tech.com
{ XML, Java, Cocoon, OpenSource }*{ Training, Consulting, Projects }


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