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From Sylvain Wallez <sylv...@apache.org>
Subject Re: [C3] StAX research reveiled!
Date Sun, 28 Dec 2008 20:47:11 GMT
Steven Dolg wrote:
> Sylvain Wallez schrieb:
>> <snip/>
>>
>> Steven Dolg wrote:
>>> Basically you're providing a buffer between every pair of components 
>>> and fill it as needed.
>>
>> Yes. Now this buffer will always contain a very limited number of 
>> events, corresponding to the result of processing an amount of input 
>> data that is convenient to process at once to avoid complex state 
>> management (e.g. an <i18:text> tag with all its children). And so 
>> most often, this buffer will contain just one event.
>>
>> Think of it as being just a bridge between the writer view used by a 
>> producer and the reader view used by its consumer. These are in my 
>> opinion the most convenient views to write StAX components.
>>
>>> But you need to implement both XMLStreamWriter and XMLStreamReader 
>>> and optimize that for any possible thing a transformer might do.
>>> In order to buffer all the data from the components you will have to 
>>> create some objects as well - I guess you will end up with something 
>>> like the XMLEvent and maintaining a list of them in the StaxFIFO.
>>> That's why I think an efficient (as in faster than the Event API)  
>>> implementation of the StaxFIFO is difficult to make.
>>
>> It's certainly less trivial than maitaining a list of events, but 
>> should be doable quite efficiently by using an int FIFO (to store 
>> event types and attribute counts) and a String FIFO (for everything 
>> else). I'll try find a couple of hours to prototype this.
>>
>>> On the other hand I do think that the cursor API is quite a bit 
>>> harder to use.
>>> As stated in the Javadoc of XMLStreamReader it is the lowest level 
>>> for reading XML data - which usually means more logic in the code 
>>> using the API and more knowledge in the head of the developer 
>>> reading/writing the code is required.
>>> So I second Andreas' statement that we will sacrifice simplicity for 
>>> (a small amount of ?) performance.
>>
>> I understand your point, even if I don't totally agree :-) Now it 
>> should be mentioned that if even with events, my proposal still 
>> stands: just replace XMLStream{Reader|Writer} with 
>> XMLEvent{Reader|Writer}.
>>
>>> The other thing is that - at least the way you suggested - we would 
>>> need a special implementation of the Pipeline interface.
>>> That is something that compromises the intention behind having a 
>>> Pipeline API.
>>> Right now we can use the new StAX components and simply put them 
>>> into any of the Pipeline implementations we already have.
>>> Sacrificing this is completely out of the question IMO.
>>
>> Actually, I'm wondering if wanting a single API is not wishful 
>> thinking and will in the end lead to something that is overly 
>> abstract and hence difficult to understand and use, or where 
>> underlying implementations will leak in the high-level abstraction.
>>
>> There is already some impedence mismatch appearing between pull and 
>> push in the code:
>> - a StAXGenerator has to call initiatePullProcessing() on its 
>> consumer, which in turn will have to call it on it's own consumer, 
>> etc until we reach the Finisher that will finally start pulling 
>> events. This moves a responsibility that belongs to the pipeline down 
>> to its components.
>
> Well I don't see the problem with that.
> From the pipeline's point of view those are normal components just 
> like all the other.
> The pipeline was never intended to "care" about the internals of the 
> components - so why bothering that the StAXGenerator calls 
> "initiatePullProcessing" on its consumer instead of calling some other 
> method like e.g. "startDocument".

Hmm... the fact that every implementation has to copy/paste the exact 
same call to initiatePullProcessing (or has extend a common abstract 
class that does it) because the pipeline expects processing to be 
started on the first component is a sign of a design problem to me.

Some responsibilities of the pipeline creep into its components because 
the pipeline is too abstract, or because there's an intermediate 
adaptation layer that's missing.

>> - an AbstractStAXProducer only accepts a StAXConsumer, defeating the 
>> idea of a unified pipeline implementation that will accept everything.
>
> The idea was to have pipelines being capable of processing virtually 
> any data.
> But that is not the same as combining components in an arbitrary way, 
> e.g. there is no sense in linking a FileGenerator with an (not yet 
> existing) ImageTransformer based on Java's Imaging API.
>
> The components must be "compatible" - that is they must understand the 
> data they exchange with each other.
> We may however provide some adapters/converters to make certain 
> "types" of components compatible, e.g. SAX <--> StAX.
>
>> So we should either have several APIs specifically tailored to the 
>> underlying push or pull model, or make sure the unified API and its 
>> implementations accept any kind of component and set the appropriate 
>> conversion bridges between them.
>
> As I tried to state above: that will not be possible for every 
> conceivable combination of components.
> At least not when thinking beyond XML - which I do.

Again I doubt of the real value of a common unified pipeline if all the 
responsibility of ensuring proper compatibility between components 
(including possible data conversion) is delegated to components. This 
leaves a lot of complexity to component implementers (except in the 
simple straightforward push scenario), and the features of the pipeline 
will be limited to linking the components together and caching.

Furthermore, people will have to take great care of choosing components 
that fit together, or they will get exceptions at pipeline execution 
time. Hmm... reminds me of some criticism about the StAX stream API :-D

So let's agree that we disagree. I'll see what you guys come up with and 
hope I'll change my mind then.

Sylvain

-- 
Sylvain Wallez - http://bluxte.net


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