Sylvain Wallez schrieb:
Andreas Pieber wrote:
On Saturday 27 December 2008 10:36:07 Sylvain Wallez wrote:
Michael Seydl wrote:
Hi all!
One more mail for the student group! Behind this lurid topic hides our
evaluation of the latest XML processing technologies regarding their
usability in Cocoon3 (especially if there are suited to be used in a
streaming pipeline).
As it's commonly know we decided to use StAX as our weapon of choice
to do the XML, but this paper should explain the whys and hows and
especially the way we took to come to our decision, which resulted in
using the very same API.
Eleven pages should be a to big read and it contains all necessary
links to all the APIs we evaluated and also line wise our two cents
about the API we observed. Concludingly we also tried to show the
difference between the currently used SAX and the of us proposed StAX
API.
I hope this work sheds some light on our decision making and taking
and that someone dares to read it.
That's from me, I wish you all a pleasant and very merry Christmas!
Regards,
Michael Seydl
Good work and interesting read, but don't agree with some of its
statements!
The big if/else or switch statements mentioned as a drawback of the
cursor API (XMLStreamReader) in 1.2.4 also apply to the event API,
since
it provides abstract events whose type needs also to be inspected to
decide what to do.
Of course, you're right!
The drawbacks of the stream API compared to the event API are, as you
mention, that some methods of XMLStreamReader will throw an exception
depending on the current event's type and that the event is not
represented as a data structure that can be passed directly to the next
element in the pipeline or stored in an event buffer.
The first point (exceptions) should not happen, unless the code is
buggy
and tries to get information that doesn't belong to the context. I have
used many times the cursor API and haven't found any usability problems
with it.
Also here you're right, but IMHO it is not necessary to add another
source for bugs if not required...
Well, there are so many other sources of bugs... I wouldn't sacrifice
efficiency for bad usage of an API. And when dealing with XML, people
should know that e.g. calling getAttribute() for a text event is
meaningless.
The second point (lack of data structure) can be easily solved by using
an XMLEventAllocator [1] that creates an XMLEvent from the current
state
of an XMLStreamReader.
Mhm but if we use an XMLEventAllocator, y not directly use the
StAXEvent api?
Sorry, I wasn't clear: *if* and XMLEvent is needed, then it's easy to
get it from a stream.
The event API has the major drawback of always creating a new object
for
every event (since as the javadoc says "events may be cached and
referenced after the parse has completed"). This can lead to a big
strain on the memory system and garbage collection on a busy
application.
Thats right, but having in mind to create a pull pipe, where the
serializer pulls each event from the producer through each
transformer and writing it to an output stream we don't have any
other possibility than creating an object for each event.
Think about it a little more in detail. To be able to pull each event
you have to have the possibility to call a method looking like:
Object next();
on the parent of the pipelineComponent. Doing it in a StAX cursor way
means to increase the complexity from one method to 10 or more which
have to be available through the parent...
Not necessarily, depending on how the API is designed. Let's give it a
try:
/** A generator can pull events from somewhere and writes them to an
output */
interface Generator {
/** Do we still have something to produce? */
boolean hasNext();
/** Do some processing and produce some output */
void pull(XMLStreamWriter output);
}
/** A transformer is a generator that has an XML input */
interface Transformer extends Generator {
void setInput(XMLStreamReader input);
}
class StaxFIFO implements XMLStreamReader, XMLStreamWriter {
Generator generator;
StaxFIFO(Generator generator) {
this.generator = generator;
}
// Implement all XMLStreamWriter methods as writing to an
// internal stream FIFO buffer
// Implement all XMLStreamReader methods as reading from an
// internal stream FIFO buffer, except hasNext() below:
boolean hasNext() {
while (eventBufferIsEmpty() && generator.hasNext()) {
// Ask the generator to produce some events
generator.pull(this);
}
return !eventBufferIsEmpty();
}
}
Building and executing a pipeline is then rather simple :
class Pipeline {
Generator generator;
Transformer transformers[];
XMLStreamWriter serializer;
void execute() {
Generator last = generator;
for (Transformer tr : transformers) {
tr.setInput(new StaxFIFO(previous);
last = tr;
}
// Pull from the whole chain to the serializer
while(last.hasNext()) {
last.pull(serializer);
}
}
}
Every component gets an XMLStreamWriter where to write their output
(in a style equivalent to SAX), and transformers get an
XMLStreamReader where to get their input from.
The programming model is then very simple: for every call to pull(),
read something in, process it and produce the corresponding output
(optional, since end of processing is defined by hasNext()). The
buffers used to connect the various components allow pull() to read
and process a set of related events, resulting in any number of events
being written to the buffer.
So the cursor API is the most efficient IMO when it comes to consuming
data, since it doesn't require creating useless event objects.
Now in a pipeline context, we will want to transmit events untouched
from one component to the next one, using some partial buffering as
mentioned in earlier discussions. A FIFO of XMLEvent object seems to be
the natural solution for this, but would require the use of events at
the pipeline API level, with their associated costs mentioned above.
I'm not sure if I get the point here, but we do not like to
"transmit" events. They are pulled. Therefore in most cases we simply
do not need a buffer, since events could be directly returned.
In the previous discussions, it was considered that pulling from the
previous component would lead that component to process in one pull
call a number or related events, to avoid state handling that would
make it even more complex than SAX. And processing these related
events will certainly mean returning ("transmitting" as I said)
several events. So buffering *is* needed in most non-trivial cases.
So what should be used for pipelines ? My impression is that we should
stick to the most efficient API and build the simple tools needed to
buffer events from a StreamReader, taking inspiration from the
XMLBytestreamCompiler we already have.
Maybe some events could be avoided using the cursor API, but IMO the
performance we could get is not worth the simplicity we sacrifice...
I don't agree that we sacrifice simplicity. With the above, the
developer only deals with XMLStreamWriter and XMLStreamReader objects,
and never has to implement them.
We just need an efficient StaxFIFO class, which people shouldn't care
about since it is completely hidden in the Pipeline object.
Thoughts?
Well the approach outlined about will certainly work.
Basically you're providing a buffer between every pair of components and
fill it as needed.
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.
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.
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.
Steven
Sylvain
