From: Bob Hutchison <hutch@...>
Date: 2008-01-14T21:44:44+09:00
Subject: Re: Stream Parsing with REXML

Hi Robert,

On 14-Jan-08, at 2:50 AM, Robert Klemme wrote:

> Yep, looks pretty similar.  To be honest I never used an XML pull  
> parser myself.  Personally I prefer the push parser a bit because it  
> avoids the looping and decision logic based on event and element  
> type (in #work). Are there major advantages of pull parsers over  
> push parsers that I have overlooked so far?


I use both and I'm comfortable with both. The combination of sax and  
pull gives you a much more complete toolkit for streaming XML.

The biggest advantage (for me) to a pull parser is when you are  
parsing an XML file and using those events to drive some kind of  
algorithm. Either the parser or the algorithm is going to have to be  
able to suspend processing (and I mean save state by that) and resume  
when the other needs it to. This can be really hard to do for some  
algorithms, in the case of some it isn't possible without  
fundamentally changing the algorithm. If you read about pull parsers  
they will talk about where the flow of control (the 'outermost' loop)  
is in the program: in the parser or in the application. With sax the  
parser controls the flow, with pull parsers the application does. A  
subset of that situation is that the pull parser is just an object  
that you can pass around, and that can be *really* handy. This also  
means you can have more than one pull parser operating at a time, and  
that's incredibly confusing with sax.

You can fake a pull parser by putting a sax parser in a different  
thread and streaming the events over some kind of channel with some  
kind of blocking protocol. Some implementations of pull parsers do  
exactly that, and that is *slow*, yet if you need it people will put  
up with it. If you've ever had the temptation to do that then you  
might want to think pull parser.

It is pretty much trivial to write a sax parser based on a pull parser  
at very small cost (xampl-pp ships with an example that does that). So  
one trick that you can play is switching back and forth between sax  
and pull where appropriate.

Junior programmers have a much easier time with pull parsers. It seems  
to provide a gentler route to sax based software. XML is a big enough  
step. There is some advantage to avoiding the need for a beginner to  
learn both XML and event based programming at the same time.

Historically pull parsers tend to be very fast compared to sax  
parsers. This is not going to last, there's no technical reason for  
such a difference. In the Java world the speed difference is eroding  
for the good of all. Anyway, speed isn't the issue it used to be. If  
you poke around the web you'll still see a bunch of references to  
performance advantages, this doesn't apply anymore if you choose your  
parser correctly.

Cheers,
Bob

----
Bob Hutchison                  -- tumblelog at http://www.recursive.ca/so/
Recursive Design Inc.          -- weblog at http://www.recursive.ca/hutch
http://www.recursive.ca/       -- works on http://www.raconteur.info/cms-for-static-content/home/