[xml] New user, evaluating XML libraries

["Will Sappington" <wsappington ndma us>]

Hello to all,

 

I’m a new user of libxml and new to XML in general.  I’ve been asked to evaluate XML libraries, preferably Open Source projects, for some things we want to do with XML in our products.  We provide an archival/retrieval system for medical records and images and we use XML for attaching metadata to the files we store.  We have some front-end UI components that make some use of XML but currently most of the work is done in the transport layer and the backend database components.  Due to the volume of data involved, efficiency and execution speed is a prime concern, though not necessarily an overriding one.  Most of the XML work being done now is with roll-your-own string processing.  Going forward we will need to be more sophisticated and standards-compliant.

 

Of the packages that turned up when I did a search, Xerces and libxml are the leading candidates.  I’ve downloaded, installed, built, and written test code for both and based on my findings, I’m leaning very heavily toward recommending libxml.  The person I report to has a very strong bias toward Xerces in general, and the W3C DOM standard in particular, as the hammer with which to pound all nails, even if the problem isn’t a nail.  I’ve also received feedback from some of the users in the Xerces group and they make some points that I should at least consider.  What I’d like to do is present my reasons for recommending libxml, given the job we need to do as described above, include some of the Xerces users’ comments, and hopefully get your thoughts as well.  I like libxml because:

 

• It’s fast, about 3x faster than Xerces in some fairly rudimentary tests
• It supports XPath (one of our big requirements) on its own, Xerces requires a bolt-on component like Xalan or Pathan to do XPath.
• Being written in C, it has a much simpler programming interface than Xerces’ C++ object model.  Nothing against C++, it’s my primary language and I like it, but the interface to Xerces is more complicated, perhaps unnecessarily so, than most of the C++ I’ve been exposed to.  To me, a simpler interface translates to better understanding by a wider range of programmers, faster up-and-running time, and potentially better, safer code.
• It’s better documented.  In addition to the API reference manual, there’s the let-me-walk-you-through-it tutorial, well documented sample code, and many pages of additional information on a variety of topics.  The information presented in all areas is more thorough.  Xerces has the Doxygen-generated ref. man., a Programming Guide (equivalent to the tutorial, but sparse by comparison), and some commented sample code.
• (I may be mistaken about this, but…) for character encodings libxml uses a standard library (iconv) that is distributed with most versions of Linux and Unix (and has been ported to Win32), Xerces uses its own internal routines (?).
• In addition to a DOM-like interface and SAX support, libxml has the XMLTextReader interface which I haven’t tried yet, but I’m assuming is a fast efficient way to do simple XML queries.  Xerces has only DOM and SAX.

 

I’ve likened the use of big packages like Xerces for some of the things we need to “using a blowtorch to light a cigarette”.  Here is one response from a Xerces user:

 

“Libxml is a great library with somewhat different goals than Xerces.  I

don't think it's explicitly stated on the Web site, but Xerces and other

projects that build on it tend to implement W3C standards (DOM, XML

Schema), while libxml implements what its maintainer prefers (a unique

API, RelaxNG), with a focus on efficiency.  Both approaches are

reasonable, and which is appropriate depends on your needs.

 

In your shoes, if I were certain that lighting a cigarette is all I

would ever need to do, I'd probably use libxml.  In my experience,

though, XML is useful for so many things that I'd probably want to be

prepared to bake, boil, weld, and power fighter jets as well - in a

variety of local languages.  I'm a nut for portability, and a DOM

interface has the advantage of being similar or identical in a wide

range of environments (C++, C#, _javascript_, etc).”

 

What about this?  Is Xerces that much more powerful, as the writer suggests?  Is portability the only advantage to W3C-compliant interfaces like DOM?

 

And then this:

 

“In cases where performance is critical, I think you'd be best off

avoiding XPath altogether. (snip) An optimal Xerces SAX parser might well be more efficient than

libxml parsing + XPath evaluation.”

 

Finally:

 

“One big difference between Xerces-C++ and Libxml2 is that the latter

does not have a functional XML Schema validator. I don't know if it

is important to you or not. Also note that much of the speed-up of

Libxml2 compared to Xerces-C++ comes from the fact that Xerces-C++

uses 2-byte characters (UTF-16) while Libxml2 uses 1-byte characters

(UTF-8). Since most performance tests that I am aware of are done

on XML files that are either ASCII or UTF-8, Libxml2 has a natural

advantage here. This is also something to consider depending on the

type of applications you are planning to build.”

 

I’m unsure of the importance of an XML Schema validator so I can’t comment on this.  I don’t think I agree with the comment about speed vis a vis UTF-8/16.  Encoding conversions using UTF-8 are more computationally intensive than UTF-16 so what you lose by moving around double the number bytes would, I think be offset by the greater CPU requirement for translating the data.  Does Xerces’ use of UTF-16 provide support for a wider range of encodings and local languages?

 

I know this is rather long and I apologize in advance if it is too much so, but obviously there’s a lot to be considered, this is a hefty decision, and I want to provide anybody who might be inclined to help with as much to go on as possible.  Thanks in advance for any responses,

 

-will