Jabbering Giraffe

A Year in XQuery

About a year ago, I wrote The year of XQuery?. I’ve just finished my involvement with the large project at $WORK that was using XQuery. So it’s time to reflect over it a little.

First, a bit of background. The site I was developing is essentially a more-or-less static view of 112,1491XML documents in 4 collections2. The main interface is a search page, plus there’s a browse-by-title if you really fancy getting lost. It sounds simple, but as with any large collection of data, there is lots of variation in the data, leading to unexpected complications. Plus there are several different views of each document, just to make life more fun.

The site is based on Cocoon talking to an XML database (MarkLogic Server in this instance). We get XML back from the database, and render it through an XSLT pipeline into HTML[3]. Plus there’s some nice jQuery on top to smooth the ride.

Looking inside the codebase, we’re presently running at 5200 lines of XQuery code across 39 files (admittedly, that includes a 1000 line “lookup table”). But that doesn’t include any of the search code, which is dynamically generated using Java.

But in many senses, that’s not the remarkable thing about the XQuery. One of the most important aspects has been the ability to query the existing data. This may not sound particularly remarkable for a query language. But for a collection of XML this size, the ability to do ad-hoc queries that understand the document structure is truly remarkable.

For example, several months ago, I received a bug report stating “the tables are not rendering in article 12345.” I was able to look at the article, see that there were no tables, examine the source markup in the database and discover a TAB element that I’d never seen before4. But how widespread is this problem? Three seconds later, I have:

  distinct-values(
    for $tab in //TAB
    return base-uri($tab)
  )

Which tells me the 99 affected articles. Now I know that I only need to reload those from the source data instead of all 112,149.

Looking back over my original criticisms, how do they stand up to my experience?

But what new things have I learned?

Overall, I have to ask myself: would I do it the same again? And I probably would. For this particular project, I would try to place more emphasis on the XQuery than the XSLT (this was down to our inexperience—you should always try to work as close to the data store as possible). Despite the initial strong learning curve, the XQuery itself was rarely the main problem. But that’s leading into a whole new post…

In short: if you have a bunch of XML data lying around, XQuery is an excellent way to get the most use out it5.

1count(/doc)

2 There’s also a second site, almost identical, but on a different topic which has 46,876 documents.

3 HTML 4.01. Sadly, XHTML and browsers still interact badly.

4 This is a rather baroque DTD unfortunately.

5 If you’re not up to paying for a MarkLogic licence (it’s pricey), then eXist might be worth checking out.