Evergreen Exposed: introduction to Evergreen development (OLA 2009)

Posted on Sun 01 February 2009 in Libraries

Update 2009-02-19: uploaded diffs from Evergreen (EG_exposed.tar.gz) for adding details to record summary; and Bill Erickson's slides and code examples are also available for download

The slides: Evergreen exposed, part 1 (OpenOffice)

My second presentation at the OLA SuperConference 2009 was Evergreen Exposed: hacking the open library system, which promised to “take attendees on a tour of the architecture and source code of the Evergreen library system”. I was very fortunate to have Bill Erickson, one of the original Evergreen developers, agree to join me as a co-presenter. Given the hour-and-fifteen-minute time slot that we were allotted, we opted to take an incremental approach to introducing parts of Evergreen to the audience, starting with basic tasks and working up to more complex customisations. We also tried to focus on answering questions that had been posted to the Evergreen mailing lists to ensure that we would satisfy our target audience's interests.

Dan starts with the basics

I started the session with an introduction of how to create a different skin for the catalogue, starting with text, CSS, JavaScript, and images and extending to the translation and customization framework. We talked about how to future-proof your customizations against future upgrades and how consortia can use skins to provide not just different look-and-feel, but different functionality, for each member of the consortium. Not much more than XML entities defined by DTDs, massaged via Apache server side includes (SSI), but it's an important conceptual building block for both the catalogue and the staff client.

I then ran through the exercise of adding a new metadata export format that brought the Federal Geographic Data Committee's Content Standard for Geospatial Data Metadata (FGDC CSGDM) format to Evergreen's existing list of supported formats. On the one hand: big deal, another metadata format. Hold that thought in that one hand; we'll come back to it later.

I also walked through two other common requests on the mailing lists: how do I define a new index or tweak the behaviour of an existing index and how do I hide or show more information on the detailed record display page? I'll follow up with separate posts for each of these pieces to augment what you have before you in the slides; suffice to say that there's a lot of MODS, a little bit of JavaScript, a smidgin of XPath, a dollop of Evergreen's interface definition language (IDL), and a slice of Perl mixed together. Along the way, I peeled back the covers to show a bit of OpenSRF in operation, setting up Bill's part of the show...

Bill leads us into the promised land

Note I'll update this with a link to Bill's slides when he manages to post them!

Bill gave a quick "big picture" view of how OpenSRF operates, including a much clearer overview of Evergreen's object-relational IDL that maps objects to relational tables. He also covered the cstore OpenSRF application that offers access to the underlying database without requiring SQL but still with support for full transactions (commit/rollback) and sub-transactions (savepoints). During Bill's demonstrations of these features, he exercised srfsh in a way that was new to me - he used the introspect command with a partial method name to perform a left-anchored search for matching method names. Cool!

Oh, and he also showed that if OpenSRF would normally return a reference to an object defined in the IDL, you can ask it to flesh the object in-place with its complete set of attributes instead; and of course if any of those attributes are object references, you have the option of fleshing those as well. It's a lovely way to cut down on chattiness in your application.

From there, Bill whipped out DojoSRF, the OpenSRF-aware extensions for dojo, the JavaScript toolkit that Evergreen adopted as its core JavaScript framework in release 1.4. In 90 lines of HTML and JavaScript code, he implemented a basic but workable catalogue - and then, with a few more lines of code, he gave the audience the payoff for that FGDC CSGDM (geographic metadata) format that I had earlier hacked into Evergreen. As part of the transform separates out the geographic coordinates of the subject matter (in the case of our demo data, maps of Northern California), Bill was able, in just a few more lines of code, to easily extract the coordinates from the FGDC CSGDM representation of the bibliographic material and plot the bounding box for the coverage area on a Google Map image. Very cool.

We had about 15 to 20 people attend our session, and I was happy with that attendance given the extremely technical content and relatively niche product. If as a result we end up adding just one more developer to the Evergreen community, that would be a great outcome. And for myself, I was forced to learn much more of Evergreen - just in time for Project Conifer, I hope :-)