Since the last release of the draft I’ve done a fair amount of work to simplify the format even further; simplifying terminology, clearing up some inconsistencies and dropping namespaces altogether.
You’ll notice that we still don’t define any terms. As Ben touched on in a recent post, we decided to not confuse the format by trying to tie it to any one application, while keeping it as easy as possibly to actually use. I’ll cover this in more detail a bit later…
For those who don’t already know, Elgg is an Open source social networking application engine. The previous version has been downloaded over 100K times, and Import and export was one of the most frequently requested enhancements.
Export was a fairly trivial matter. The new version of Elgg employs a flexible event system, so all I had to do was trigger an “export” event.
This event is passed a GUID – an identifier identifying the thing you are exporting, and elements of the system (and thirdparty plugins) can listen for this event and react accordingly.
The event is essentially asking all parts of the Elgg application – core and plugins – “Tell me all you know about X”. The export listens to the answers and converts it into an ODD document that looks something like this:
<odd version="1.0" generated="Wed, 30 Apr 2008 22:21:55 +0100">
<entity uuid="http://example.com/odd/78/" class="object" subclass="blog" published="Fri, 18 Apr 2008 11:45:50 +0100" />
<metadata uuid="http://example.com/odd/78/attr/owner_uuid/" entity_uuid="http://example.com/odd/78/" name="owner_uuid" published="Fri, 18 Apr 2008 11:45:50 +0100" >http://example.com/odd/77/</metadata>
<metadata uuid="http://example.com/odd/78/attr/title/" entity_uuid="http://example.com/odd/78/" name="title" published="Fri, 18 Apr 2008 11:45:50 +0100" >test</metadata>
<metadata uuid="http://example.com/odd/78/attr/description/" entity_uuid="http://example.com/odd/78/" name="post" published="Fri, 18 Apr 2008 11:45:50 +0100" >First post</metadata>
<metadata uuid="http://example.com/odd/78/metadata/35/" entity_uuid="http://hexample.com/odd/78/" name="tags" type="metadata" owner_uuid="http://example.com/odd/77/" published="Fri, 18 Apr 2008 11:45:50 +0100" >wibble</metadata>
Here we see an entity (in this case a blog post), and some details about it (the metadata).
Import is traditionally the more complicated part of the equation. ODD is trivial to parse, each tag is atomic and represents exactly one thing, this is a big advantage from the point of view of anyone implementing a reader for it since it makes the whole thing pretty much stateless.
ODD tags arrive, whether as a file to import or as a live feed, and an event is triggered. This event passes around the tag and essentially asks the question “Does anyone know how to handle this?”.
The stateless nature of ODD of course meaning that you don’t have to process the entire file, making it a trivial matter to implement a reader using a SAX parser.
That just about covers it, I’ll be posting some example code in a few days (workload permitting) so hopefully people can start getting stuck in. If you want to get involved in development, please head over to the ODD group.
A final note: I will be in San-Francisco all next week, so if you are in the bay area and feel like having a chat about ODD or Elgg, then please get in touch!