Wednesday, September 12nd, 2007

FlatAtompub

A little while ago I decided to whip up a small Atompub server to get my head around the Atom Publishing Protocol. I called it FlatAtomPub because it was just storing stuff in flat files. I’m not committing to that name. It was also a chance to kick the tires on WebOb.

What I take out of the process:

  • APE was very handy. I lazily did very little unit testing, instead relying on APE to do the work for me. This seemed to work quite well. It is fun doing test driven development when someone else develops the tests.
  • I wrote a little decorator that serves as a kind of framework. It worked pretty well, I think. This might be the prototype of what the PylonsController.__call__ method does in some WebOb-using future.
  • Stuff like conditional requests and responses are mostly implemented in WebOb itself, which works well. HTTP is clear about how conditional requests work, so if you can just setup the basic info (ETag, Last-Modified) you can let the library handle the rest. I could probably save a little work by paying closer attention to ETags and Last-Modified up-front, but since there’s no complicated template rendering the work saved doesn’t seem significant.
  • The atom library removed most concern about the XML itself.
  • I don’t know what to with collections. I guess I could allow multiple collections via configuration. If the store wasn’t a dumb store (e.g., it was plugged directly into a blog, it didn’t just passively store things) it would be clearer what a collection would mean. As it is, collections are just a way of aggregating multiple Atompub servers into one service document, which doesn’t seem very useful.
  • Handling links and slugs is kind of annoying. I took the lazy way out for this, using relative links and treating the slug and link as the same thing. This isn’t a good long-term solution, as it can mess things up if you start handing entries around, or worse move a server, and I don’t even set xml:base on elements. In theory it should all work, but it makes the client do more effort than I would like. On the other hand, I suppose a client should do that extra work anyway, as it shouldn’t rely on the server not being lazy. So maybe I’m better off sticking with a lazy solution, and making sure I work with non-lazy clients.
  • I considered pluggable storage, but ultimately decided it didn’t matter. Storing entries in files is fine; files are easy, and they work. I put in pluggable indexing instead. Amplee is another Python Atompub server, and I looked at Amplee storage backends. It’s kind of clever to have things like svn or S3 backends. But I’m not sure what use I’d actually do with that.
  • I haven’t yet considered transactions; if something fails part way through, stuff will be inconsistent. Admittedly this is where files make things harder. Probably a clear way to re-index would be useful too, as at least there’s a clear location for the canonical data (the files).
  • The dependencies are still a little tangled; even though the library doesn’t use a great deal of stuff, there’s enough pieces that some people have had a hard time getting it setup.
  • Ignoring authentication is nice. I should see what it takes to setup some authentication, but implementing it directly is out of scope for FlatAtomPub.

Interested people can look at the svn repository.

This makes me wonder how hard WebDAV would be…

Comments !

tweets

This is the personal site of Ian Bicking. The opinions expressed here are my own.