I had a look at a few microblogging APIs, hoping to find one that's extensible and RESTful.

That world is fairly fragmented at the moment, which is not surprising considering how new the whole microblogging thing is. Having a common API would be good, and the newly incubating ESME project might be a nice place to make this happen.

I don't think anyone expects microblogging interactions (except for the very basic ones maybe) to happen directly from a naked browser, so a clean RESTful interface, that does not have to work around current browser's HTTP weaknesses, should be possible.

Here's a quick list of my findings - I spent less than an hour on this so it's just a quick "first impressions" tour. Feedback is welcome, as are pointers to APIs that I might have overlooked.

The Jaiku API uses hostnames to differentiate between users, as in http://johndoe.jaiku.com/presence/json. That won't work in the general case, you can't force everybody who installs a microblogging server to setup virtual hosts. The API is also not RESTful when it comes to posting, as it uses method names in request parameters.

The current ESME API is not RESTful, and it uses HTTP sessions. But it's simple to understand and describe, which is cool - microblogging is not *that* complicated.

The Twitter API is the most complete, and provides various useful formats including Atom and JSON. The resource URLs are nice, but an URL can have a different meaning based on the name of the authenticating user, which is not good for caching, debugging, etc.

Dick Hirsch (who often writes at blog.esme.us) mentioned the Yammer API. The spec is simple and clear, with nice URLs, but like in Twitter a resource URL can have a different meaning depending on the username used to sign the request.

I'm not a REST guru, but common sense makes me wonder why an URL like http://myservice.com/messages.json is better than http://myservice.com/johndoe/messages.json? Expressing the johndoe username in the URL makes it very clear what is what. No username, as in http://myservice.com/messages.json, means "all users" which maps nicely to the public timeline.

I'll have a closer look, with the idea (at some point) of creating a microblogging server based on Sling. And talk to the ESME folks, as I'm a mentor for ESME's incubation.

Update: Lars suggests having a look at openmicroblogging as well. Will do.