Restful Versioning

In a discussion of restful features the other day, a good question came up: where's the back button? POST and PUT are easy to understand, but how about rolling back or undoing changes? I have a simple solution which is going to look pretty familiar to Subversion users.

Let's beginning by modifying a feature:

PUT /features/1
{ ... }

The curly braces and ellipsis stand in for a JSON object, the details of which aren't important now. From a versioning feature resource, I'd want to get a response like:

200 OK
feature: http://example.org/features/1
revision: http://example.org/features/1/revisions/1

Representations of the feature and its most recent revision are identical. Want to rollback this change? First, let's get the previous revision:

GET /features/1/revisions/0
{ ... // revision 0 }

and then put it back:

PUT /features/1
{ ... // revision 0 }

With the response:

200 OK
feature: http://example.org/features/1
revision: http://example.org/features/1/revisions/2

That's the restful equivalent of:

$ svn merge -r 1:0 http://example.org/features
$ svn commit

OSM, by the way, has node history, but no URIs for node revisions.