Does anybody know a wiki engine that can be built on top of a RESTful application? I have a restful application, that exposes a document resource, I want the wiki engine to use the REST API to persist the documents, instead of saving them to a DB. I am also open for suggestions of an open-source wiki engines that can be easily modified to support such functionality.
Okay, this is a puzzler. Wikis in general are more or less the canonical example of a RESTful approach. The page name names a resource. What do you want that isn't in, eg, Twiki?
While you could try to find a wiki that can use a REST backend, it may be better to write a small wiki yourself. Because even though a your backend has REST interface, that doesn't mean you can put some other application in front of it.
dokuwiki does not use a database. It is a filesystem based wiki. I don't know its internal code structure but you might be able to use it as your base.
As Wikis were originally designed to work in a standard browser, and most browsers did not support anything except GET and POST, REST is not a concept used a lot on the wiki world. However, nowadays, some wikis (foswiki - the community fork of TWiki for instance) provide you a REST API to it http://foswiki.org/System/CommandAndCGIScripts#rest ).
But you need the opposite: a wiki with a customizable backend (storage) that could be plugged on top of a REST storage service. As wikis with a pluggable backend, I know only of pmwiki http://www.pmwiki.org/wiki/PmWiki/PmWiki and foswiki (the open fork of TWiki) http://foswiki.org.