CouchDB supports undo as part of their core storage model. http://guide.couchdb.org/draft/btree.html http://guide.couchdb.org/draft/btree.html The short answer

CouchDB supports undo as part of their core storage model. http://guide.couchdb.org/draft/btree.html http://guide.couchdb.org/draft/btree.html The short answer is that because CouchDB uses append-only files, the B-tree root node must be rewritten every time the file is updated. However, old portions of the file will never change, so every old B-tree root, should you happen to have a pointer to it, will also point to a consistent snapshot of the database.