If you are looking for portable storage for your topic map application PouchDB may be of interest:
PouchDB is a complete implementation of the CouchDB storage and views API that supports peer-to-peer replication with other CouchDB instances. The browser version is written for IndexedDatabase (part of HTML5). An additional implementation is in progress for leveldb.
Storage and Consistency
Unlike the other current couch-like browser APIs built on WebStorage (http://dev.w3.org/html5/webstorage/) PouchDB’s goal is to maintain the same kinds of consistency guarantees Apache CouchDB provides across concurrent connections across the multiple-tabs a user might be using to concurrently access an PouchDB database. This is something that just isn’t possible with the BrowserStorage API previous libraries like BrowserCouch and lawnchair use.
PouchDB also keeps a by-sequence index across the entire database. This means that, just like Apache CouchDB, PouchDB can replicate with other CouchDB instances and provide the same conflict resolution system for eventual consistency across nodes.
At this time PouchDB is completely independent from BrowserCouch. The main reason is just to keep the code base concise and focused as the IndexedDatabase specification is being flushed out.
After IndexedDatabase is more solidified it’s possible that BrowserCouch and PouchDB might merge to provide a simple fallback option for browsers the do not yet support IndexedDatabase.