The goal is for each client to have a local personalized copy of the database. This personal local database is a subset of all the existing documents. Clients can share documents or not.
Each client intiates an authenticated session with the server. On each session, server sets up personalized read and write filters. These filters only propagate changes that the user is allowed / interested in.
There is a central CouchDB database server, containing allthe documents for every user. Node.js server listens for _changes on all databases. Each change is forwarded to client filters.