Well, it is. Just like Meteor. There are dozens of possibilities how you could do it nowadays. You can even use your own Node.js server with WebSockets, use CouchDB, etc.

But nothing comes close to the first experience and ease of development you have with Firebase. And I'd love to build my products around Firebase and embrace their service if there was not that risk of them shutting down some day.

An API that works as a drop-in replacement would let us developers sleep much better, wouldn't it?

So I'd like to know: Which open-source product would you consider best as the foundation for something like that?

Couchbase? hood.ie? Meteor? CouchDB?

I'd love to work on such an open-source "clone", and maybe we'll even find some more collaborators.

CouchDB could be the starting point that we can build on top of.

Can you explain why you would suggest CouchDB? Of course, it seems to be a great project. Nevertheless, it has no built-in support for publishing and subscribing, and real-time notifications.

We would have to built this ourselves. However, CouchDB has a "_changes" API that can even be turned into "continuous" mode [1]. This is the perfect data source for pub/sub.

hood.ie, Meteor, etc. offer more features, but they are also more cluttered and not as clean as CouchDB.

[1] http://docs.couchdb.org/en/latest/api/database/changes.html

Well I mean if you just want to use replace Firebase you might as well use hood.ie directly, unless you want to build it yourself (in which case using just the database and layering functionality on top of that might be best). Hood.ie can be an out-of-the-box replacement for most of Firebase's functionality as it is.