After a long time, I’ve decided to go back to realtime collaboration on Phoenix. The plan is to implement something compatible with ShareDB’s protocol. There are already some tools using ShareDB in the wild, and the most important case (collaborative editing of text) is already covered by ´ShareDB` pretty well (there’s a demo under 20 LOC doing exactly that with some extensions).
ShareDB has a client and a server implementation, both written in Javascript. In an ideal world, I’d port the protocol to Elixir and reuse the client without requiring any changes. However, the protocol expects messages to be sent through “bare” websockets, and Phoenix expects messages to be sent through “channels”. Channels are ingrained so deeply in Phoenix that I don’t think it’s possible to use bare websockets wthout reimplementing the whole channel machinery. In fact, Phoenix doesn’t even know what an websocket is.
The client-side API for ShareDB starts with something like this:
var ShareDB = require('sharedb/lib/client');
var socket = new WebSocket('ws://localhost:8080');
var connection = new ShareDB.Connection(socket);
The socket really must be a WebSocket, or at least something that implements the websocket methods. That’s actually very easy with Phoenix channels: you just have to create an object (class, prototype, whatever) which implements those methods and behaves as a webocket. However, there is a problem.
ShareDB allows you to create several “documents”, which can be updated in parallel. That means each document should live in its own process and be connected to the client though its own topic. Our “custom” websocket implementation would have to route each message into the appropriate phoenix topic. But the WebSocket object is sent raw JSON and doesn’t know which document/topic the message refers to. The only way to know that is to parse the JSON again and extract the document’s ID, which is unacceptable.
This means I will have to rewrite both the Connection object and the WebSocket object so that the WebSocket is compatible with Phoenix channels and the Connection object can send objects into the WebSocket (so that it can route the messages into the correct topics).
