Tried to read socket.io source code

Some important parts of socket.io.

Manager

This is the main interface of socket.io. require('socket.io').listen() returns a Manager (the io object).

Manager holds a server, which is the instance of nodejs server; a namespaces, which is the collections of all namespaces; a settings, which holds the customizable setting info. a sockets, which is the default namespace.

new exports.Manager(server, options) workflow:

• init all the members including server, namespaces, sockets, settings.
• server.on('request'... use handleRequest to handle server request.
• sockets is a bad name, because is actually refers to “” namespace, as this.sockets = this.of('') shows.
• init store
  • here the store subscribes lots of messages like handshaken, connect, open, join, leave, etc. and call the corresponding method
  • init handshaken, connected, open, closed, rooms, roomClients. These are important controll objects.
• init transports, transports are underlying data transporation modules.

handshake

The first step of websocket is shandshake. client inits a http request inicating itself as a websocket request, the server indentifies it and start websocket communication. Here is how it works in socket.io:

• handleRequest: when request comes, this method is called. In this method, checkRequest is used to extract messsages from rquest data. When request data doesn’t have an id, then it is considered as a handshake, and handleHandshake will be called.
• handleHandshake: This method first extracts handshake data from request data with handshakeData method.
  • authorize method can be used to authoriztion. authoriztion method can be passed from outside with set method.
  • If authorization succeeded, then success is returned. Here the unique id for this connection is created, and be used everywhere.
  • onHandshake: It is called after response, sets handshaken object with id as key, and handshake data as value.
  • Also, handshake message is published.
  • Now connection is established.

send and receive message

To receive messages from client, the workflow is the same as handshake, except that handleHTTPRequest is used instead of handleHandshake.

handleHTTPRequest

handleClient will do the actual work.

handleClient

why onHandshake for second time here??

Because when client get disconnected, it will try to reconnect for sometime, and will enter handleClient.

Important parts are: Here is where connect event of client is fired:

if (i === '') {
  this.namespaces[i].handlePacket(data.id, { type: 'connect' });
}

Here is how message of specific connection can be listend:

this.store.subscribe('message:' + data.id, function (packet) {
   self.onClientMessage(data.id, packet);
});

I think we can use this to communicate with connection from outside of nodejs. For example, when redis store is used, we can publish message:111 from other program, say a php batch, to notify something to the connection with id 111. Update: too simple, sometimes naive: After a close look at the code, I found this is difficut as the data of message is the packet itself.

The real code to send packet back to user is done by namespaces.

SocketNamespace

The packet is handled by handlePacket method. The important member is socket, which is the socket we got in io.sockets.on('connection', funtion(socket) {}. socket.io has one socket for each connection identified by id. In handlePacket method, itdoes different things based on pacekt.type. Here we can find why socket.on(xxx, function() {}) can work, in fact it calls socket.$emit.apply(socket, params).

connect

In packet method, it does different things based on packet.type. When type is connect, then connection event is emitted, and our io.sockets.on('connection') will be called. Also here, we will call socket.packet({ type: 'connect'}) to send connect packet back to client, so that client’s ‘on connect’ will be called.

TODO ack

The packet method of SocketNamespace calls Manager’s onDispath which in turn calls transport’s onDispatch to send the packet.

Transport

Transport is where the transition of data is carried out. We only focus on websocket. There’re several subclass represents different sub protocal of websocket, the onDispatch called write method of its sub class. I only searched in default.js. In default.js, there’s a write method finally writes data buffer into socket.