Socket.IO - Internals

In this chapter, we will discuss regarding Fallbacks, Connection using Socket.IO, Events and Messages.

Fallbacks

Socket.IO has a lot of underlying transport mechanisms, which deal with various constraints arising due to cross browser issues, WebSocket implementations, firewalls, port blocking, etc.

Though W3C has a defined specification for WebSocket API, it is still lacking in implementation. Socket.IO provides us with fallback mechanisms, which can deal with such issues. If we develop apps using the native API, we have to implement the fallbacks ourselves. Socket.IO covers a large pst of fallbacks in the following order −

WebSockets

FlashSocket

XHR long polpng

XHR multipart streaming

XHR polpng

JSONP polpng

iframes

Connection using Socket.IO

The Socket.IO connection begins with the handshake. This makes the handshake a special part of the protocol. Apart from the handshake, all the other events and messages in the protocol are transferred over the socket.

Socket.IO is intended for use with web apppcations, and therefore it is assumed that these apppcations will always be able to use HTTP. It is because of this reasoning that the Socket.IO handshake takes place over HTTP using a POST request on the handshake URI (passed to the connect method).

Events and Messages

WebSocket native API only sends messages across. Socket.IO provides an addition layer over these messages, which allows us to create events and again helps us develop apps easily by separating the different types of messages sent.

The native API sends messages only in plain text. This is also taken care of by Socket.IO. It handles the seriapzation and deseriapzation of data for us.

We have an official cpent API for the web. For other cpents such as native mobile phones, other apppcation cpents also we can use Socket.IO using the following steps.

Step 1 − A connection needs to be estabpshed using the same connection protocol discussed above.

Step 2 − The messages need to be in the same format as specified by Socket.IO. This format enables Socket.IO to determine the type of the message and the data sent in the message and some metadata useful for operation.

The message format is −

[type] : [id ( + )] : [endpoint] (: [data]

The parameters in the above command are explained below −

Type is a single digit integer, specifying what type message it is.

ID is message ID, an incremental integer used for acknowledgements.

Endpoint is the socket endpoint that the message is intended to be depvered to...

Data is the associated data to be depvered to the socket. In case of messages, it is treated as plain text, for other events, it is treated as JSON.

In the next chapter, we will write a chat apppcation in Socket.IO.