Socket.io

Overview

Description Here

Class Outline

Learning Objectives

Students will be able to

Describe and Define

• The OSI Networking Model
• TCP and UDP Protocol
• Stateful Networking
• Packets
• Buffered Transfer
• Web Sockets
• Socket.io
• Network Events

Execute

• Implement a standalone Socket.io server for handling events and real time messaging.
• Use events to properly route incoming messages and payload

Notes

OSI Model

#	Layer	Protocol Data Unit	Function	Examples
7	Application	Data	Height Level APIs	HTTP, IMAP, POP, SSH
6	Presentation	Data	Data translating, including encryption, character encoding, and compression	Strings encoded with null terminated strings vs Strings defined by an Integer Length
5	Session	Data	Manages a session though passing data back and fourth	NetBios and Remote Procedure Protocol (RPC)
4	Transport	Segment / Datagram	Reliable transmission of data between points on a network	TCP and UDP
3	Network	Packet	Managing the network through addressing, routing, and traffic control	IP and ICMP
2	Data Link	Frame	Reliable transmission of frames between to physical layer nodes	Ethernet and IEEE 802.11 wireless LAN
1	Physical	bit	transmission and reception of raw data streams over a physical medium	USB, Bluetooth, Ethernet physical layer, SMB, Telephone network modem

Internet Protocol Suite

The Internet Protocol Suite is the conceptual model for the protocols used by the internet. It is often referred to as TCP/IP because the IP and TCP were the original protocols in the suite. The Internet Protocol Suite is described using four layers - Link, Internet, Transport, and Application. Web developers often reference the Internet Protocol Suite model when discussing network communication and data exchange.

Layer	Function	Examples
Application	Provides high level data exchange for use in user application development	HTTP, SMTP, FTP, DHCP
Transport	Provides process to process data exchange	TCP, UDP, µTP
Internet	Maintains computer addressing and identification and manages packet routing	IPv4, IPv6, ICMP
Link layer	Used to move packets between two different hosts	MAC, ARP, DSL, Ethernet

TCP

The Transmission Control Protocol (TCP) is widely used by application layer protocols in the Internet Protocol Suite. TCP creates a two way communication between two hosts and provides reliable, ordered, and error checked byte streams between the applications. TCP data transfers manage network congestion and use flow control to limit the rate a sender transfers data to guarantee reliable delivery. Each IP packet between the hosts carries a single TCP Segment. A TCP segment is made up of header and data sections.

Web Sockets

A communication Protocol which provides bidirectional communication between the Client and the Server over a TCP connection, WebSocket remains open all the time so they allow the real-time data transfer. When clients trigger the request to the Server it does not close the connection on receiving the response, it rather persists and waits for Client or server to terminate the request.

Socket.io Library

It is a library which enables real-time and full duplex communication between the Client and the Web servers. It uses the WebSocket protocol to provide the interface. Generally, it is divided into two parts, each of which use WebSockets, but also provide additional functionality such as broadcasting, namespacing, and other means of segmenting connected clients into groups.

• Client Side: it is the library that runs inside the browser or a “satellite” server that connects to a “hub”
• Server Side: It is the library for Node.js that serves as “hub” or “traffic cop”

Connections

With TCP, you connect directly to a server with a keep-alive type of connection.

With Socket.io, you connect to a server over HTTP. The session is “kept alive” through it’s internal use of the WebSocket Protocol, with session information being preserved.

Messaging

Standard node events are sent with emit() and received with on() … Socket.io uses the same methodology/terminology.

In an event driven node app, the entire app is in memory, and (through a common event pool), all parts of your application can emit and hear events, communicating with each other. However, no outside application can participate in these events natively.

With Socket.io, the entire purpose is to have events shared between ‘disconnected’ participants. Through a mediator (server), clients connect, emit events, and respond to events from the server. A typical flow works like this:

• Client Applications 1, 2, 3, x … connect to a running Socket.io server
  • Clients can join the common pool of connections or coalesce into groups/subgroups, if the server has been setup in this manner
• Client Application 1 emits an event called ‘speak’ to the server, with the data ‘Hello World’
• Server has an on('speak', (data) => {}) which “hears” that event
• Upon processing the event, the server may elect to
  • broadcast() the event itself or emit() an event of it’s own.
    • i.e. socket.emit('heard', data) or socket.emit('speak',data)
  • Messages can be sent to individual clients, groups, or sub-groups of clients
• Other client applications that have connected into the server may have a listener on that event type, can then “hear” it as well…
  • i.e. socket.on('incoming-message', text => console.log(text)
• Not every client will have a listener for every event.
• The server may not have a listener for every event a client sends.

Server

server.on('connection', (socket) => {
  // When clients "emit" an event called 'some-event', this code on the server handles it
  socket.on('some-event',  (payload) => {
    // do something with the event
  }
})

Client

// When anyone "emits" or "broadcasts" an event called 'cool-thing-happened', this code on the client handles it
// Note: Not all clients will subscribe to that event. This is the whole point!
client.on('cool-thing-happened', (payload) => {
  // do something with the event
})

Broadcasting

Servers or clients can “emit” or “broadcast” events in Socket.io by issuing either command with a payload

client.emit('some-event', "I just wanted to say hello");

```socket.broadcast(‘cool-thing-happened’, “WOW!”);

Refer to the emit cheatsheet for examples and use cases.