web sockets primer

WebSockets 101dhaval.dalal@software-artisan.com

@softwareartisan

AReal-time Webapp

Polling• “Simulate” real-time by making series of XHRs

at regular intervals.

• Pulls Data from the server.

Web Server

Browser

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Problems with Polling• Its a wasted request if there are no updates.

• Vice-versa, users may be working on “stale” data since the last update

• Each request creates new connection to the server.

• Each request causes exchange of HTTP headers (request and response), they consume Bandwidth.

• Not scalable

Long-Polling• Solves the problem of extremes (wasted request

or stale data) by pushing updates to clients as they happen.

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Browser

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How Long-Polling works• Browser makes request to the server.

• Connection is kept open between the server and the browser.

• When an update arrives the connection is closed (sent as complete response 200 OK).

• The browser then initiates a new long-polling request for subsequent updates.

• Techniques

• XHR Style LP

• Script Tag LP

• IFrame

Long-Polling Pros & Cons

• Pros

• Reduces latency for data-delivery.

• Cons.

• Each request creates new connection and causes exchange of HTTP headers, they consume Bandwidth.

• Not Scalable

• Request hangs until a response is ready to be delivered.

• Old Server software won’t work with this approach as they hold the thread for each request.

Long-Polling Scalability

• It demands a server software that does not hold thread on server for each request.

• Instead move towards asynchronous event-driven server architecture

• For e.g Nginx, Netty, Node.js etc... Servers

• Addresses the C10K (Concurrent 10,000 connections) problem

Separate Upstream & Downstream connections• Downstream Connection.

• Is kept open between the server and the browser.

• Regular updates pushed through this connection.

• Messages are continuously parsed by the client as they arrive.

• Upstream Connection

• Browser makes Ajax requests to send commands (i.e events that trigger action) to server.

Separate Connections:Pros & Cons

• Pros

• Offers lowest latency.

• Best for streaming needs.

• Cons

• Uses 2 Half-Duplex connections to simulate Full-Duplex.

• HTTP is a request/response protocol (Half-Duplex), not bi-directional

• Co-ordination overheads between two connections.

• Some browsers may not support more than two simultaneous connections.

Enter WebSockets• Full-Duplex

• Exchange HTTP headers initially when the connection is established, after that its all data exchange.

• Uses less bandwidth.

• Significant Scalability with reduction in network traffic

• 100 clients.

• 100 * 10 clients.

• 100 * 10 * 10 clients.

Fun time! Lets create a

WebSocket connection

Check Browser Support• Use window.WebSocket to find out

• What to do if the browser does not support it?

• use polyfills (github.com/Modernizr/Modernizr/wiki/HTML5-Cross-Browser-Polyfills)

• SockJS

• Atmosphere jQuery Plugin (Fallback to LP)

• Socket.io

• web-socket-js (Flash based)

• Kaazing Websocket Gateway (Commercial)

• Graceful Websocket jQuery Plugin (Fallback to LP)

• jQuery Socket (Supports WebSocket, Server-Sent Events, Streaming and LP)

WebSocket ServersJava .NET Python Ruby PHP

Jetty SuperWebSocket websockify GoliathExtendible

Web Socket Server

jWebSocket Nugget pywebsocketsweb-socket-

ruby

Netty XSockets

GlassFish

Apache Active MQ

Tomcat

Architecture

WebServer

WebSocket Server

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Serve Static Resources

Initiate WebSocket Connection

Upgrade Connection

5 Send Data

6 Receive Data

Event driven on both sides of the WebSocket connection.

Async API

• Server Callbacks

• onopen - when the connection is established

• onclose - when the connection is closed

• onerror - when an error occurs

• onmessage - when a message is received from the server

• Client Transmissions

• send

How WS protocol works• When making a WS

connection, initiated by HTTP request, client sends a connection “upgrade” request.

• Server responds by upgrading the connection and switching protocols

• Handshake is complete when both client & server switch protocols

• After this, client and server can begin sending messages until

• either of them closes the connection

• there is some network problem

More fun!Using Jetty’s

WebSocketServlet.

Same-Origin Policy• Prevents client-Side Webapp running from one

origin to obtain data retrieved from another origin.

• Limits unsafe HTTP requests launched towards destinations that differ from the running application’s origin

• <script> element can execute content retrieve from foreign origins.

• Requires you to trust the server

• Bypass using JSONP

• Server needs to implement some logic to allow you to do this.

Cross-Origin Resource Sharing (CORS)

• Extends Same-Origin Policy

• Its a way of restricting the domains that can access services. 

• White-list domains that are allowed to access services

• The browser and in-browser applications are supposed to respect that, and sometimes the Services (server-side) may protect themselves.

• Use a CORS Filter

• Flash uses crossdomain.xml

Thank-you!

References• blog.caplin.com/2009/04/20/what-is-the-real-time-web/

• leggetter.co.uk/2012/04/22/websockets-realise-comet-theyre-not-an-alternative.html

• infrequently.org/2006/03/comet-low-latency-data-for-the-browser

• jstorimer.com/js/2009/01/12/what-the-heck-is-jsonp.html

• carloscarrasco.com/blog/posts/read/implementing-script-tag-long-polling-for-comet-applications

• en.wikipedia.org/wiki/Comet_(programming)

• en.wikipedia.org/wiki/C10k_problem

• tomcatexpert.com/blog/2012/04/24/websockets-tomcat-7

• github.com/Modernizr/Modernizr/wiki/HTML5-Cross-Browser-Polyfills

• Peter Lubbers, HTML5 WebSocket DZone Refcardz