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Core

Web browsers and web servers

URIs

HTTP

Client-Side

Basic: HTML, CSS

Advanced: JavaScript, DOM, AJAX, HTML 5, RIA

Server-Side technologies for Web Applications

CGI

PHP

Java Servlets

JavaServer Pages (JSPs)‏

Unit 2: Web Technologies (1/2)‏

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“The World Wide Web (known as "WWW', "Web" or "W3") is the universe of network-accessible information, the embodiment of human knowledge” (W3C)‏

The Web was created in 1989 by Tim Berners-Lee and Robert Cailliau working at the European Organization for Nuclear Research (CERN) in Geneva.

The three basic components of the initial WWW proposal:

HTML: Markup language for formatting hypertext documents

URI: Uniform notation scheme for addressing accessible resources over the network

HTTP: Protocol for transporting messages over the network

World Wide Web

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World Wide Web: Typical Interaction

FTP HTTPDNS …

TCP

IP

WEB SERVER

http://www.essi.upc.edu

Departament d’ESSI

HTTP …

TCP

IP

BROWSER

147.83.20.44

index.html

port 80

GET /index.html

Physical Net

1

2

3

4

5

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Web Browsers provide an –usually graphical- interface to obtain, interpret, format and render HTML documents.

HTML documents usually have links, hyperlinks, to other documents

Each link is encoded through an URI (Uniform Resource Identifier)‏

HTML documents may contain images, audio files, etc that are also referred through URIs and that are rendered jointly by the browser.

Other features: Cache, bookmarks, printing, off-line operation, … Client-side scripting support: JavaScript/ VBscript RIA support Helper Applications Plug-ins

The Universal Client: The Web Browser

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Web Servers “listen to” a TCP port -usually 80- waiting for a client (web browser) to initiate a connection.

Once the connection is established, the web browser sends a request and the web server returns a response. Then the connection is released

HTTP is the protocol that defines such a request/response communication

Initially, requests were about “static” resources -HTML documents, binary pictures, etc- stored in files reachable by the web server.

Other features: Logging

Server-side scripting: CGI, FastCGI, SSI, ASP.NET, Java Web Containers, PHP, etc.

The Web Server

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“The Web is an information space. Human beings have a lot of mental machinery for manipulating, imagining, and finding their way in spaces. URIs are the points in that space” (W3C)‏

A URI is a compact sequence of characters that identifies an abstract or physical resource. Its generic syntax defines four parts:

<scheme name> : <hierarchical part> [ ? <query> ] [ # <fragment> ]

Example:

http://user:pass@example.com:992/animal/bird?species=seagull#wings

login host port

authority path

scheme hierarchical part query fragment

Uniform Resource Identifier (URI)‏

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A "Uniform Resource Locator" (URL) is a URI that provides also a mean for locating the resource by describing its primary access mechanism. Examples:

ftp://ftp.is.co.za/rfc/rfc1808.txt

ldap://[2001:db8::7]/c=GB?objectClass?one

mailto:John.Doe@example.com

ed2k://|file|Jim%20Conallen%20­%20Building%20Web%20Applications%20with%20UML%202nd%20Ed.chm|6685541|74112A8EDD20B521B4BCB052D0416FE7|/

A "Uniform Resource Name" (URN) refers to a URI under the "urn" scheme (e.g urn:isbn:1892295490) or to any other URI with the properties of a name

The Internationalized Resource Identifier (IRI) is a URI that may contain characters from the Universal Character Set (Unicode/ISO 10646)‏

URLs, URNs and IRIs

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HTTP is the transfer protocol used throughout the Web: It specifies what messages web browsers may send to servers and what responses they get back in return.

Typical interaction (HTTP/1.0 or lower):

Web browser establishes a TCP/IP connection with the target Web server

Web browser sends a HTTP ASCII request over the TCP/IP connection.

Web server returns a HTTP MIME-like response and releases the TCP/IP connection

HTTP/1.1 improvements:

Persistent connections: several request/response interactions in a single connection.

Request pipelining: multiple HTTP requests are sent without waiting for the corresponding responses.

HyperText Transfer Protocol (HTTP)‏

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<METHOD> <Resource_Path> <HTTP_Version> <CR> // Request

( <Attribute>: <Value> <CR> )* // Parameters

<CR> // Blank line

[Body] // If needed

METHOD := GET | POST | HEAD | …

Example:

GET /index.html HTTP/1.1

Host: www.essi.upc.edu // Compulsory if HTTP/1.1

The HTTP Request

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<HTTP_Version> <Result_Code> [explanation] <CR>( <Attribute>: <Value> <CR> )*<CR>[Body]

Result_Code := 200 [OK] | 400 [Bad Request] | 404 [Not found] | …

Example:HTTP/1.1 200 OKDate: Fri, 19 Feb 2010 16:48:36 GMTServer: Apache/2.2.14 (Unix) mod_ssl/2.2.14

OpenSSL/0.9.8l PHP/4.4.9 mod_perl/2.0.4 Perl/v5.8.9Last-Modified: Tue, 02 Feb 2010 14:36:59 GMTETag: "22a7f-4270-47e9f08d3ad1f"Accept-Ranges: bytesContent-Length: 17008Content-Type: text/html<HTML>… </HTML>

The HTTP Response

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HTTP is stateless:

The Web Server handles each HTTP request independently and there is no easy way to keep track of each client request and to associate it with a previous one.

However, managing state is important for many applications: a single use case scenario often involves navigating through a number of Web pages. Without a state management mechanism, you would have to continually supply all previous information entered for each new Web page.

HTTP is one-way:

Clearly separated roles: Clients -Web browsers- make requests to -Web- Servers, no vice versa.

HTTP: Two Important Remarks

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HTML is the main publishing language of the Web.

HTML is based on SGML (Standard Generalized Markup Language, ISO 8879)‏

HTML Document = Content (mostly text) + Markup

HTML Markup allows to define

How content must be structured

How to render each piece of structured content

Links to other HTML documents

Embedded objects: images, audio, video, forms, scripts, applets, …

Markup is performed using tags:

<TAG (ATRIBUTE=VALUE)*> Content </TAG>

HyperText Markup Language (HTML)‏

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Despite an strong effort for standardization … Hypertext Markup Language (First Version), published as an Internet

Engineering Task Force (IETF) working draft (June 1993). HTML 2.0, IETF RFC 1866 November (1995)‏ HTML 3.2, W3C Recommendation (January 1997)‏ HTML 4.0, W3C Recommendation (December 1997)‏ HTML 4.01, W3C Recommendation (December 1999)‏ ISO/IEC 15445:2000, based on HTML 4.01 Strict (May 2000)‏ XHTML 1.0, W3C Recommendation (Published January 2000, revised

August 2002)‏ XHTML 1.1, W3C Recommendation (May 2001)‏ XHTML 2.0, W3C Working Draft (July 2006)‏Abandoned in 2009 (X)HTML 5.0 W3C Working Draft

… HTML programmers are constantly faced with the problem of using HTML features that are not supported consistently, if at all, across all the target browser platforms.

HTMLs

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HTML Forms

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HTML Forms: Processing

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Some HTML markup already defines how to render the affected content …

… however, CSS is preferable because:

provide more flexibility and control in the specification of presentational characteristics.

enable a clean separation of document content from document presentation.

allow the reuse of a same style sheet in different HTML documents: shared look and feel.

Cascading here refers to a priority scheme to determine which style rules apply if more than one rule matches against a particular element. (≈ polymorphism rules in OO generalization hierarchies).

Cascading Style Sheets (CSS)‏

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Inline<h1 style="color: red;">CSS test</h1>

<p style="font-size: 12pt; font-family: Verdana, sans-

serif">This a test to show the different ways of using

<em style="color: green;">Cascading Style

Sheets</em>.</p>

Embedded<html>

<head>

<STYLE TYPE="text/css">

H1 { color: red}

P { font-size: 12pt; font-family: Verdana, sans-serif;}

EM { color: green}

</STYLE>

</head>

<body>

</body>

</html>

CSS: Ways of Use (1/2)‏

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Linked

<html>

<head>

<LINK REL="STYLESHEET" HREF="prova.css"

TYPE="text/css">

</head>

Imported

<html>

<head>

<STYLE>

@import url(http://localhost/prova.css);

@import url(prova2.css);

</STYLE>

</head>

CSS: Ways of Use(2/2)‏

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JavaScript is an interpreted programming language that allows to embed executable code -scripts- into a HTML document.

Such scripts are executed by the web browser when rendering the HTML document.

JavaScript is a “light” version of Java: Type control is not strong. There are “objects”, but programmers cannot define their own

classes.

ECMAScript: JavaScript “Standard” (ECMA-262 specification and ISO/IEC 16262). Main dialects: JavaScript (it’s a trademark of Sun Microsystems, used under

license by Netscape and Mozilla) Microsoft’s JScript Adobe’s ActionScript

JavaScript

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Put dynamic text into an HTML page

Interact with the web browser: open new windows, show alerts, redirect to another URI, etc.

Read and write HTML elements (e.g. form fields)‏

Validate form data

Handle events : onClick, onLoad, onMouseOver, onMouseOut, onSubmit, etc.

Create/read/write cookies

Cookie: information sent by a server to a web browser and then sent back unchanged by the browser each time it accesses that server. Cookies are used for authenticating, tracking, and keeping data about users.

Interact with applets

JavaScript: What Can It Do?

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<html><head><title>La Data d’avui</title>

<script language="JavaScript">

function print_todays_date()‏

{ today = new Date();

days_ca = new Array("Diumenge", "Dilluns", "Dimarts",

"Dimecres", "Dijous", "Divendres", "Dissabte");

months_ca = new Array("gener", "febrer", "març", "abril",

"maig", "juny", "juliol", "agost", "setembre",

"octubre", "novembre", "desembre");

document.write(days_ca[today.getDay()]+", ");

document.write(today.getDate()+" de ");

document.write(months_ca[today.getMonth()]+" de ");

document.write(today.getFullYear());}

</script></head>

<body>

<hr>La data d’avui &eacute;s:<br><b>

<script language="JavaScript"> print_todays_date(); </script>

</b><hr></body></html>

JavaScript: Example

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DOM “is a platform- and language-neutral interface that will allow programs and scripts to dynamically access and update the content, structure and style of documents. The document can be further processed and the results of that processing can be incorporated back into the presented page”. (W3C)‏

HTML/XML documents and their inner elements are handled as objects with data and behavior (operations). The whole structure is represented -and accessed- as a object tree.

Dynamic HTML (DHTML): “is a term used by some vendors to describe the combination of HTML, style sheets and scripts that allows documents to be animated” (W3C).

Examples: www.w3schools.com/dhtml/

Document Object Model / DHTML

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DOM: Object tree

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Asynchronous JavaScript And XML

It is indeed a “mix” of – previous – technologies:

standards-based presentation using (X)HTML and CSS;

dynamic display and interaction using DOM;

data interchange and manipulation using XML and XSLT;

asynchronous data retrieval using XMLHttpRequest;

and JavaScript binding everything together.

AJAX

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AJAX: before and after

before

The browser sends a request to the server whenever the user selects a link or fills a form.

The web browser remains idle while the server is processing the request and building the response.

The response – a whole HTML page – is completely build on the server side.

after User interaction with the current

page displayed by the browser triggers presentation events

Event handlers may perform asynchronous calls to the server to request new/updated data.

Such calls are sent to and processed by the server without end users noticing it. The web browser is not idle

Server response is minimal (not a whole HTML page)‏

The new data is incorporated dynamically on the current page with JavaScript + DOM (DHTML)‏

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AJAX: Example

XMLHttpRequest

function doResp() {

if _ajax.readyState == 4 and

_ajax.status != 200 {

div=document.getElementById(‘status’)

div.innerHTML = _ajax.responseText;

Server Script: /validate

Database

Web ServerWeb Browser

onChange event:

_ajax = new XMLHTTPRequest();

_ajax.onreadystatechange = doResp;

url = ‘./validate?field=’

+this.name+‘&value=’+this.value;

_ajax.open(‘GET’, url, true )

Not a valid name

_ajax.send()Get parameters…do some work

Return something…

a text message

an XML document

an HTML snippet

a javascript method

whatever you want…

Message

status=999

msg=Not a valid name

Name:

Manolito

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Auto-complete

Full words are suggested as the user types the first letters

Progress bar

To show the status of the “work in progress” in the server side

“Real-time” validation of forms

Without needing to fill all the form fields

Updating the information displayed on the current page

Without “refresh”

Rich Internet Applications (RIA)‏

AJAX: uses

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The XMLHttpRequest object only can make requests to the same server that provided the web page

AJAX Programming “from scratch” is not advised.

Unexpected results when pressing the Back and Refresh buttons of the browser.

Downloading time takes longer as AJAX-enabled pages require larger amounts of code.

Debugging gets harder as code is executed in both browser and server sides

AJAX code is visible, prone to be copied and hacked.

Servers may get overloaded for abuse of asynchronous calls.

AJAX: Some considerations

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History:

Project started by the Web Hypertext Application Technology Working Group (WHATWG) in 2004:

Web Applications 1.0

Web Forms 2.0

In conjunction with the W3C since 2007

Main objective: Provide good support for modern documents and web applications (The browser as web application platform).

Other objectives:

Support legacy web content and provide backward compatibility

Cover common modern browser functionality

Make web content authoring more uniform

Clarify processing model

HTML 5

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New markup

Structure: section, article, header, footer, ...

Multimedia: audio, video, embed, ...

Graphics: canvas, figure, ...

New API’s

Timed media playback: embed interactive video and audio easily, without plugins

Offline Web applications: ApplicationCache

Client-side data storage: per-session (sessionStorage) and persistently across sessions (localStorage and client-side SQL database storage)

Document editing, Drag-and-drop, etc.

More flexibility in handling syntax errors

HTML 5 Features

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Features:

New input types

Basic client side validation

Repetition blocks

Access to remote data (for e.g. select elements)

Examples:

<input type="email” value="a@b">

<input pattern="[1-9]{10}" value="1234567891">

<input type="number" min="7" max="25” step="2">

<input type="date” required>

HTML 5 Forms (formerly Web Forms 2.0)

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RIA are Web applications that have the features and functionality of traditional desktop applications, by executing most of their - rich -user interfaces on the client side.

Indeed, the RIA paradigm is a new version of the Remote User Interface pattern for Client/Server architectures

RIAs typically:

run in a Web browser with/without extra software installation

run locally in a secure environment called a sandbox

“First generation” RIA platforms:

Java Applets, (Microsoft) ActiveX Controls, (Macromedia) Flash

“Second generation” RIA platforms:

AJAX (frameworks), Adobe Flex/AIR, JavaFX, (Microsoft) Silverlight, OpenLaszlo, …

Rich Internet Applications (RIA)‏

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Java Applets: Example

<html>

<head>

<title>TicTacToe v1.1</title>

</head>

<body>

<center>

<h1>TicTacToe v1.1</h1>

<hr>

<OBJECT

codetype="application/java"

classid="java:TicTacToe.class"

width=120 height=120>

</OBJECT>

</center>

</body>

</html>

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SHKLAR, Leon et al. Web Application Architecture: Principles, Protocols and Practices, Second Edition. John Wiley & Sons, 2009.

www.w3c.org

www.w3schools.com

http://www-128.ibm.com/developerworks/

References