Module: Software Engineering of
Web Applications
Chapter 2:
Technologies
1
Web Applications Evolution steps





Extending the presentation capability of HTML
through a better separation of content and
presentation.
The emergence of the eXtensible Markup
Language (XML).
Server-side scripting
extended architectures for Web servers (such
as Java2EE or Microsoft .NET)
Web services
These slides are designed to accompany module: Software Engineering of Web Applications
2
2.1 The HyperText Transfer
Protocol (HTTP)




The HyperText Transfer Protocol (HTTP) is the
very basic ingredient on which the Web is
founded.
It is a client-server application protocol that
defines a standard format for specifying the
request of resources on the Web.
Through HTTP a user using a client application
(e.g., a browser) can request resources
available on a remote server (the Web server).
Typical resources exchanged through HTTP
are HTML pages
These slides are designed to accompany module: Software Engineering of Web Applications
3
HTTP



More generally, a request may be related to a
file of any format stored on the Web server or
to the invocation of a program to be executed
on the server side.
Since such resources are distributed over the
Internet, they need an identication mechanism
to be located and accessed.
The identifier for referencing resources is a
string, called the Uniform Resource Locator
(URL)
These slides are designed to accompany module: Software Engineering of Web Applications
4
URL


URL species the protocol used for the resource transfer
(e.g., HTTP for Web page exchange, or other protocols,
such as FTP, supported by the browser), the name or
the IP address of the machine hosting the resource, an
optional port number denoting the access to a specific
server port, and the document name and location in the
Web server's file system.
For example, the URL
http://home.dei.polimi.it/matera/index.html denotes the
file named index.html stored in the directory named
matera/ in the file system managed by the Web server
installed on the host named home.dei.polimi.it.
These slides are designed to accompany module: Software Engineering of Web Applications
5

Additionally, parameters (the so-called query
string") can follow to enable, for example, the
transfer of processing instructions or simple
data provided by users through forms.
These slides are designed to accompany module: Software Engineering of Web Applications
6
Request-response cycle of
HTTP
These slides are designed to accompany module: Software Engineering of Web Applications
7
How does HTTP work?

when the user types a URL into the browser
address line or when the user clicks on an
anchor within a page representing a URL for a
given resource, the browser issues an HTTP
request, in which a request line species an
HTTP method, the URL of the requested
resource, and the protocol version.
These slides are designed to accompany module: Software Engineering of Web Applications
8
HTTP methods




The most important HTTP methods are GET and
POST.
The GET method submits a plain request for a
resource to the Web server, and allows the user to
submit simple inputs via the query string.
The POST method submits a request that allows
the user to submit complex inputs (for example, a
long text or a file) to be processed by the server.
With the POST method, the user input is packaged
as an attachment to the request, and constitutes
the so-called request body.
These slides are designed to accompany module: Software Engineering of Web Applications
9


When receiving the request, a server locates
the resource and sends a response to the
client.
The response message includes a status line,
which includes information about the protocol
version and a numeric status code with its
associated message (for example, HTTP/1.1
404 Not found), and a message body carrying
the actual resource to be exchanged.
These slides are designed to accompany module: Software Engineering of Web Applications
10
2.2 The HyperText Markup
Language (HTML)



Besides managing the request and transfer of
resources through the HTTP protocol, a Web
browser also handles the visual presentation of the
resources.
The HyperText Markup Language (HTML) is used
to express the content and the visual formatting of
Web pages.
The document presentation is managed by a
processor embedded in the Web browser, which
receives as input the marked-up content and
transforms it into a rendered document by
interpreting the meaning of the tags.
These slides are designed to accompany module: Software Engineering of Web Applications
11
A simple HTML page with an embedded
image and a clickable hyperlink
<HTML>
<HEAD>
<TITLE>Inserting an image and an anchor </TITLE>
<META name="keywords" content="HTML, example">
</HEAD>
<BODY>
<H1>A simple HTML page</H1>
<P>With an embedded image:</P>
<P align="left">
<IMG src="http://www.dei.polimi.it/images/logo.gif">
</P>
<P align="left">And with a link to an external resource: </P>
<P align="left"><FONT face="Arial" size="+1">
<A href="http://www.polimi.it">Click here</A>
to open the home page of Politecnico di Milano...
</FONT>
</P>
</BODY>
</HTML>
These slides are designed to accompany module: Software Engineering of Web Applications
12