Web Development

Dr Derek Bridge
School of Computer Science & Information Technology
University College Cork

Lecture Objectives

  • learn the difference between absolute URLs and relative URLs
  • learn when to use absolute URLs and when to use relative URLs
  • learn how servers respond to various requests

Absolute URLs

Absolute URLs use absolute pathnames.

But they start at the document root, not the filesystem root.

Relative URLs

Relative URLs use relative pathnames.

They start at the directory that contains the current document.

The browser converts the relative URL into an absolute URL before sending a request to the server.

Web Servers

There are two main Web servers:
Apache
and
Microsoft Internet Information Server (IIS)
(and lots of ones that are more niche).

For concreteness, we'll look at the Apache Web Server.

The Apache Web Server

  • Apache is modular, and you decide which of its modules to install, e.g.
    • mod_mime;
    • mod_ssl.
  • You enable/disable and configure the modules that you have installed by placing directives into configuration files.
  • The main configuration file is called httpd.conf

httpd.conf Example


ServerName www.example.org
        
Listen 80

DocumentRoot "/var/www/html"
  • The ServerName directive specifies the hostname.
  • The Listen directive specifies the port that the server is listening on.
  • DocumentRoot is the directory that hosts all the web resources.

The DocumentRoot


ServerName www.example.org
        
Listen 80

DocumentRoot "/var/www/html"
  • By default, if a browser makes a request, the URL pathname is appended to the DocumentRoot pathname.
  • E.g. if a browser requests http://www.example.org/fish/guppies.html, the server will serve /var/www/html/fish/guppies.html

Example of Absolute URL

Assume the server's DocumentRoot is /var/www/html.
A browser sends a GET request for the following URL:
http://www.example.org/cats/persian.html
The server sends back /var/www/html/cats/persian.html

Example of Absolute URL

Similarly, if a browser sends a GET request for the following URL:
http://www.example.org/cats/manx.html
The server's response is a 404.

Example of Absolute URL

Similarly, if a browser sends a GET request for the following URL:
http://www.example.org/dogs/poodle.html
The server's response is a 404.

Example of a Relative URL

The browser has just fetched the following web page: http://www.example.org/cats/persian.html

The web page contains a hyperlink:
<a href="siamese.html>Siamese!</a>

The user clicks on this hyperlink.

So the browser sends a new request to http://www.example.org/cats/siamese.html

Example of a Relative URL

Again, the browser has just fetched the following:
http://www.example.org/cats/persian.html

The web page contains a hyperlink:
<a href="../fish/guppies.html>Guppies</a>

The user clicks on this hyperlink.

So the browser sends a new request to http://www.example.org/fish/guppies.html

Absolute and Relative URLs

Advice about how to write hyperlinks in web pages.

Absolute URLRelative URL
External link
(i.e. on a different server)		scheme + hostname/IP address + absolute pathnameN/A
Internal link
(i.e. on the same server)		scheme + hostname/IP address + absolute pathnamerelative pathname
absolute pathname

External Link Example

You want a link in siamese.html to the RTE news.

An external link, hence an absolute URL:
scheme + hostname + absolute pathname

<a href="https://www.rte.ie/news/index.html">RTE News</a>

Internal Link Example

You want a link in siamese.html to guppies.html

An internal link, hence three options.

either
<a href="http://www.example.org/fish/guppies.html">Click me</a>
or
<a href="/fish/guppies.html">Click me</a>
(N.B. The forward slash at the start.)
or
<a href="../fish/guppies.html">Click me</a>
(N.B. No forward slash at the start; no scheme, no host.)

Some Other Apache Directives

Some other common directives.

HTTP Response Status Codes

2XX Success e.g. 200 OK
3XX Redirection e.g. 301 Moved Permanently
4XX Client error e.g. 400 Bad Request
e.g. 403 Forbidden
e.g. 404 Not Found
5XX Server error e.g. 500 Internal Server Error

Error Pages

For client and server errors, some web developers like to serve customized error pages.

To do this, use an ErrorDocument directive in httpd.conf

Requests for Directories

If a browser requests a file, the server serves that file.

But what does a server do if a browser requests a directory?
E.g. http://www.example.org/fish/

  • It may return an error (usually 404).
  • It may return a default page, usually from the directory.
  • It may return a page that lists the contents of the directory.

Index Files

If a browser requests a directory, one option is for the server to return a default page. 


ServerName www.example.org
        
Listen 80
        
DocumentRoot "/var/www/html"
        
DirectoryIndex index.html

If this is your httpd.conf, then Apache will serve you a file called index.html from the directory you requested.

Directory Listings

Another option is for the server to return a page that lists the contents of the directory. 


ServerName www.example.org
        
Listen 80
        
DocumentRoot "/var/www/html"
        
<Directory />
    Options +Indexes
</Directory>

Here, Apache will serve you a list of the contents of the directory that you requested.

Be wary of setting things up in this way!

Example

Example

Example

G'luck!