How web servers and clients (browsers) interact.
"Sending Requests Using the HTTP Protocol," discusses the nitty-gritty of HTTP requests in more detail. For now, suffice to say that your HTTP request contains several pieces of information needed so that your page may be correctly identified and served to you, including the following:
• The domain at which the page is stored (for example, mydomain.com)
• The name of the page (This is the name of a file in the web server's file systemfor example, mypage.html.)
• The names and values of any parameters that you want to send with your request
Web Page
Anyone with some experience using the World Wide Web will be familiar with the term web page. The traditional user interface for websites involves the visitor navigating among a series of connected pages each containing text, images, and so forth, much like the pages of a magazine.
Generally, each web page is actually a separate file on the server. The collection of individual pages constituting a website is managed by a program called a web server.
Web Servers
A web server is a program that interprets HTTP requests and delivers the appropriate web page in a form that your browser can understand. Many examples are available, most running under either UNIX/Linux operating systems or under some version of Microsoft Windows.
The term web server is often used in popular speech to refer to both the web server programsuch as Apacheand the computer on which it runs.
Perhaps the best-known server application is the Apache Web Server from the Apache Software Foundation (http://www.apache.org), an open source project used to serve millions of websites around the world
Another example is Microsoft's IIS (Internet Information Services), often used on host computers running the Microsoft Windows operating system
Not all Windows-based web hosts use IIS. Various other web servers are available for Windows, including a version of the popular Apache Web Server.
Server-Side Programming
Server-side programs, scripts, or languages, refer to programs that run on the server computer. Many languages and tools are available for server-side programming, including PHP, Java, and ASP (the latter being available only on servers running the Microsoft Windows operating system). Sophisticated server setups often also include databases of information that can be addressed by server-side scripts.
Server-side programming in this book is carried out using the popular PHP scripting language, which is flexible, is easy to use, and can be run on nearly all servers. Ajax, however, can function equally well with any server-side scripting language
The purposes of such scripts are many and various. In general, however, they all are designed to preprocess a web page before it is returned to you. By this we mean that some or all of the page content will have been modified to suit the context of your requestperhaps to display train times to a particular destination and on a specific date, or to show only those products from a catalog that match your stated hobbies and interests.
In this way server-side scripting allows web pages to be served with rich and varied content that would be beyond the scope of any design using only static pagesthat is, pages with fixed content.
Web Browsers
A web browser is a program on a web surfer's computer that is used to interpret and display web pages. The first graphical web browser, Mosaic, eventually developed into the famous range of browsers produced by Netscape.
By graphical web browser we mean one that can display not only the text elements of an HTML document but also images and colors. Typically, such browsers have a point-and-click interface using a mouse or similar pointing device.
There also exist text-based web browsers, the best known of which is Lynx (http://lynx.browser.org/), which display HTML pages on character-based displays such as terminals, terminal emulators, and operating systems with command-line interfaces such as DOS.
The Netscape series of browsers, once the most successful available, were eventually joined by Microsoft's Internet Explorer offering, which subsequently went on to dominate the market.
Recent competitive efforts, though, have introduced a wide range of competing browser products including Opera, Safari, Konqueror, and especially Mozilla's Firefox, an open source web browser that has recently gained an enthusiastic following
Browsers are readily available for many computer operating systems, including the various versions of Microsoft Windows, UNIX/Linux, and Macintosh, as well as for other computing devices ranging from mobile telephones to PDAs (Personal Digital Assistants) and pocket computers
Client-Side Programming
We have already discussed how server scripts can improve your web experience by offering pages that contain rich and varied content created at the server and inserted into the page before it is sent to you.
Client-side programming, on the other hand, happens not at the server but right inside the user's browser after the page has been received. Such scripts allow you to carry out many tasks relating to the data in the received page, including performing calculations, changing display colors and styles, checking the validity of user input, and much more.
Nearly all browsers support some version or other of a client-side scripting language called JavaScript, which is an integral part of Ajax and is the language we'll be using in this book for client-side programming.
DNS The Domain Name Service
Every computer connected to the Internet has a unique numerical address (called an IP address) assigned to it. However, when you want to view a particular website in your browser, you don't generally want to type in a series of numbersyou want to use the domain name of the site in question. After all, it's much easier to remember www.somedomain.com than something like 198.105.232.4.
When you request a web page by its domain name, your Internet service provider submits that domain name to a DNS server, which tries to look up the database entry associated with the name and obtain the corresponding IP address. If it's successful, you are connected to the site; otherwise, you receive an error.
The many DNS servers around the Internet are connected together into a network that constantly updates itself as changes are made. When DNS information for a website changes, the revised address information is propagated throughout the DNS servers of the entire Internet, typically within about 24 hours.
