DATABASE SYSTEMS AND THE WEB - Database Design and Development: An Essential Guide for IT Professionals - page 590

Establish
Connection

Connection
Established
Send
Request
Message

Request
Received

Web
Client

Respond to
Request OR
Send Error
Message

Web
Server

Close
Connection

Figure 19-2 Stages of an HTTP transaction.

HyperText Transfer Protocol (HTTP)

HTTP is a simple, object-oriented protocol governing the exchange of hypertexts
written in HTML on the Web. The protocol stipulates how clients and servers can
communicate and exchange information. This protocol was developed especially for
the Web. Its simplicity is its strength. HTTP may be thought of as a great equalizer;
it allows you to create HTML documents on any computing system and deliver them
in a consistent way to any other system. If a server can deliver the document, any
browser can view it the way you created it.
With HTTP, exchange of information takes places in four stages or phases. An
HTTP transaction operates on a request-response method. Figure 19-2 presents the
stages of an HTTP transaction.
Here is a brief description of the actions in the four stages:

Connection. The client establishes connection with the identified Web server. You
may note the status line of most browsers indicating this action in the status line
as “connecting to....” If client cannot establish the connection, nothing more
happens. The connection attempt times out.

Request. If the connection is established, the client sends a request message to the
Web server. The request includes information about the transmission protocol being
used with the version number, the object requested, and how the server must
respond to the request.

Response. The Web server sends the requested HTML document to the client indi-
cating the protocol being used. If the server cannot fulfill the request, it sends back
an error message. As soon as the document is received, the browser launches the
appropriate software to display its text and the multimedia components.