internet

Web Client / Server Communication and architecture The web client requests information from a particular web server on a distant computer using the internet as the transportation medium. The request is formulated into an HTTP request and sent to the target computer (server).When the server receives the request ,it retrieves the page or the other information that the server requested ,formulates it as an HTML page and sends it back to the requester client via the Internet. When the requested information in the form of HTML page arrives at the client computer the web browser displays the page on the client machine according to directions laid out in the page’s HTML code. Some times a simple client request results in a dozens or even hundreds of separate server responses to locate and deliver information. Two-Tier Client / Server This model involves only a client and server, All communication takes place between the client on the internet and the target server at the other end. Other computers are involved in the process of transporting packets of information across the internet. A two tier architecture is one in which only a client (tier 1) and a server(tier 2) are involved in the request and responses that flow between them over the internet When a server recognize a client request it establishes a temporary connection with the client. Once the server sends the message back the connection between them is broken. A typical request message from a client to a server consists of 3 major parts 1) A request line 2) Optional request header 3) Optional entity body The request contains the command, the name of the target resource (with out the protocol or domain name), and the protocol name and version. Following the request line are name/value pairs that comprise the request header, if it exists. The request header contains additional information about the client and more information about the request. The optional entity body is sometimes used to pass bulk of information to the server; a blank line separates the entity body from the header information The GET command asks the server to retrieve a file. Following the command is the path and filename of the needed file.”HTTP/1.0” at the end of the request indicates the client is using version 1 of the hypertext protocol. Message lines 2 and 3 indicate that the client accepts text in HTML format and accepts a specific audio format. Once the request is received by the server it executes the command, retrieves a particular web page from it trove of pages and then formulates a properly formulated response to send back to the requesting client. A server’s response consists of three parts that are identical in structure to a request message: a response header line, one or more response header fields and an optional entity body. The response header line indicates the HTTP version used by the server, the status of the response and an explanation of status information. Following the response header line is the response header field, which returns information describing the servers attributes. The entity body returns the HTML page requested by the client machine. THREE – TIERED CLIENT/SERVER Three - tier architecture builds on the traditional two-tier approach. The first tier is the client, the second tier is the web server and the third tier consists of the application s and their associated databases that supply non HTML information to the server on request. From software perspective three tiers are client processes, web services and data services. Interactions between the server and client operate in the same way as they do in the two-tier architecture. The third tier provides comprehensive data services including database operations supported by database software, enterprise resource planning software services and other services needed to support a robust e-commerce server. The client request is formulated into an HTTP message ,sent over the internet to the server and examined by the server. If analysis of the request reveals that the help of server’s database is required, then the server sends a request to the database to search and retrieve the required information. Database information flows back to the server and the server formats the response into properly formed HTML page and sends a message back to the client over the Internet.