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A Web server is a program running on the server machine, which accepts requests from a Web browser and sends back results in the form of HTML documents The browser and Web server communicate by a protocol called the HyperText Transfer Protocol (HTTP) HTTP provides powerful features, beyond the simple transfer of documents The most important feature is the ability to execute programs, with arguments supplied by the user, and deliver the results back as an HTML document As a result, a Web server can easily act as an intermediary to provide access to a variety of information services A new service can be created by creating and installing an application program that provides the service The common gateway interface (CGI) standard de nes how the Web server communicates with application programs The application program typically communicates with a database server, through ODBC, JDBC, or other protocols, in order to get or store data Figure 213 shows a Web service using a three-tier architecture, with a Web server, an application server, and a database server Using multiple levels of servers increases system overhead; the CGI interface starts a new process to service each request, which results in even greater overhead
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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HTTP browser
Three-tier Web architecture
Most Web services today therefore use a two-tier Web architecture, where the application program runs within the Web server, as in Figure 214 We study systems based on the two-tier architecture in more detail in subsequent sections Be aware that there is no continuous connection between the client and the server In contrast, when a user logs on to a computer, or connects to an ODBC or JDBC server, a session is created, and session information is retained at the server and the client until the session is terminated information such as whether the user was authenticated using a password and what session options the user set The reason that HTTP is connectionless is that most computers have limits on the number of simultaneous connections they can accommodate, and if a large number of sites on the Web open connections, this limit would be exceeded, denying service to further users With a connectionless service, the connection is broken as soon as a request is satis ed, leaving connections available for other requests Most information services need session information For instance, services typically restrict access to information, and therefore need to authenticate users Authentication should be done once per session, and further interactions in the session should not require reauthentication To create the view of such sessions, extra information has to be stored at the client, and returned with each request in a session, for a server to identify that a request is part of a user session Extra information about the session also has to be maintained at the server This extra information is maintained in the form of a cookie at the client; a cookie is simply a small piece of text containing identifying information The server sends a cookie to the client after authentication, and also keeps a copy locally Cookies sent to different clients contain different identifying text The browser sends the cookie automatically on further document requests from the same server By comparing the cookie with locally stored cookies at the server, the server can identify the request as
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
VII Other Topics
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The McGraw Hill Companies, 2001
Web Interfaces to Databases
network
web server and application server database server
HTTP browser data server
Figure 214 Two-tier Web architecture
part of an ongoing session Cookies can also be used for storing user preferences and using them when the server replies to a request Cookies can be stored permanently at the browser; they identify the user on subsequent visits to the same site, without any identi cation information being typed in
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