1 MSCS 237 Overview of web technologies (A specific type of distributed systems)

2 OVERVIEW OF WEB SERVER TECHNOLOGY The World Wide Web (WWW) builds on existing computers, networks, and services to provide universal access to information through the use of Web browsers.

3 WWW TECHNOLOGIES The WWW builds on three important technologies: Universal Resource Locators (URLs). Hypertext Transfer Protocol (HTTP). Hypertext Markup Language (HTML).

4 UNIVERSAL RESOURCE LOCATOR URLs provide an address system for locating information anywhere on the Internet. The following URL points to a file at the domain yahoo.com. It specifies a web page that should be retrieved using the HTTP protocol.

5 HYPERTEXT TRANSFER PROTOCOL HTTP provides the communication framework between clients and servers on the Web. Defines a simple request/response conversation between the client and the server using methods. The GET method is used to retrieve information (e.g., a Web page). The POST method is used to store information on a server (e.g., input to a script).

6 HYPERTEXT MARKUP LANGUAGE HTML is used to create the multimedia hypertext documents that are displayed on Web browsers. HTML defines the structure and layout of a Web document by using a variety of tags and attributes. The following example shows the correct structure of a simple html page. (What the document is about) (All the information of the current document)

7 STATIC VERSUS DYNAMIC WEB PAGES Information displayed on a Web browser can either be retrieved from: Static files that are retrieved and displayed in a browser, or Dynamically generated files where the contents of a page displayed on a browser can vary due to the execution of software components.

8 DYNAMIC WEB PAGES Dynamically generated pages are especially useful for electronic commerce and other Web applications that use information from a database as a backend to drive the information displayed on a Web page. Examples: amazon.com ebay.com Numerous real-time airline flight tracking systems.

9 METHODS FOR DYNAMIC WEB PAGE GENERATION Common Gateway Interface (CGI) Java Applets Java Servlets Active Server Pages (ASP) Server-side JavaScript Java Server Pages (JSP)

10 COMMON GATEWAY INTERFACE (CGI) CGI scripts were one of the first methods for dynamic Web page generation. A Web server passes requests to an external program. The output of the external program is returned as a static file. Typically used with the Perl programming language. Inefficient since it creates a separate process for each request. CGI scripts pose security concerns since they execute as regular programs on the Web server system.

11 CGI - BASED WEB SERVER (Diagram from Web Server Technology, Morgan Kaufmann, N. Yeager and R. McGrath, 1996)

12 JAVA APPLETS Applets are Java programs that are embedded directly into HTML pages. An application designed to be executed from within another application. Applet byte code is downloaded to the client and executed by the browser when a browser downloads a page with a reference to an applet. Download time can be unacceptable for large applets. The applet will not run if the client is not using a compatible browser.

13 JAVA SERVLETS A Java servlet is a Java class that can be loaded dynamically to expand the functionality of a server. The Java servlet generates full HTML pages to be displayed on a Web browser. Executes in the Java Virtual Machine (JVM) on the server (thus solving the security problem associated with CGI).

14 JAVA SERVLETS vs. CGI & APPLETS Executes on the server (thus solving browser compatibility problems associated with applets). More efficient than CGI since servlets do not create separate processes for each request. Portable across operating systems and Web servers.

15 JAVA SERVLET-BASED WEB SERVER (Diagram from Web Server Technology, Morgan Kaufmann, N. Yeager and R. McGrath, 1996)

16 JAVA SERVLETS EXAMPLE Java servlets can also be used as server-side includes where the servlet is embedded inside of HTML. The JDBC example is a servlet based application that provides the capability to connect to any relational database registered as an ODBC data source.

17 ACTIVE SERVER PAGES (ASP) An HTML page that contains embedded code, such as VBScript or JScript. Separates the user interface from the underlying content generation. The embedded code is executed by the Web server before the page is sent to the browser.

18 SERVER-SIDE JAVASCRIPT A server-side scripting language from Netscape that is embedded in HTML pages. Web pages are pre-compiled on the server-side to improve performance. Limited Support.

19 JAVA SERVER PAGES (JSP) Extension of the Java Servlet API. Uses Java as a scripting language. Separates the user interface from the underlying content generation. Platform and server independent. Usually used in combination with Java Beans, which are Java objects with set and get methods.

20 References 3, ,00.asphttp:// 3, ,00.asp htmlhttp://java.sun.com/features/1997/nov/rmi. html ng/Programming/JDCBook/dist.htmlhttp://java.sun.com/developer/onlineTraini ng/Programming/JDCBook/dist.html r_proceeding.pdf