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Web Application Architectures

This note discusses various web application architectures, including CGI, server extension APIs, and Java servlets, with examples such as Amazon.com and Google.com.

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Web Application Architectures

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  1. CS122B: Projects in Databases and Web ApplicationsWinter2018 Notes 06: Web-App Architectures Professor Chen Li Department of Computer Science

  2. Outline • Web applications: overview • CGI and its variants • Server Extension APIs • Java servlets • Comparisons

  3. Web Applications • Create dynamic content for a web page • keyword search on a document archive • E-commerce • … • Example: • Amazon.com • Google.com

  4. CGI (Common Gateway Interface) One of the first techniques for creating dynamic HTML pages Goal: Define a standard method for a server to talk with external applications Example: http://www.tigernt.com/cgi-bin/ecdict.cgi CGI

  5. CGI life cycle CGI-based Web Server e.g., apache Main Process Request for CGI 1 Child Process for CGI1 Request for CGI 2 Child Process for CGI2 Request for CGI 1 Child Process for CGI1

  6. Server receives a request for a CGI program Server creates a new process to run the CGI program Server passes information to the program: via environment variables and standard input. CGI Life Cycle (cont)

  7. Comparison between processes and threads http://www.cs.berkeley.edu/~istoica/classes/cs194/05/notes/4-ProcThread.pdf Processes are expensive

  8. Expensive to create a process for each request: requires time and significant server resources, limits the # of requests a server can handle concurrently Stateless: No history remembered “Memento”  A big problem for web-based development Solutions: cookies Still expensive Problems with CGI

  9. Once the CGI program starts: It cannot interact with the web server It cannot take advantage of the server's abilities once it begins execution Reasons: the program is running in a separate process. Example, a CGI script cannot write to the server's log file. Problems with CGI (cont)

  10. FastCGI Other solutions Ways to improve cgi performance

  11. FastCGI CGI-based Web Server Main Process Request for CGI 1 Single Child Process for CGI1 Request for CGI 2 Child Process for CGI2 Request for CGI 1 http://www.rebol.com/docs/fastcgi.html

  12. Developed by a company “Open Market” FastCGI creates a single persistent process for each FastCGI program It eliminates the need to create a new process for each request. FastCGI (cont)

  13.  No need to start multiple processes for different requests for the same fastcgi program  Still needs one process for each cgi program. Thus not scalable to handle many concurrent requests  It does nothing to help the FastCGI program more closely interact with the server. Not implemented by some of the more popular servers, I.e., Microsoft's Internet Information Server. Not very portable FastCGI: /

  14. Proprietary solutions that work only with certain web servers. Platform dependent Examples: ASP (Microsoft) Other solutions

  15. Server Extension APIs life cycle Web Server with Server Extension API Main Process Server Extension 1 Server Extension 2 Request for Server Extension 1 Request for Server Extension 2 Request for Server Extension 1

  16. Write server extensions that enhance or change the base functionality of the server Allowing the server to handle tasks that were once relegated to external CGI programs. Use linked C or C++ code Thus can run extremely fast; and make full use of the server's resources. Server Extension APIs 

  17. Difficult to develop and maintain Pose significant security and reliability hazards a crashed server extension can bring down the entire server. Proprietary server extensions are tied to the server API on a particular OS Not very portable Server Extension APIs: 

  18. Developed by Microsoft With ASP, an HTML page on the web server can contain snippets of embedded code: usually VBScript or Jscript it's possible to use nearly any language This code is read and executed by the web server before it sends the page to the client. ASP is optimized for generating small portions of dynamic content. Support for ASP is built into Microsoft Internet Information Server Version 3.0 and above Active Server Pages (ASP)

  19. http://www.w3schools.com/asp/showasp.asp?filename=demo_text http://www.w3schools.com/asp/showasp.asp?filename=demo_formatting <html><body><%response.write("Hello World!")%></body></html> The browser shows: Hello World! ASP example

  20. A Servlet is a Service Unit Java Servlet Based Web Server Web Server Main Process JVM Request for Servlet 1 Servlet1 Thread Request for Servlet 2 Thread Servlet2 Thread Request for Servlet 1

  21. Life With Servlets A servlet is a service unit which runs inside a web server, extending its functionality beyond the basic GET, POST, DELETE and PUT operations.

  22. Inside a Servlet A servlet does not have a main() method. a Java program: yes an applet: no Certain methods of a servlet are invoked by the server in the process of handling requests. Each time the server dispatches a request to a servlet, it invokes the servlet's service() method.

  23. Servlet Package Hierarchy JAVAX Servlet GenericServlet HttpServlet Form Servlet AdminServlet CGIServlet ErrorServlet FileServlet Image Servlet

  24. The Servlet Interface Servlet interface { void init(ServletConfig sc) throws ServletException; void service(ServletRequest req, ServletResponse res); throws ServletException, IOException; void destroy(); }

  25. GenericServlet Class http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/GenericServlet.html GenericServlet subclass Server request service( ) response Override the service() method

  26. HttpServlet Class http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpServlet.html java.lang.Object javax.servlet.GenericServletjavax.servlet.http.HttpServlet A function: protected  void doGet(HttpServletRequest req, HttpServletResponse resp)

  27. Request and Response Objects They form the core objects for communication. HttpServletRequest contains all the details about the client request to the servlet. E.g., request.getParameter(”paramName”); HttpServletResponse contains all the information for replying back to the client. For e.g., the output stream.

  28. Comparisons • CGI/FastCGI: • use multiple processes to handle separate programs and/or separate requests • Outside the server • Servlets: • handled by separate threadswithin the web server process. • Thus more efficient and scalable. • can interact very closely with the server to do things that are not possible with CGI scripts.

  29. Comparisons (cont) • Java Servlets are portable: • across operating systems (java) • across web servers, since all major web servers support servlets. • Thus, Java servlets offer a good platform for web-application development

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