CS122B: Projects in Databases and Web Applications Winter 2019 Notes 06: Web-App Architectures Professor Chen Li Department of Computer Science

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

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

CGI CGI (Common Gateway Interface) Goal: Example: 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 life cycle Main Process CGI-based Web Server e.g., apache Request for CGI 1 Child Process for CGI1 Request for CGI 2 Child Process for CGI2 Request for CGI 1 Child Process for CGI1

CGI Life Cycle (cont) 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.

Processes vs Thread Process Thread Typically independent Has considerably more state info than thread Separate address spaces Interact only through system inter-process communication (IPC) Thread Subsets of a process Multiple threads within a process share process state, memory, etc. Threads share their address space Process is very expensive from the perspective of the OS!

Problems with CGI 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 (cont) 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.

Ways to improve cgi performance FastCGI Other solutions

FastCGI Main Process CGI-based Web Server 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

FastCGI (cont) 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: /  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

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

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.

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.

Servlet Package Hierarchy JAVAX Servlet GenericServlet HttpServlet Form Servlet Admin Servlet CGI Servlet Error Servlet File Servlet Image Servlet

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

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

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

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.

Sharing stateful info within a session The first thread of an HTTP request keeps stateful information as key-value pairs on the heap of the server Since they are on the heap, they remains on the server even after the first thread is finished Threads of later requests can retrieve these key-value pairs from the heap

Comparisons CGI/FastCGI: Servlets: use multiple processes to handle separate programs and/or separate requests Outside the server Servlets: handled by separate threads within 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.

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

Other techniques: JSP

ASP example Other techniques: ASP (Microsoft) 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!