Web Information Systems (WE-DINF-11912): Lecture 02 - Web Architectures

Web Information Systems Web Architectures Prof. Beat Signer Department of Computer Science Vrije Universiteit Brussel http://vub.academia.edu/BeatSigner 2 December 2005

Web Information Systems  A web information system uses web technologies for information and service delivery  Modern web information systems and web architectures have to  be extensible to cater for emerging technolgies and new forms of interaction (e.g. multimodal interaction)  manage heterogenous information such as documents, structured data, multimedia resources, semi-structured information, ...  integrate various sources (e.g. DBs) via multi-tier architectures  offer a notion of state to reflect the current application context  deal with information about users and their environment (context)  ... October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 2

Basic Client-Server Web Architecture HTTP Request Internet HTTP Response Client Server  Effect of typing http://www.vub.ac.be in the broswer bar (1) use a Domain Name Service (DNS) to get the IP address for www.vub.ac.be (answer 134.184.129.2) (2) create a TCP connection to 134.184.129.2 (3) send an HTTP request message over the TCP connection (4) visualise the received HTTP response message in the browser October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 3

Web Servers  Tasks of a web server (1) setup connection (2) receive and process HTTP request (3) fetch resource (4) create and send HTTP response Worldwide Web Servers, http://news.netcraft.com (5) logging  The most prominent web servers are the Apache HTTP Server and Microsoft's Internet Information Services (IIS)  A lot of devices have an embedded web server  printers, WLAN routers, TVs, ... October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 4

Example HTTP Request Message GET / HTTP/1.1 Host: www.vub.ac.be User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-gb,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 5

Example HTTP Response Message HTTP/1.1 200 OK Date: Wed, 30 Sep 2009 13:01:41 GMT Server: Apache/1.3.33 (Unix) PHP/5.2.8 X-Powered-By: PHP/5.2.8 Keep-Alive: timeout=15, max=1000 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Vrije Universiteit Brussel</title> <link rel="shortcut icon" href="/favicon.ico" type="image/x-icon" /> ... </html> October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 6

HTTP Protocol  Request/response communication model  HTTP Request  HTTP Response  Communication always has to be initiated by the client  Stateless protocol  HTTP can be used on top of various reliable protocols  TCP is by far the most commonly used one  runs on TCP port 80 by default  Latest version: HTTP/1.1  Use HTTPS scheme for encrypted connections October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 7

Uniform Resource Identifier (URI)  A Uniform Resources Identifier (URI) uniquely identifies a resource.  There are two types of URIs  Uniform Resource Locator (URL) - contains information about the exact location of a resource - consists of a scheme, a host and the path (resource name) - e.g. http://wise.vub.ac.be/members/beat/ - problem: URL changes if resource is moved! • idea of Persistent Uniform Resource Locators (PURLs) [http://purl.oclc.org]  Uniform Resource Name (URN) - unique and location independent name for a resource - consists of a scheme name, a namespace identifier and a namespace-specific string (separated by colons) - e.g. urn:ISBN:3837027139 October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 8

HTTP Message Format HTTP/1.1 200 OK start line Date: Wed, 30 Sep 2009 13:01:41 GMT Server: Apache/1.3.33 (Unix) PHP/5.2.8 X-Powered-By: PHP/5.2.8 header field(s) Transfer-Encoding: chunked Content-Type: text/html blank line (CRLF) <html> ... message body (optional) </html>  Request and response messages have the same format HTTP_message = start_line , {header} , "CRLF" , {body}; October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 9

HTTP Request Message  Request-specific start line start_line = method, " " , resource , " " , version; method = "GET" , "HEAD" , "POST" , "PUT" , "TRACE" , "OPTIONS" , "DELETE"; resource = complete_URL | path; version = "HTTP/" , major_version, "." , minor_version;  Methods  GET : get a resource from the server  HEAD : get the header only (no body)  POST : send data (in the body) to the server  PUT : store request body on server  TRACE : get the "final" request (after it has potentially been modified by proxies)  OPTIONS : get a list of methods supported by the server  DELETE: delete a resource on the server October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 10

HTTP Response Message  Response-specific start line start_line = version , status_code , reason; version = "HTTP/" , major_version, "." , minor_version; status_code = digit , digit , digit; reason = string_phrase;  Status codes  100-199 : informational  200-299 : success (e.g. 200 for 'OK')  300-399 : redirection  400-499 : client error (e.g. 404 for 'Not Found')  500-599 : server error (e.g. 503 for 'Service Unavailable') October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 11

HTTP Header Fields  There exist general headers (for requests and responses), request headers, response headers, entity headers and extension headers  Some important headers  Accept - request header definining the Multipurpose Internet Mail Extensions (MIME) that the client will accept  User-Agent - request header specifying the type of client  Keep-Alive (HTTP/1.0) and Persistent (HTTP/1.1) - general header helping to improve the performance since otherwise a new HTTP connection has to be established for every single webpage element  Content-Type - entity header specifing the body's MIME type October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 12

HTTP Header Fields  Some important headers ...  If-Modified-Since - request header that is used in combination with a GET request (conditional GET); the resource is only returned if it has been modified since the specified date October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 13

MIME Types  The MIME type defines the request or response body's content and is used for the appropiate processing mime = toplevel_type , "/" , subtype;  Standard MIME types are registered with the Internet Assigned Numbers Authority (IANA) [RFC-2045] MIME Type Description text/plain Human-readable text without formatting information text/html HTML document image/jpeg JPEG-encoded image ... ... October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 14

HTTP Message Information  Various tools for HTTP message logging  e.g. HttpFox add-on for Firefox browser  Simple telnet connection telnet wise.vub.ac.be 80 (press Enter) GET /members/beat/ HTTP/1.1 Host: wise.vub.ac.be (press Enter 2 times)  Until 1999 the W3C has been working on the HTTP Next Generation (HTTP-NG) protocol as a replacement for HTTP/1.1  never introduced October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 15

Proxies Internet Proxy Client Server  Web proxy between client and server  acts as a server to the client and as a client to the server and may, for example, be specified in the browser settings; used for - firewalls and content filters - transcoding (on the fly transformation of HTTP message body) - content router (e.g. select optimal server in content distribution networks) - anonymous browsing, ... October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 16

Caches Client 1 1 Internet 2 1 2 Client 2 Server Proxy Cache  A proxy cache is a special type of proxy server  can reduce server load if multiple clients share the same cache  often multi-level hierarchies of caches (e.g. continent, country and regional level) with communication between sibling and parent caches as defined by the Internet Cache Protocol (ICP)  passive and active (prefetching) caches October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 17

Caches ...  Special HTTP cache control header fields  Expires - expiration date after which the cached resource has to be refetched  Cache-Control: max-age - maximum age of a document (in s) after it has been added to the cache  Cache-Control: no-cache - response cannot be directly served from the cache (has to be revalidated first)  ...  Validators  Last-modified time as validator - cache with resource that has been last modified at time t uses an If-Modified-Since t request for updates  Entity tags (ETag) - changed by the publisher if content has changed; If-None-Match etag request October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 18

Caches ...  Advantages  reduces latency and used network bandwidth  reduces server load (client and reverse proxy caches)  transparent to client and server  Disadvantages  additional resources (hardware) required  might get stale data out of the cache  creates additional network traffic if we use an aggressive caching approach (prefetching) but achieve a low cache hit rate  server loses control (e.g. access statistics) since no longer all requests have to be sent to the server October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 19

Tunnels SSL Internet SSL HTTP SSL Client HTTP[SSL] HTTP[SSL] SSL Server  Implement one protocol on top of another protocol  e.g. HTTP as a carrier for SSL connections  Often used to "open" a firewall to protocols that would otherwise be blocked  e.g. tunneling of SSL connections through an open HTTP port October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 20

Gateways HTTP Internet FTP HTTP/FTP HTTP Client FTP Server Gateway  A gateway can act as a kind of "glue" between applications (client) and resources (server)  translate between two protocols (e.g. from HTTP to FTP)  security accelerator (e.g. HTTPS/HTTP on the server side)  often the gateway and destination server are combined in a single application server (HTTP to server application translator) October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 21

Session Management  HTTP is a stateles protocol  Session (state) tracking solutions  use of IP address - problem: IP address is often not uniquely assigned to a single user  browser login - use of special HTTP authenticate headers - after a login the browser sends the user information in each request  URL rewriting - add information to URL in each request  hidden form fields - similar to URL rewriting but information can also be in body (POST request)  cookies - the server stores a piece of information on the client which is then sent back to the server with each request October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 22

Cookies  Introduced by Netscape in June 1994  A cookie is a piece of information that is assigned to a client on their first visit  list of <key,value> pairs  often just a unique identifier  sent via Set-Cookie or Set-Cookie2 HTTP response headers  The browser stores the info in a "cookie database" and sends it back every time the same server is accessed  Potential privacy issues  third-party websites might use persistent cookies for user tracking  Cookies can be disabled in the browser settings October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 23

Hypertext Markup Language (HTML) <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>Beat Signer: Interactive Paper, PaperWorks, Paper++, ...</title> </head> <body> Beat Signer is Assistant Professor of Computer Science at the VUB ... </body> </html>  Dominant markup language for webpages  If you never heard about HTML have a look at  http://www.w3schools.com/html/  More details in the exercise and in the next lecture October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 24

Dynamic Web Content  Often it is not enough to serve static web pages and content should be changed on the client or server side  Server-side processing  Common Gateway Interface (CGI)  Java Servlets  JavaServer Pages (JSP)  PHP: Hypertext Preprocessor (PHP)  ...  Client-side processing  JavaScript  Java Applets  Adobe Flash  ... October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 25

Common Gateway Interface (CGI) Program in CGI HTTP Request Perl, Tcl, C, C++, Java, .. Internet HTTP Response HTML Pages Client Server  CGI was the first server-side processing solution  transparent to the user  certain requests (e.g. /account.pl) are forwarded over CGI to a program by creating a new process  program processes the request and creates an answer and optional HTTP response headers October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 26

Common Gateway Interface (CGI) ...  CGI Problems  a new process has to be started for each request  if the CGI program for example acts as a gateway to a database, a new DB connection has to be established for each request which results in a very poor performance  FastCGI solves some of the problems by introducing persistent processes and process pools  CGI/FastCGI becomes more and more replaced by other technologies (e.g. Java Servlets) October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 27

Java Servlets Servlet Container HTTP Request Servlets Internet HTTP Response HTML Pages Client Server  A Java servlet is a Java class that has to extend the abstract HTTPServlet class  The Java servlet class is loaded by a servlet container and relevant requests (based on servlet binding) are forwarded to the servlet instance for further processing October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 28

Java Servlets ...  Main HttpServlet methods doGet(HttpServletRequest req, HttpServletResponse resp) doPost(HttpServletRequest req, HttpServletResponse resp) init(ServletConfig config) destroy()  Servlet life cycle  a servlet is initialised once via the init() method  the doGet(), doPost() methods may be executed multiple times (by different HTTP requests)  finally the servlet container may unload a servlet (upcall of the destroy() method before that happens)  Servlet container (e.g. Apache Tomcat) integrated with Web Server or as standalone component October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 29

Java Servlet Example package org.vub.wise; import java.io.*; import java.util.Date; import javax.servlet.http.*; import javax.servlet.*; public class HelloWorldServlet extends HttpServlet { public void doGet (HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { PrintWriter out = res.getWriter(); out.println("<html>"); out.println("<head><title>Hello World</title></head>"); out.println("<body>The time is " + new Date().toString() + "</body>"); out.println("</html>"); out.close(); } }  In the exercise you will learn how to process parameters etc. October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 30

JavaServer Pages (JSP)  A "drawback" of Java Servlets is that the whole page (HTML) has to be defined within the servlet  not easy to share tasks between web designer and programmer  Add program code through scriptlets and markup to existing HTML pages  These JSP documents are then either interpreted on the fly (Apache Tomcat) or compiled into Java Servlets  The JSP approach is similar to PHP or Active Server Pages (ASP)  Note that Java Servlets become more and more and enabling technology (as with JSP) October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 31

JavaScript  Interpreted scripting language for client-side processing  JavaScript functionality often embedded in HTML documents but can also be provided in separate files  JavaScript often used to  validate data (e.g. in a form)  dynamically add content to a webpage  process events (onLoad, onFocus, etc.)  change parts of the original HTML document  create cookies  ...  Note: Java and JavaScript are completely different languages! October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 32

JavaScript Example <html> <body> <script type="text/javascript"> document.write("<h1>Hello World!</h1>"); </script> </body> </html>  Please have a look at the following JavaScript tutorial to learn some of the basic constructs (operators, control statements, etc.)  http://www.w3schools.com/JS/  In the exercise session you will use JavaScript to implement a web application October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 33

Java Applets  A Java applet is a program delivered to the client-side in the form of Java bytecode  executed in the browser using a Java Virtual Machine (JVM)  an applet has to extend the Applet or JApplet class  runs in the sandbox  Advantages  the user automatically has always the most recent version  high security for untrusted applets  full Java API available  Disadvantages  requires a browser Java plug-in October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 34

Java Applets ...  Disadvantages ...  Only signed applets can get more advanced functionality - e.g. network connections to other machines than the source machine  More recently Java Web Start (JavaWS) is replacing Java Applets  program no longer runs within the browser - less problematic security restrictions - less browser compatibility issues  Java Chess Applet Example  http://english.op.org/~peter/ChessApp/ October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 35

Exercise 2  Hands-on experience with various web technologies  HTTP  Java Servlets  JavaServer Pages  Apache Tomcat  HTML  JavaScript  ... October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 36

References  David Gourley et al., HTTP: The Definitive Guide, O'Reilly Media, September 2002  R. Fielding et al., RFC2616 - Hypertext Transfer Protocol - HTTP/1.1  http://www.faqs.org/rfcs/rfc2616.html  N. Freed et al., RFC2045 - Multipurpose Internet Mail Extensions (MIME)  http://www.faqs.org/rfcs/rfc2045.html  HTML and JavaScript Tutorials  http://www.w3schools.com October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 37

References ...  Java Servlet Tutorial  http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/Servlets.html October 1, 2009 Beat Signer - Department of Computer Science - bsigner@vub.ac.be 38

Next Week Markup Languages 2 December 2005

Web Information Systems (WE-DINF-11912): Lecture 02 - Web Architectures

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