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Web Architectures - Lecture 2 - Web Technologies (1019888BNR)

Web Architectures - Lecture 2 - Web Technologies (1019888BNR)

This lecture forms part of the course Web Technologies given at the Vrije Universiteit Brussel.


Beat Signer

October 07, 2021


  1. 2 December 2005 Web Technologies Web Architectures Prof. Beat Signer

    Department of Computer Science Vrije Universiteit Brussel
  2. Beat Signer - Department of Computer Science - 2

    October 8, 2021 Basic Client-Server Web Architecture ▪ Effect of typing in the browser bar (1) use a Domain Name Service (DNS) to get the IP address for (answer (2) create a TCP connection to (3) send an HTTP request message over the TCP connection (4) visualise the received HTTP response message in the browser Internet Client Server HTTP Request HTTP Response
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    October 8, 2021 Web Server ▪ Tasks of a web server (1) setup connection (2) receive and process HTTP request (3) fetch resource (4) create and send HTTP response (5) logging ▪ The most prominent web servers are nginx and the Apache HTTP Server ▪ A lot of devices have an embedded web server ▪ printers, WLAN routers, TVs, ... Worldwide Web Servers,
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    October 8, 2021 Example HTTP Request Message GET / HTTP/1.1 Host: User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:92.0) Gecko/20100101 Firefox/92.0 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, br Connection: keep-alive Upgrade-Insecure-Requests: 1 DNT: 1 Cache-Control: no-cache ...
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    October 8, 2021 Example HTTP Response Message HTTP/1.1 200 OK Date: Thu, 07 Oct 2021 10:02:33 GMT Content-Encoding: gzip Content-Language: nl Content-Type: text/html; charset=utf-8 Cache-Control: public, max-age=10800 Keep-Alive: timeout=15, max=100 Etag: "1633600847-1 " Connection: Keep-Alive Transfer-Encoding: chunked ... <!DOCTYPE html> <html lang="nl" dir="ltr"> <head> ... <title>Vrije Universiteit Brussel | Redelijk eigenzinnig</title> <meta name="Description" content="Welkom aan de VUB" /> ... </html>
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    October 8, 2021 HTTP Protocol ▪ Request/response communication model ▪ HTTP Request ▪ HTTP Response ▪ Communication always has to be initiated by the client ▪ Stateless protocol (no sessions) ▪ 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 ▪ HTTPS scheme used for encrypted connections
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    October 8, 2021 Uniform Resource Identifier (URI) ▪ A Uniform Resource 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. - problem: the URL changes if resource is moved! • Persistent Uniform Resource Locators (PURLs) [] ▪ 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
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    October 8, 2021 HTTP Message Format ▪ Request and response messages have the same format <html> ... </html> HTTP/1.1 200 OK Date: Thu, 07 Oct 2021 10:02:33 GMT Server: Apache/2.4.50 (Ubuntu) X-Powered-By: PHP/7.4.24 Transfer-Encoding: chunked Content-Type: text/html header field(s) blank line (CRLF) message body (optional) start line HTTP_message = start_line , {header} , "CRLF" , {body};
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    October 8, 2021 HTTP Request Message ▪ Request-specific start line ▪ 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 start_line = method, " " , resource , " " , version; method = "GET" , "HEAD" , "POST" , "PUT" , "TRACE" , "OPTIONS" , "DELETE"; resource = complete_URL | path; version = "HTTP/" , major_version, "." , minor_version;
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    October 8, 2021 HTTP Response Message ▪ Response-specific start line ▪ 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') start_line = version , status_code , reason; version = "HTTP/" , major_version, "." , minor_version; status_code = digit , digit , digit; reason = string_phrase;
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    October 8, 2021 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 defining the Media Type (formerly MIME type) 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 specifying the body's MIME type
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    October 8, 2021 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
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    October 8, 2021 Media Types (MIME Types) ▪ The Media Type defines the request or response body's content (used for appropriate processing) ▪ 7 top-level media types ▪ Standard Media Types are registered with the Internet Assigned Numbers Authority (IANA) [RFC-6838] mediaType = toplevel_type , "/" , subtype; Media Type Description text/plain Human-readable text without formatting information text/html HTML document image/jpeg JPEG-encoded image ... ...
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    October 8, 2021 HTTP Message Information ▪ Various developer tools for HTTP message logging ▪ e.g. developer console window in Chrome (Ctrl+Shift+I) ▪ Simple telnet connection ▪ Until 1999 the W3C has been working on HTTP Next Generation (HTTP-NG) as a replacement for HTTP/1.1 ▪ never introduced ▪ HTTP/2.0 since May 2015 ▪ inspired by Google’s development of SPDY telnet 80 (press Enter) GET /beat-signer HTTP/1.1 (press Enter) Host: (press Enter 2 times)
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    October 8, 2021 Proxies ▪ A web proxy is situated between the client and the server ▪ acts as a server to the client and as a client to the server ▪ can 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, ... Internet Client Server Proxy
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    October 8, 2021 Caches ▪ 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 or active (prefetching) caches Internet Client 1 Server Proxy Cache Client 2 1 2 1 2
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    October 8, 2021 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 seconds) 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
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    October 8, 2021 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 active 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
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    October 8, 2021 Tunneling ▪ Transmit one protocol encapsulated inside 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 Internet SSL Client SSL Server SSL HTTP SSL HTTP[SSL] HTTP[SSL]
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    October 8, 2021 Gateways ▪ 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) Internet HTTP Client FTP Server HTTP/FTP Gateway HTTP FTP
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    October 8, 2021 Session Management ▪ HTTP is a stateless 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 the 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
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    October 8, 2021 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 HTTP response headers ▪ Browser stores the information 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
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    October 8, 2021 Hypertext Markup Language (HTML) ▪ Dominant markup language for webpages ▪ If you never heard about HTML have a look at ▪ ▪ More details in the exercise and in the next lecture <!DOCTYPE html> <html lang="en"> <head> <title>Beat Signer: Cross-Media Technology, DataPhys, ...</title> </head> <body> Beat Signer is Professor of Computer Science at the VUB and co-director of the WISE laboratory ... </body> </html>
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    October 8, 2021 Dynamic Web Content ▪ Often it is not enough to serve static web pages but 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) ▪ Node.js ▪ … ▪ Client-side processing ▪ JavaScript ▪ Java Applets ▪ ...
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    October 8, 2021 Common Gateway Interface (CGI) ▪ CGI was the first server-side processing solution ▪ transparent to the user ▪ certain requests (e.g. / are forwarded via CGI to a program by creating a new process ▪ program processes the request and creates an answer with optional HTTP response headers Internet Client Server HTTP Request HTTP Response Program in Perl, Tcl, C, C++, Java, .. HTML Pages CGI
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    October 8, 2021 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)
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    October 8, 2021 Java Servlets ▪ 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 a servlet binding) are forwarded to the servlet instance for further processing Internet Client Web Server HTTP Request HTTP Response HTML Pages Servlet Container Servlets
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    October 8, 2021 Java Servlets ... ▪ Main HttpServlet methods ▪ 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) either integrated with web server or as standalone component doGet(HttpServletRequest req, HttpServletResponse resp) doPost(HttpServletRequest req, HttpServletResponse resp) init(ServletConfig config) destroy()
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    October 8, 2021 Java Servlet Example ▪ In the exercise you will learn how to process parameters etc. package org.vub.wise; import*; 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(); } }
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    October 8, 2021 JavaServer Pages (JSP) ▪ A "drawback" of Java servlets is that the whole page (e.g. 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 an enabling technology (as with JSP)
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    October 8, 2021 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!
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    October 8, 2021 JavaScript Example ▪ More details about JavaScript in lecture 6 and in the exercise session <html> <body> <script type="text/javascript"> document.write("<h1>Hello World!</h1>"); </script> </body> </html>
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    October 8, 2021 Node.js ▪ Server-side JavaScript ▪ low-level, comparable to functionality offered by Servlets ▪ handling post/get requests, database, sessions, … ▪ Write your entire app in one language ▪ however, server-side and client-side code still separated ▪ Built-in web server (no need for Apache, Tomcat, etc.) ▪ High modularity ▪ packages can be added for additional functionality (via npm) ▪ many available frameworks (Express, Passport, Sequelize,…) ▪ HTTP utility methods (sessions, routing, ...) ▪ template engines (Jade, EJS, …)
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    October 8, 2021 Java Applets ▪ A Java applet is a program delivered to the client side in the form of Java bytecode and runs in a sandbox ▪ executed in the browser using a Java Virtual Machine (JVM) ▪ an applet has to extend the Applet or JApplet class ▪ Advantages ▪ the user automatically always has the most recent version ▪ high security for untrusted applets ▪ full Java API available for trusted signed applets ▪ Disadvantages ▪ requires a browser Java plug-in ▪ only signed applets can get more advanced functionality - e.g. network connections to other machines than the source machine
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    October 8, 2021 Java Applets ... ▪ 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 ▪ Math and Physics Applet Examples ▪
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    October 8, 2021 Web Application Frameworks ▪ There exist dozens of web application frameworks and we will present various of these framework in 2 weeks! A web application framework is a software framework that is designed to support the development of dynamic web- sites, web applications, web services and web resources. The framework aims to alleviate the overhead associated with common activities performed in web development. For example, many frameworks provide libraries for database access, templating frameworks and session management, and they often promote code reuse. []
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    October 8, 2021 Exercise 2 ▪ Hands-on experience with the HTTP protocol
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    October 8, 2021 References ▪ David Gourley et al., HTTP: The Definitive Guide, O'Reilly Media, September 2002 ▪ M. Belshe et al., RFC7540 - Hypertext Transfer Protocol Version 2 (HTTP/2.0) ▪ ▪ N. Freed et al., RFC6838 - Media Type Specifications and Registration Procedures ▪ ▪ HTML and JavaScript Tutorials ▪
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    October 8, 2021 References ... ▪ M. Knutson, HTTP: The Hypertext Transfer Protocol (refcardz #172) ▪ ▪ W. Jason Gilmore, PHP 5.4 (refcardz #23) ▪ ▪ Java Servlet Tutorial ▪
  40. 2 December 2005 Next Lecture HTML5 and the Open Web