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

Beat Signer
October 02, 2023

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 02, 2023
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  1. 2 December 2005
    Web Technologies
    Web Architectures
    Prof. Beat Signer
    Department of Computer Science
    Vrije Universiteit Brussel
    beatsigner.com

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  2. Beat Signer - Department of Computer Science - [email protected] 2
    October 3, 2023
    Basic Client-Server Web Architecture
    ▪ Effect of typing http://www.vub.be in the browser bar
    (1) use a Domain Name Service (DNS) to get the IP address for
    www.vub.ac.be (answer 134.184.0.178)
    (2) create a TCP connection to 134.184.0.178
    (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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  3. Beat Signer - Department of Computer Science - [email protected] 3
    October 3, 2023
    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, https://news.netcraft.com

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  4. Beat Signer - Department of Computer Science - [email protected] 4
    October 3, 2023
    Example HTTP Request Message
    GET / HTTP/1.1
    Host: www.vub.be
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0)
    Gecko/20100101 Firefox/118.0
    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
    ...

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  5. Beat Signer - Department of Computer Science - [email protected] 5
    October 3, 2023
    Example HTTP Response Message
    HTTP/1.1 200 OK
    Date: Mon, 03 Oct 2022 14:39:43 GMT
    Content-Encoding: gzip
    Content-Language: en
    Content-Type: text/html; charset=UTF-8
    Cache-Control: public, max-age=10900
    Keep-Alive: timeout=15, max=100
    Etag: "1633600847-1 "
    Connection: Keep-Alive
    Transfer-Encoding: chunked
    ...



    ...
    Vrije Universiteit Brussel | Redelijk eigenzinnig

    ...

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  6. Beat Signer - Department of Computer Science - [email protected] 6
    October 3, 2023
    HTTP Protocol
    ▪ Request/response communication model
    ▪ HTTP Request
    ▪ HTTP Response
    ▪ Communication always must 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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  7. Beat Signer - Department of Computer Science - [email protected] 7
    October 3, 2023
    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. https://vub.academia.edu/BeatSigner
    - problem: the URL changes if resource is moved!
    • Persistent Uniform Resource Locators (PURLs) [https://archive.org/services/purl/]
    ▪ 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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  8. Beat Signer - Department of Computer Science - [email protected] 8
    October 3, 2023
    HTTP Message Format
    ▪ Request and response messages have the same format

    ...

    HTTP/1.1 200 OK
    Date: Mon, 02 Oct 2023 16:59:21 GMT
    Server: Apache/2.4.56 (Ubuntu)
    X-Powered-By: PHP/8.2.11
    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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  9. Beat Signer - Department of Computer Science - [email protected] 9
    October 3, 2023
    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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  10. Beat Signer - Department of Computer Science - [email protected] 10
    October 3, 2023
    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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  11. Beat Signer - Department of Computer Science - [email protected] 11
    October 3, 2023
    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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  12. Beat Signer - Department of Computer Science - [email protected] 12
    October 3, 2023
    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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  13. Beat Signer - Department of Computer Science - [email protected] 13
    October 3, 2023
    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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  14. Beat Signer - Department of Computer Science - [email protected] 14
    October 3, 2023
    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 wise.vub.ac.be 80 (press Enter)
    GET /beat-signer HTTP/1.1 (press Enter)
    Host: wise.vub.ac.be (press Enter 2 times)

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  15. Beat Signer - Department of Computer Science - [email protected] 15
    October 3, 2023
    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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  16. Beat Signer - Department of Computer Science - [email protected] 16
    October 3, 2023
    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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  17. Beat Signer - Department of Computer Science - [email protected] 17
    October 3, 2023
    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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  18. Beat Signer - Department of Computer Science - [email protected] 18
    October 3, 2023
    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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  19. Beat Signer - Department of Computer Science - [email protected] 19
    October 3, 2023
    Tunnelling
    ▪ 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. tunnelling of SSL connections through an open HTTP port
    Internet
    SSL Client SSL Server
    SSL
    HTTP
    SSL
    HTTP[SSL] HTTP[SSL]

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  20. Beat Signer - Department of Computer Science - [email protected] 20
    October 3, 2023
    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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  21. Beat Signer - Department of Computer Science - [email protected] 21
    October 3, 2023
    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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  22. Beat Signer - Department of Computer Science - [email protected] 22
    October 3, 2023
    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 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
    ▪ persistent cookies with long lifetime
    ▪ third-party cookies for user tracking across different websites
    ▪ Cookies can be disabled in the browser settings

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  23. Beat Signer - Department of Computer Science - [email protected] 23
    October 3, 2023
    Hypertext Markup Language (HTML)
    ▪ Dominant markup language for webpages
    ▪ If you never heard about HTML have a look at
    ▪ https://www.w3schools.com/html/
    ▪ More details in lecture 4 and the exercise session



    Beat Signer: Cross-Media Technology, DataPhys, ...


    Beat Signer is Professor of Computer Science at the VUB
    and director of the WISE laboratory ...


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  24. Beat Signer - Department of Computer Science - [email protected] 24
    October 3, 2023
    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
    ▪ Jakarta Server Pages (JSP)
    ▪ PHP: Hypertext Preprocessor (PHP)
    ▪ Node.js
    ▪ …
    ▪ Client-side processing
    ▪ JavaScript
    ▪ Java Applets
    ▪ ...

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  25. Beat Signer - Department of Computer Science - [email protected] 25
    October 3, 2023
    Common Gateway Interface (CGI)
    ▪ CGI was the first server-side processing solution
    ▪ transparent to the user
    ▪ certain requests (e.g. /account.pl) 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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  26. Beat Signer - Department of Computer Science - [email protected] 26
    October 3, 2023
    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 became more and more replaced by other
    technologies (e.g. Java Servlets)

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  27. Beat Signer - Department of Computer Science - [email protected] 27
    October 3, 2023
    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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  28. Beat Signer - Department of Computer Science - [email protected] 28
    October 3, 2023
    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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  29. Beat Signer - Department of Computer Science - [email protected] 29
    October 3, 2023
    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("");
    out.println("Hello World");
    out.println("The time is " + new Date().toString() + "");
    out.println("");
    out.close();
    }
    }

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  30. Beat Signer - Department of Computer Science - [email protected] 30
    October 3, 2023
    Jakarta Server 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 have become more and more an
    enabling technology (as with JSP)

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  31. Beat Signer - Department of Computer Science - [email protected] 31
    October 3, 2023
    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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  32. Beat Signer - Department of Computer Science - [email protected] 32
    October 3, 2023
    JavaScript Example
    ▪ More details about JavaScript in lecture 6 and in the
    exercise session


    <br/>document.write("<h1>Hello World!</h1>");<br/>


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  33. Beat Signer - Department of Computer Science - [email protected] 33
    October 3, 2023
    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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  34. Beat Signer - Department of Computer Science - [email protected] 34
    October 3, 2023
    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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  35. Beat Signer - Department of Computer Science - [email protected] 35
    October 3, 2023
    Java Applets ...
    ▪ Java Web Start (JavaWS) replaced Java Applets
    ▪ program no longer runs within the browser
    - less problematic security restrictions
    - less browser compatibility issues
    ▪ Math and Physics Applet Examples
    ▪ http://www.falstad.com/mathphysics.html

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  36. Beat Signer - Department of Computer Science - [email protected] 36
    October 3, 2023
    Web Application Frameworks
    ▪ There exist dozens of web application frameworks and
    we will present various of these frameworks next week
    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.
    [https://en.wikipedia.org/wiki/Web_application_framework]

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  37. Beat Signer - Department of Computer Science - [email protected] 37
    October 3, 2023
    Exercise 2
    ▪ Hands-on experience with the HTTP protocol

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  38. Beat Signer - Department of Computer Science - [email protected] 38
    October 3, 2023
    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)
    ▪ http://www.faqs.org/rfcs/rfc7540.html
    ▪ N. Freed et al., RFC6838 - Media Type Specifications
    and Registration Procedures
    ▪ http://www.faqs.org/rfcs/rfc6838.html
    ▪ HTML and JavaScript Tutorials
    ▪ https://www.w3schools.com

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  39. Beat Signer - Department of Computer Science - [email protected] 39
    October 3, 2023
    References ...
    ▪ M. Knutson, HTTP: The Hypertext Transfer
    Protocol (refcardz #172)
    ▪ https://dzone.com/refcardz/http-hypertext-transfer-0
    ▪ W. Jason Gilmore, PHP 5.4 (refcardz #23)
    ▪ https://dzone.com/refcardz/php-54-scalable
    ▪ Java Servlet Tutorial
    ▪ https://www.tutorialspoint.com/servlets/

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  40. 2 December 2005
    Next Lecture
    Web Application Frameworks

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