web architectures - web technologies (1019888bnr)
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2 December 2005
Web TechnologiesWeb Architectures
Prof. Beat Signer
Department of Computer Science
Vrije Universiteit Brussel
http://www.beatsigner.com
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 2October 7, 2016
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 heterogeneous 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)
...
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 3October 7, 2016
Basic Client-Server Web Architecture
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
Internet
Client Server
HTTP Request
HTTP Response
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 4October 7, 2016
Web Server
Tasks of a web server(1) setup connection
(2) receive and processHTTP request
(3) fetch resource
(4) create and sendHTTP response
(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, ...
Worldwide Web Servers, http://news.netcraft.com
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 5October 7, 2016
Example HTTP Request Message
GET / HTTP/1.1Host: www.vub.ac.beUser-Agent: Mozilla/5.0 (Windows NT 6.1; rv:24.0) Gecko/20100101
Firefox/24.0Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8Accept-Language: en-gb,en;q=0.5Accept-Encoding: gzip, deflateAccept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7Connection: keep-alive
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 6October 7, 2016
Example HTTP Response Message
HTTP/1.1 200 OKDate: Thu, 03 Oct 2013 17:02:19 GMTServer: Apache/2.2.14 (Ubuntu)X-Powered-By: PHP/5.3.2-1ubuntu4.15Content-Language: nlSet-Cookie: lang=nl; path=/; domain=.vub.ac.be; expires=Mon, 18-Sep-2073
17:02:16 GMTContent-Type: text/html; charset=utf-8Keep-Alive: timeout=15, max=987Connection: Keep-AliveTransfer-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>
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 7October 7, 2016
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/2.0 (May 2015)
HTTPS scheme used for encrypted connections
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 8October 7, 2016
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!
• idea of Persistent Uniform Resource Locators (PURLs) [https://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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 9October 7, 2016
HTTP Message Format
Request and response messages have the same format
<html>...</html>
HTTP/1.1 200 OK
Date: Thu, 03 Oct 2013 17:02:19 GMTServer: Apache/2.2.14 (Ubuntu)X-Powered-By: PHP/5.3.2-1ubuntu4.15Transfer-Encoding: chunkedContent-Type: text/html
header field(s)
blank line (CRLF)
message body (optional)
start line
HTTP_message = start_line , {header} , "CRLF" , {body};
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 10October 7, 2016
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;
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 11October 7, 2016
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;
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 12October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 13October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 14October 7, 2016
Media Types
The Media Type (MIME type) defines the request or
response body's content (used for appropiate processing)
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
... ...
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 15October 7, 2016
HTTP Message Information
Various tools for HTTP message logging e.g. HttpFox add-on for Firefox browser
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
recently HTTP/2.0 has been released
- inspired by Goggle’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)
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 16October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 17October 7, 2016
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, countryand 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
ServerProxy CacheClient 2
1
2
12
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 18October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 19October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 20October 7, 2016
Tunnels
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
Internet
SSL Client SSL Server
SSL
HTTP
SSL
HTTP[SSL] HTTP[SSL]
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 21October 7, 2016
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 ServerHTTP/FTP
Gateway
HTTP
FTP
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 22October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 23October 7, 2016
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
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 24October 7, 2016
Hypertext Markup Language (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
<!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 Associate Professor of Computer Science at the VUB ...</body></html>
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 25October 7, 2016
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)
...
Client-side processing JavaScript
Java Applets
Adobe Flash
...
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 26October 7, 2016
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
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 27October 7, 2016
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)
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 28October 7, 2016
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 Server
HTTP Request
HTTP Response
HTML Pages
Servlet
Container
Servlets
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 29October 7, 2016
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()
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 30October 7, 2016
Java Servlet Example
In the exercise you will learn how to process parameters etc.
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();}}
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 31October 7, 2016
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)
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 32October 7, 2016
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!
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 33October 7, 2016
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>
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 34October 7, 2016
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 always has the most recent version
high security for untrusted applets
full Java API available
Disadvantages requires a browser Java plug-in
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 35October 7, 2016
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
Math and Physics Applet Examples http://www.falstad.com/mathphysics.html
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 36October 7, 2016
Exercise 2
Hands-on experience with the HTTP protocol
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 37October 7, 2016
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., RFC6838 - Media Type Specifications
and Registration Procedures http://www.faqs.org/rfcs/rfc6838.html
HTML and JavaScript Tutorials http://www.w3schools.com
Beat Signer - Department of Computer Science - bsigner@vub.ac.be 38October 7, 2016
References ...
M. Knutson, HTTP: The Hypertext Transfer
Protocol (refcardz #172) http://refcardz.dzone.com/refcardz/http-hypertext-transfer-0
W. Jason Gilmore, PHP 5.4 (refcardz #23) http://refcardz.dzone.com/refcardz/php-54-scalable
Java Servlet Tutorial http://www.tutorialspoint.com/servlets/
2 December 2005
Next LectureHTML5 and the Open Web Platform
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