Part 1: Overview of Web Systems Part 2: Peer-to-Peer Systems

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Part 1 Overview of Web SystemsPart 2
Peer-to-Peer Systems

• Internet Computing Workshop
• Tom Chothia

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Introduction

• Part 1
• Summary how Web pages, JavaScript, Servlets and
  JSP are related.
• How to run your own Servlets and JSP in Netbeans
• Part 2
• Peer to Peer Systems.
• Very high level overview of the design of
  Napster, Gnutella and Bittorrent.
• P2P case studies, BBCs iPlayer.

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4 Key Web Technologies

• Web Pages
• HTML file passed via HTTP.
• JavaScript
• Code in HTML file, runs on client
• Servlets
• Server code that generates HTML file.
• JavaServer Pages
• Java code in HTML that runs on the server.

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Web Pages
HTTP GET
Client
Server
HTTP file
HTML
HTML
Display
HTML file
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JavaScript

• HTML is very basic.
• JavaScript is a simple scripting language.
• You can put into HTML web pages.
• It runs on the client.

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JavaScript
HTTP GET
Client
Server
HTMLJS
HTTP file
HTMLJS
JS interpreter
HTML J.S. file
Display
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Servlets

• Sometimes you dont want to run code on the
  client.
• The webpage doesnt have to be a file.
• A servlet is Java code that runs on the server to
  handle a request from the client.
• Servlet a little be of a server.

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HTTPServlets

• The javax.servlet.http.HttpServlet class lets you
  build a servlet to handle web page request.
• When someone requests the page the doGet method
  is called.
• The doGet method must write HTML output.

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Servlets
HTTP GET
Client
Server
HTTP file
HTML
HTML
Java code Servlet doGet
Display
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JavaServer Pages (JSP)

• When writing servlets you can end up writing lots
  of HTML.
• JSP lets you put the Java in a HTML file.
• Great for pages with a lot of HTML and a little
  Java.

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JSP
Server
HTTP GET
Client
HTML
HTTP file
HTML
Java code Servlet doGet
Display
HTML Java file
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4 Key Web Technologies

Language Where Processed
Web Pages HTML N/A
JavaScript HTML JavaScript Client
Servlets Java Server
JavaServer Pages HTML Java Server
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Containers

• To run JSP or Servlets you need a JSP/Servlets
  container.
• This is a web server that can run servlets and
  compile JSP.
• E.g. Apache TomCat, Glassfish.
• Good IDEs can run these for you.

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Running JSP in Netbeans

• Create a Web Application project
• File gt New Project gt Java Web gt Web Application
• This creates the JSP page index.jsp
• Edit index.html (it can call other Java code).
• Click Run, to start a web server and open the
  page in a browser.

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Running Servlets in NetBeans

• Create a web app, as before.
• Create a servlet file File gt New File gt Web gt
  Servlet
• Click Next, Next and enter the URL Pattern
• The servlet is run when a client requests this
  URL.
• Edit the Servlet file. E.g. write the doGet()
  method.
• Click run to start up the server.
• Enter the URL in the browser to see your page.

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Part 2 Peer-to-Peer Systems

• Internet Computing Workshop

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File Sharing P2P Networks

• Napster
• The first big file sharing network.
• Online between 1999-2001.
• Central Coordinator.
• Gnutella
• Most popular file-sharing network in mid 2000s.
• No central coordinator.
• Unstructured Network.
• BitTorrent
• Lots of little P2P networks.
• Single tracker coordinates each download.
• Rewards up-loaders.

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Napster
I

• Peer A asks the database for x.
• The database tells A which peer has the file.
• A downloads the file from that peer.
• A tells the database it has the file.
• Other peers can then download the file from A

H
G
F
E
x
D
x
C
x?
B
A,E
A
E
x?
x?
I have x
x?
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Napster User Figures
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Original Gnutella

• In newer networks peers record the IP address of
  other peers.
• A searcher sends a request to all of its
  neighbours.
• This is forwarded to all of there neighbours, up
  to a fixed hops.

A
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Original Gnutella

• The search request includes As IP address.
• Any peer with the requested file contacts A
  directly.
• Peer A may then request the file.

A
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BitTorrent Setting Up
Tracker
Web Server

• Peer A wants to share a find.
• A must find a tracker to coordinate the
  download.
• A makes a torrent file,
• The name of the file.
• The hash of the data.
• The IP address of the tracker.
• A must put the torrent somewhere people can
  find it. E.g., on a web server.
• A tells the tracker is wants to share the file.

A
T
A
T
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BitTorrent Sharing
Tracker
Web Server

• Peer B finds the torrent file.
• B finds the tracker IP and connects to it.
• The tracker
• tells B the IP of the file sharers.
• And adds B address to the list of sharers
• B connects to A
• and then gets some pieces of the file.

A
,B
T
B
T
A
A
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BitTorrent Sharing
Tracker
Web Server

• Peer C connects in the same way.
• A will try to hand out different copies of the
  file.
• A may disconnect.
• C may get the rest of the file from B.
• D can connect and the sharing continues without
  A.

A,
B
,C
T
B
T
A
A
C
A,B
T
D
B,C
T
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Case Study BBCs iPlayer

• The BBC wants to provide TV programs via the Web.
• Streaming all that data costs a lot.
• Therefore the BBC tried out a peer-to-peer
  system for downloads.
• They used a centrally controlled coordinator and
  digital rights management.

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Case Study BBCs iPlayer

• The idea is
• If Im only downloading then I cost the BBC money
  (they pay for bandwidth).
• If I upload as much as I download then I cost the
  BBC very little.
• If I upload more than I download then I am
  providing a service for the BBC.
• In theory, a company can save millions.

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Case Study BBCs iPlayer

• In fact
• iPlayer users like to stream TV, not download.
• The P2P system cant provide the exact
  information at the exact time to watch live.
• In December 2008 the BBC changed their download
  service to direct download via HTTP.
• Key point P2P has great potential, but it has
  its problems

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Next Time

• AJAX and other new web systems.