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DistributedProgramming in Java

Networking (4)

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Peer-to-Peer

A peer-to-peer (or P2P ) computer network is anetwork that relies primarily on the computing power andbandwidth of the participants in the network rather thanconcentrating it in a relatively low number of servers.

P2P networks are typically used for connecting nodes vialargely ad hocconnections. Such networks are useful formany purposes. Sharing content files (see file sharing) containing audio,

video, data or anything in digital format is very common, and real-time

data, such as telephony traffic, is also passed using P2Ptechnology

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Properties of P2P

A pure peer-to-peer network does not have the notion ofclients or servers, but only equal peernodes thatsimultaneously function as both "clients" and "servers" tothe other nodes on the network.

This model of network arrangement differs from the client-server model where communication is usually to and from acentral server.

A typical example for a non peer-to-peer file transfer is anFTP server where the client and server programs are quitedistinct, and the clients initiate the download/uploads andthe servers react to and satisfy these requests.


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Acceptor-Connector

Decouple the connection of peercomponents from the processing the peers

perform after they are connected

[BH]

active

passive

passive

active

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Case Study

JSquiggle is a peer-to-peer program written inJava for handheld devices.

Runs on desktops and PDAscan be used to write to any other JSquiggle

within the same local network or far away by

simply put their IP addresses.

No centralized control each device is a peerin a P2P network

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Open protocols uses HTTP and XML formessaging between peers

Ordered messages uses logical timestampto ensure messages are in order

http://www.aspectsolution.com/sketch


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Usage Scenario

Peer connect to any other peer, eg using awireless network (WLAN)

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How to use it

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Use Cases

DiscussedDraw SketchAdd PeerAdd MessageNot discussed:Search Peer, Get Info,Save Sketch, Open Sketch, andPrint Sketch

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Draw Sketch

Precondition Peers are added in the peer lists

Postcondition Sketch message is constructed and sent to peer

1. Capture sketch activites until pen is released 2. Increase the logical time for new event 3. Construct sketch message 4. Send the message to each peer on the list

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Add Peer

Precondition Peer's IP address is known

Post condition

Peers are added to peer list

1. User inputs peer's IP address 2. User inputs peer's port number 3. Add the peer into list of peers


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Add Message

Precondition Peer is started at correct port

Post condition Sketch messages are processed/drawn on the screen

1. Remote peer sends out sketch message 2. System gets and parses the message 3. If logical time is bigger, advance the logical clock 4. Execute the message (ie update sketch)

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Sending a Message

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SketchCanvas

Press mouse: start, dragging: add new points,release mouse: create and send path


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Command Builder

Separate the construction of commandsfrom their representation

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SketchSender

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SketchSender

Send command to all known peers


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SketchPeer

Follows Half-Sync/Half-Async pattern

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SketchPeer.SenderThread

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Scalable Vector Graphics

Scalable Vector Graphics (SVG) is anXML markup language for describing two-

dimensional vector graphics, both static and

animated, and either declarative or scripted.

It is an open standard created by the WorldWide Web Consortium.


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Command Format

Commands are encoded in XML using theScalable Vector Graphics SVG standard forvector graphics

M = move-to, L = link

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Command Format

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Receiving a Message

parse


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Acceptor-Connector

Decouple the connection of peercomponents from the processing the peers

perform after they are connected

[BH]

active

passive

passive

active

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JSquiggle

Creates a SketchCanvas (a ServiceHandler) anda SketchServer (an Acceptor)

The drama unfolds when the SketchServerreceives a connection request ...

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SketchServer

A typical Acceptor, SketchServer accepts aclient request and hands it over to a

SketchService to process (which will invoke

our SketchCanvas ... eventually)


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SketchService

Parse the command, then process it ...

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Command Format

Commands are encoded in XML using theScalable Vector Graphics SVG standard for

vector graphics

M = move-to, L = link

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SketchParser

Decipher the command using the XmlParserfrom org.kxml (small footprint)


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SketchReceiver

Complete the Command Message patternHistory of commands for redrawing

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Command Message

Send command (with state) in a messageBoth sender and receiver contain code for

processing the command

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Command Message

Invokes a command on a remote serverMessage handling code separate from code

to process command messages


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Message Ordering

Correct ordering of messages ensures byusing Lamports timestamp algorithm

Each peer increments clock between anytwo successive events

When sending message m, use current clockvalue as timestamp Tm

Upon receiving message m, check thetimestamp Tm, and match the clock to Tm,

if current clock is less than Tm

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Lamports Algorithm

if a before b

then Ta < Tb

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Lamport Clock


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Use in JSquiggle

State contains the peers local state,including its timestamp

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