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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 hoc connections. 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-timedata, 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 peer nodes 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 peersperform 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 PDAs

•can be used to write to any other JSquigglewithin the same local network or far away bysimply 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

•Discussed•Draw Sketch•Add Peer•Add Message

•Not discussed:•Search Peer, Get Info,•Save Sketch, Open Sketch, and•Print 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 commands

from 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 andanimated, 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 the

Scalable 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 peersperform 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 aSketchService to process (which will invokeour 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 the

Scalable Vector Graphics SVG standard forvector 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 pattern

•History of commands for redrawing

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Command Message

•Send command (with state) in a message

•Both sender and receiver contain code forprocessing the command

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Command Message

• Invokes a command on a remote server

•Message handling code separate from codeto process command messages

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Message Ordering

•Correct ordering of messages ensures byusing Lamport’s 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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Lamport’s Algorithm

if a before bthen Ta < Tb

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Lamport Clock

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Use in JSquiggle

•State contains the peer’s local state,including its timestamp