09 P2P Pastry

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Peer-to-Peer Architectures

Programmazione Avanzata di Rete

a.a. 2003/04

Mauceri Calogero

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16 May 2007 Mauceri Calogero P2P Architectures 2 / 29

Outline Peer to peer architecture

DHT

Pastry

Scribe

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Publish/Subscribe SystemsI want to receive msg with topic X

?P

S

S

S

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Client-Server Architecture

ServerP1

P2

S1

S2

S3

Simple

A lot of traffic and computation in the server No scalable

Single point of failure

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Peer-to-peer Architecture

Distributed system architecture:

No centralized control

Nodes are symmetric in function

Large number of heterogeneous and unreliable

nodes

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Lookup Problem

P

?

Store msgs with topic X

I want to receive msgs with topic X

S

R

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Centralized Approach

P

S

Single Point of failure

O(N) status in the coordinator node

R

C

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Flooding Approach

P

S

R

O(N) messages per lookup

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Distributed Hash Table

Requirements

Scalability

Adaptivity

Reliability

Efficiency Decentralized

Load Balance

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Distributed Hash Table Approach

P

S

R

Usually O(log(N)) messages per lookup

Building and maintenance of routing tables is needed

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Pastry

A peer-to-peer protocol example

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Pastry Design Node IDs and keys are 128bit numbers

with base 2b. Each node has a unique nodeID assigned

randomly when node joins the system.

Key are stored in the node with nodeID

numerically closest to the key.

Prefix routing scheme (numerically closest). Expected routing steps is O(log N).

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Pastry Routing TableNodes numerically closerto the present Node

Nodes physically closestto the present node

Prefix-based routing entries:

common prefix with10233102-next digit-rest ofNodeId

nodeId=10233102, b =2, l = 8

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Routing Check the leaf set

If destination present route to it directly Consult the routing table

Forward to a node whose identifier matches themessage key in at least one more digit

Forward to a node whose identifier is numericallycloser to the message key

This routing procedure converge

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Routing Example

65a1fc d13da3

d4213f

d462ba

d467ca

d471f1

Route(d46a1c)

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Node Arrival Example

65a1fc d13da3

d4213f

d462ba

d467ca

Route(d46a1c)

d46a1cI want to join to Pastry network

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Locality (1/2) when X joins the Pastry network, it send a join

message to A and finally the message will reachZ Because A is supposed to be physically near to X, the

entries in the 1st row of A are near to A, and thus nearto X (triangular inequality)

The nodes in the 2nd row of Bs routing table are nearto B, but much farther than X to B

These nodes are likely to be near to X, although notexactly the nearest

To compensate for the cascading error, X asks thenodes in its neighborhood set and routing table fortheir states, and find the entries nearest to X

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Locality (2/2)

XA

B

C

Z

Level 0

Level 1

Level 2

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Locating the nearest among k

nodes Goal:

among the k numerically closest nodes to a key, amessage tends to first reach a node near the client.

Problem: Since Pastry routes primarily based on nodeId

prefixes, it may miss nearby nodes with a differentprefix than the key.

Solution (using a heuristic): Based on estimating the density of nodeIds, it detects

when a message approaches the set of k and thenswitches to numerically nearest address basedrouting to locate the nearest replica.

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Scribe Topic-based publish-subscribe system

Peer-to-peer network of Pastry nodes

Application-level multicast

Best-effort dissemination of events

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Scribe Architecture Each topic has a unique topicId.

Rendezvous point The Scribe node with a nodeId numerically

closet to the topicId

Root of multicast tree.

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Create Topic

Rendez-vous point

nodeId = topicId

Route(CREATE,topicId)

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Subscription

Route(JOIN,topicId = 1100)

01111000

1001

1011

1100

11011111

0100 0111

1001

0100

1101

1100

1011

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Event Dissemination

Route through the

Pastry network using

the topicId as the

destination Dissemination along

the multicast tree

starting from the root 0111

1001

0100

1101

1100

1011

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Repairing the multicast tree

0111

1001

0100

1101

1100

1111

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References Distributed Hash Tables (DHTs): Design and Performance. Brad Karp. (ppt file)

A Survey on Distributed Hash Tables. Nitin Gupta. (html page)

Pastry: Scalable, distributed object location and routing for large-scale peer-to-

peer systems. Antony Rowstron and Peter Druschel. (pdf file)

Scribe: A large-scale and decentralized application-level multicastinfrastructure. Miguel Castro, Peter Druschel, Anne-Marie Kermarrec, and Antony

Rowstron. (pdf file)

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09 P2P Pastry