Plethora: Infrastructure andSystem Design

Introduction• Peer-to-Peer (P2P) networks:

– Self-organizing distributed systems– Nodes receive and provide services cooperatively – No predetermined client/server roles

• Key features:– Scalable– Adaptive and reconfigurable– Leverage technology trends

(network/processor/memory)

• Key problems:– Locating and routing objects efficiently– Consistency management– Fault-Tolerance

Location and Routing - DHT• Apply structure to the network:

– Inputs hashed to a key– Each node responsible for a subset of keys

• Nodes maintain small routing tables

• Queries routed to neighboring nodes that ensure progress towards the ultimate destination.

Location and Routing - DHT0XX

X1XX

X2XX

X3XX

X

2321

2032

2001

0112

START

0112 routes a message

to key 2000.

First hop fixes first digit (2)

Second hop fixes second

digit (20)END

2001 closest live node to

2000.

Motivation

• Virtualization destroys locality.

• Query responses do not contain locality information.

• Recent studies show that queries for multiple keys in P2P networks follow a Zipf-like distribution.

• Exploit geographic locality.

• Build highly-distributed collaborative environments and applications:– information lifecycle– distributed file systems– software distribution– archival storage and disaster recover

IP Addresses as Virtual IDs

• Incorporate locality into overlay networks:– Explore addressing scheme of the underlying network.

• In most cases, nodes with IP addresses that are numerically close are also physically close.

• Organization of the Internet in ASs. By correcting a few bits in each hop, the last hops would be inside an AS.

• Issues:– IP space is not uniformly populated by peers.– Load imbalance at the peers.– The upper bound of O(log n) can no longer be guaranteed.

IP Addresses as Virtual IDs

IP Addresses as Virtual IDs

IP Addresses as Virtual IDs

• 2,420 nodes. 20 keys per node.

Plethora• Two-level overlay

– One global overlay– Several local overlays

• Global overlay is the main repository of data. – Global overlay helps nodes organize themselves into local

overlays.

• Local overlays exploit the organization of the Internet in ASs .

– Size of the local overlay is controlled by an overlay leader.– Uses efficient distributed algorithms for merging and splitting

local overlays.

Cache Organization

Simulation Setup• Internet topology generated using GT-ITM

topology generator.

• 10,000 overlay nodes selected randomly from the hosts.

• NLANR web proxy trace with 500,254 objects.

• Zipf distribution parameters: {0.75, 0.80, 0.85, 0.90, 0.95}

• Local cache size: 5MB (LRU replacement policy).

IP Addresses as Virtual IDs

Simulation Results

Zipf-parameter Cache Hit Ratio Gain

0.75 76% 31.0%

0.80 79% 33.5%

0.85 81% 36.0%

0.90 83% 38.7%

0.95 86% 41.3%

Summary

• IP addresses as virtual IDs:– Overlays with good locality properties.– Non-uniform realworld distribution:

• severe load imbalance• no bounded latency

• Plethora Routing Core:– Two-level overlay architecture.– Local overlays are created to cluster nodes that are close in the

underlying network.

• Significant performance gains– Low maintenance overhead

Latency Hiding

• For large-scale collaborative and distributed applications:– latency effects are still an issue.– need resiliency in the presence of network

failures.

• Record updates using a transactional versioning system:– Aggregate updates– Distributed conflict resolution

Versioning and Transaction Model

T1T1

T2T2 T3T3

TTkk

TTk+1k+1 TTk+2k+2

Global homeGlobal home Local HomeLocal Home

commitcommit

Development Issues

• Implementation of versioning trees– Efficient update and commit protocols – Dealing with failures (node, network)

• Object structure of the repository to exploit versioning semantics

• Guarantees on object access and consistency of updates