Pastry
Antony Rowstron and Peter Druschel
Presented By
David Deschenes
Introduction
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General substrate for P2P applications
Provides object location and routing
Characteristics
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Completely decentralized
Fault-resilient
Scalable
Reliability
Design
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Circular Node Space
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Nodes randomly assigned unique 128 bit identifiers
Uniform distribution of identifiers
Identifiers indicate position in space
Messages and Keys
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Messages assigned N-bit keys
Each message routed to the node whose identifier is
numerically closest to its assigned key
Key generation is application specific
Node State
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Leaf Set
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Routing Table
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L closest nodes in the node space
Identifies 2b-1 hosts whose identifiers match
the current node identifier in n bits
Neighborhood Set
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M closest nodes according to a proximity
metric
Network Organization
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Node Arrival
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New node acquires the identifier X and routes a join
message with key X through node A
New nodes initialize their state based upon state
shared by nodes that route its join message
Node Departure
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If a member of a node’s leaf set is lost, the leaf set is
expanded outward to find a new node
If a routing table entry is lost a node in the same row
is consulted for a replacement
Maintaining Locality
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Proximity based on a scalar metric
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E.g. IP routing hops, geographic distance
Metric determined by application
Assumed that the triangulation inequality
holds for the metric
Connections to nodes with greater
proximity are desired
Facilitated by the neighborhood set
Message Routing
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Check the leaf set
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If destination present route to it directly
Consult the routing table
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Forward to a node whose identifier matches
the message key in at least one more bit
Forward to a node whose identifier is
numerically closer to the message key
Evaluation
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Number of routing hops scales with the
network size (log2bN) – Figure 4.
Routes are only 30% to 40% longer
despite minimal routing tables – Figure 6.
Able to locate one of the top two nodes
92% of the time
Minimally impacted by node failure –
Figure 10.