An evaluation of ring-based algorithms for the Eventually Perfect failure detector class

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An evaluation of ring-based algorithms for the
Eventually Perfect failure detector class

• Joachim Wieland
• Mikel Larrea
• Alberto Lafuente
• The University of
• the Basque Country

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Contents

• Motivation
• Unreliable failure detectors
• System model
• Communication-efficient implementations of ?P
• A non communication-efficient approach
• Performance evaluation
• Conclusion

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Motivation

• FLP impossibility result (Fischer, Lynch, and
  Paterson) Consensus cannot be solved
  deterministically in an asynchronous system
  subject to even a single process crash
• Possibility result (Chandra and Toueg) Consensus
  can be solved in an asynchronous system subject
  to failures with an unreliable failure detector

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Motivation (2)

• Evaluate different ring-based algorithms for ?P
• Two kinds of performance parameters
• Communication efficiency
• Quality of service
• Two families of algorithms
• Communication-efficient ?P some optimizations
• Non communication-efficient ?Q transformation
  to ?P
• modular approach
• designed with quality of service in mind

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Unreliable failure detectors

• Distributed oracle that provides (possibly
  incorrect) hints about the operational status of
  other processes
• Abstractly characterized in terms of two
  properties completeness and accuracy
• Completeness characterizes the degree to which
  crashed processes are suspected by correct
  processes
• Accuracy characterizes the degree to which
  correct processes are not suspected, i.e.,
  restricts the false suspicions that a failure
  detector can make

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Unreliable failure detectors (2)
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System model

• Finite set of n processes ? p1, p2, ..., pn
  that communicate only by message-passing
• Every pair of processes is connected by two
  unidirectional and reliable communication links
• Processes can fail by crashing. Once a process
  crashes, it does not recover
• Processes are arranged in a logical ring
• Partially synchronous system

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Communication-efficient implementations of ?P

• A basic communication-efficient algorithm (LLW0)
• each process p sends heartbeats to the processes
  in the ring between itself (excluded) and its
  successor succp (included)
• p monitors its predecessor predp by hearing
  heartbeats from it
• upon timeout on predp, p suspects it and monitors
  the predecessor of predp
• if p erroneously suspected q, p starts monitoring
  q again
• processes propagate the list of suspicions around
  the ring, piggybacked in the heartbeats
• upon reception of the list of suspicions from
  predp, p builds a new list by merging the list
  received with its local suspicions. Then, p sets
  succp to its nearest and non suspected process

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Communication-efficient implementations of ?P (2)
LLW0
pred1 p6, succ1 p2
p5
p5
pred2 p1 succ2 p3
pred6 p5 succ6 p1
pred6 p4 succ6 p1
p5
pred5 p4 succ5 p6
pred3 p2 succ3 p4
p5
pred4 p3, succ4 p5
pred4 p3, succ4 p6
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Communication-efficient implementations of ?P (3)

• Providing a faster stabilization of the ring
  (LLW1) sending sporadic one-to-one messages
• upon timeout on predp, p sends (START, p) to
  pred(predp). Upon reception of this message,
  pred(predp) sets its successor to p
• when p learns that it is erroneously suspecting
  q, p sends (START, q) to ps current predp. Upon
  reception of this message, ps current predp sets
  its successor to q
• Broadcasting suspicions to reduce the detection
  latency (LLW2) sending sporadic one-to-all
  messages
• upon timeout on predp, p sends (SUSPICION, predp)
  to all processes
• when a process p is being erroneously suspected,
  it sends (REFUTATION, p) to all processes

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Communication-efficient implementations of ?P (4)
LLW1
pred1 p6, succ1 p2
p5
p5
pred2 p1 succ2 p3
pred6 p5 succ6 p1
pred6 p4 succ6 p1
p5
pred5 p4 succ5 p6
pred3 p2 succ3 p4
p5
pred4 p3, succ4 p5
pred4 p3, succ4 p6
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Communication-efficient implementations of ?P (5)
LLW2
pred1 p6, succ1 p2
p5
p5
pred2 p1 succ2 p3
pred6 p5 succ6 p1
pred6 p4 succ6 p1
p5
pred5 p4 succ5 p6
pred3 p2 succ3 p4
p5
pred4 p3, succ4 p5
pred4 p3, succ4 p6
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A non communication-efficient approach

• A basic ring-based algorithm implementing ?Q
• identical monitoring schema to LLW0
• the list of suspicions does not circulate
  around the ring
• every process p periodically sends (START, p)
  to predp. When a process p receives (START,
  new_succ), it sets succp to new_succ
• Providing a faster stabilization of the ring
• Transforming ?Q into ?P
• propagating suspicions through the ring (LLWQP1)
• broadcasting suspicions to reduce the detection
  latency (LLWQP2)

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A non communication-efficient approach (2)
LLWQ
pred1 p6, succ1 p2
pred2 p1 succ2 p3
pred6 p5 succ6 p1
pred6 p4 succ6 p1
pred5 p4 succ5 p6
pred3 p2 succ3 p4
pred4 p3, succ4 p5
pred4 p3, succ4 p6
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Performance evaluation
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Performance evaluation (2)
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Performance evaluation (3)
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Performance evaluation (4)
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Performance evaluation (5)
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Performance evaluation (6)
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Conclusion

• Evaluation of two families of heartbeat-,
  ring-based algorithms implementing ?P
• Communication-efficient family
• n links are eventually used
• Non communication-efficient family
• modular approach
• n C links are eventually used (with 1 C n)
• better quality of service