Outline
• Agreement Protocols
– Introduction
– Lamport-Shostak-Pease algorithm
– Applications of agreement protocols
• Quiz
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Distributed Deadlock Detection
• In distributed systems, the system state can
be represented by a wait-for graph (WFG)
– In WFG, nodes are processes and there is a
directed edge from node P1 to node P2 if P1 is
blocked and is waiting for P2 to release some
resource
– The system is deadlocked if there is a directed
cycle or knot in its WFG
– The problem is how to maintain the WFG and
detect cycle/knot in the graph
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Distributed Deadlock Detection – cont.
• Centralized detection algorithms
• Distributed deadlock algorithms
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–
–
–
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Path-pushing
Edge-chasing
Diffusion computation
Global state detection
You need to know the basic ideas but not the
details about those algorithms
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Agreement Protocols
• In distributed systems, sites are often required to
reach mutual agreement
– In distributed database systems, data managers must
agree on whether to commit or to abort a transaction
– Reaching an agreement requires the sites have
knowledge about values at other sites
• Agreement when the system is free from failures
• Agreement when the system is prone to failure
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The System Model
• There are n sites in the distributed system and
at most m can be faulty
• Each site can communicate with all other
sites through message passing
• A receiver always knows the identity of the
sender of a message
• The communication is reliable
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The System Model – cont.
• Synchronous vs. asynchronous computations
– In a synchronous computation, processes in the
distributed system run in lock step manner
• In each step, a process receives messages from other
sites sent in the previous step, performs a
computation, and sends messages to other processes
– In an asynchronous computation, processes do
not proceed in lock steps
– Synchronous model is used for agreement
protocol
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The System Model – cont.
• Processor Failures
– Crash fault
– Omission fault
– Malicious fault
• Authenticated vs. non-authenticated messages
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The System Model – cont.
• Performance Analysis
– Time to reach an agreement under a given
protocol
– Message traffic measured by the number of
messages exchanged to reach an agreement
– Storage overhead
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Agreement Problems
• There are three well known agreement problems
– Byzantine agreement problem
– Consensus problem
– Interactive consistency problem
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Byzantine Agreement Problem
• A chosen process, called source process,
broadcasts its initial value to all other
processes
• A solution should
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The Consensus Problem
• In this problem, every process broadcasts its
initial value to all other processes, where the
initial values of different processes can be
different
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The Interactive Consistency Problem
• In this problem, every process broadcasts its
initial value to all other processes, where the
initial values of different processes can be
different
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Byzantine Agreement Problem – cont.
• The Upper Bound on the Number of Faulty Processors
– In a fully connected network, it is impossible to
reach consensus if the number of faulty processors
exceeds (n-1)/3
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Lamport-Shostak-Pease Algorithm
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Lamport-Shostak-Pease Algorithm – cont.
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Lamport-Shostak-Pease Algorithm – cont.
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Lamport-Shostak-Pease Algorithm – cont.
• Performance analysis
– Time
– Message traffic
– Storage overhead
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Fault-Tolerant Clock Synchronization
• The interactive convergence algorithm
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Atomic Commit in DDBS
• In atomic commit, sites of DDBS must agree
whether to commit or abort a transaction
– One site makes a local decision first and then it
starts the Byzantine agreement
– The sites determine a common decision based on
the agreed value
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