1. Outline Distributed Mutual Exclusion Distributed Deadlock Detection
Token-based algorithms
Distributed Deadlock Detection
Review and overview
Agreement Protocols
Introduction
Lamport-Shostak-Pease algorithm
Dolev et al’s algorithm
Applications of agreement protocols
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2. Announcements There will be a quiz next Tuesday
It will be an open book quiz
It covers the things we have covered so far
It will be at the end of the class
You can turn in your homework #2 next Tuesday
However, no submission after that date will be accepted
Please do not run your programs on diablo
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3. 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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4. Distributed Deadlock Detection – cont.
Centralized detection algorithms
Distributed deadlock algorithms
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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5. 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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6. 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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7. 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
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8. The System Model – cont. Processor Failures
Crash fault
Omission fault
Malicious fault
Authenticated vs. non-authenticated messages
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9. 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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10. Agreement Problems There are three well known agreement problems
Byzantine agreement problem
Consensus problem
Interactive consistency problem
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11. Byzantine Agreement Problem
A chosen process, called source process, broadcasts its initial value to all other processes
A solution should
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12. 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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13. 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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14. Lamport-Shostak-Pease Algorithm
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15. Lamport-Shostak-Pease Algorithm – cont.
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16. Lamport-Shostak-Pease Algorithm – cont.
Performance analysis
Time
Message traffic
Storage overhead
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17. Fault-Tolerant Clock Synchronization
The interactive convergence algorithm
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18. Fault-Tolerant Clock Synchronization – cont.
The interactive consistency algorithm
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