Lecture8 - COT 5611 Operating Systems Design Principles...

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COT 5611 Operating Systems Design Principles Spring 2012 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 5:00-6:00 PM
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Lecture 8 - Monday February 6 n Reading assignment: the class notes “Distributed systems-basic concepts” and “Petri Nets” available online. n Last time ¨ Distributed snapshots ¨ Enforced modularity  the client server paradigm ¨ Consensus protocols Lecture 8 2/16/12 2
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Today n Consensus protocols n Modeling concurrency – Petri Nets Lecture 8 2/16/12 3
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Consensus protocols and consensus service n Consensus  agree to one of several alternates proposed by a number of agents. n No fault-tolerant consensus protocol can guarantee progress but protocols which guarantee freedom from inconsistencies (safety) have been developed. n Paxos  protocols to reach consensus based on a finite state machine approach. n Consensus service clients send requests to processes and propose a value and wait for a response; the goal is to get the set of n processes to reach consensus on a single proposed value. n The messages are sent through a network. n A Byzantine failure in a distributed system could be; ¨ an omission failure, e.g., a crash failure, failure to receive a request or to send a response; ¨ it could also be a commission failure, e.g., process a request incorrectly, corrupt the local state, and/or send an incorrect or inconsistent response to a request Lecture 8 4 2/16/12
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Assumptions about the processors and the network n The processes run on processors and communicate through a network. n The processors and the network may experience failures, but not Byzantine failures n The processors: ¨ (operate at arbitrary speeds; ¨ have stable storage and may rejoin the protocol after a failure; ¨ can send messages to any other processor. n The network: ¨ may lose, reorder, or duplicate messages; ¨ messages are sent asynchronously and may take arbitrary long time to reach the destination. Lecture 8 5 2/16/12
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Actors and the flow of messages n Each process advocates a value proposed by a client and could play one, two, or all three of the following roles: ¨ Acceptors  are the persistent, or fault-tolerant, memory of the system who decide which value to choose; ¨ Proposers  propose a value sent by a client to be chosen by the acceptors; ¨ Learners learn which value was chosen and act as the replication factor of the protocol; n The leader  an elected proposer. n Quorum  a subset of all acceptors; any two quorums share at least one member. n When each process plays all three roles, proposer, acceptor, and learner, the flow of messages can be described as follows: ¨ clients send messages to a leader; ¨ during normal operations the leader receives the client's command, assigns it a new command number i, and then begins the i-th instance of the consensus algorithm by sending messages to a set of acceptor processes. n
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This note was uploaded on 02/15/2012 for the course COP 5611 taught by Professor Staff during the Spring '08 term at University of Central Florida.

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Lecture8 - COT 5611 Operating Systems Design Principles...

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