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Distributed Systems

Distributed Systems. Fault Tolerance Paul Krzyzanowski pxk@cs.rutgers.edu ds@pk.org. Except as otherwise noted, the content of this presentation is licensed under the Creative Commons Attribution 2.5 License. Faults. Deviation from expected behavior Variety of factors hardware software

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Distributed Systems

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  1. Distributed Systems Fault Tolerance Paul Krzyzanowski pxk@cs.rutgers.edu ds@pk.org Except as otherwise noted, the content of this presentation is licensed under the Creative Commons Attribution 2.5 License.

  2. Faults • Deviation from expected behavior • Variety of factors • hardware • software • operator • Network • Three categories • transient faults • intermittent faults • permanent faults

  3. Faults • Three categories • transient faults • intermittent faults • permanent faults • Any fault may be • fail-silent (fail-stop) • Byzantine • synchronous system vs. asynchronous system • E.g., IP packet versus serial port transmission

  4. Achieving fault tolerence Redundancy • information redundancy • Hamming codes, parity memory ECC memory • time redundancy • Timeout & retransmit • physical redundancy • TMR, RAID disks, backup servers

  5. How much fault tolerance? • 100 % fault-tolerance cannotbe achieved. • The closer we wish to get to 100%, the more expensive the system will be. • A system is k-fault tolerantif it can withstand k faults. • Need k+1 components with silent faultsk can fail and one will still be working • Need 2k+1 components with Byzantine faultsk can generate false replies: k+1 will provide a majority vote

  6. Active replication Technique for fault tolerance through physical redundancy No redundancy: Triple Modular Redundancy (TMR): Threefold component replication to detect and correct a single component failure

  7. Primary backup • One server does all the work • When it fails, backup takes over • Backup may ping primary with are you alive messages • Simpler design: no need for multicast • Works poorly with Byzantine faults • Recovery may be time-consuming and/or complex

  8. Agreement in faulty systems Two army problem • good processors - faulty communication lines • coordinated attack • multiple acknowledgement problem

  9. Agreement in faulty systems Byzantine Generals problem • reliable communication lines - faulty processors • n generals head different divisions • m generals are traitors and are trying to prevent others from reaching agreement • 4 generals agree to attack • 4 generals agree to retreat • 1 traitor tells the 1st group that he’ll attack and tells the 2nd group that he’ll retreat • can the loyal generals reach agreement?

  10. Agreement in faulty systems Byzantine Generals problem • Solutions require: • 3m+1 participants for m traitors (2m+1 loyal generals) • m+1 rounds of message exchanges • O(m2) messages • Costly solution!

  11. The end.

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