Distributed computing



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distcomp

CHANG SCHEME

  • Tools: Token-passing scheme + positive acknowledgments + negative acknowledgements.
  • Ack with counter
  • Sender
  • Token Site
  • Broadcast
  • Increment counter Commit message

BEAUTY OF NEGATIVE ACKNOWLEDGMENT

  • How does a site discover that it hasn’t received a message?
  • Non-token site knows that it has missed a message if there is a gap in the counter values that it has received. In that case, it requests that information from the token site (negative ack).
  • Overhead: one positive acknowledgment per broadcast message vs. one acknowledgment per site per message in naïve implementation.

TOKEN TRANSFER

  • Token transfer is a standard message. The target site must acknowledge. To become a token site, the target site must guarantee that it has received all messages since the last time it was a token site.
  • Detect failure at a non-token site, when it fails to accept token responsibility.
  • Here is token
  • I can take it
  • token
  • site
  • token

REVISIT ASSUMPTIONS

  • Sites do not lie about their state (i.e., no malicious sites; could use authentication).
  • Sites tell you when they fail (e.g. through redundant circuitry) or by not responding.
  • If there is a network partition, then no negative ack would occur, so must keep message m around until everyone has acquired the token after m was sent.

LAMPORT Times, Clocks paper

  • What is the proper notion of time for Distributed Systems?
  • Time Is a Partial Order
  • The Arrow Relation
  • Logical Clocks
  • Ordering All Events using a tie-breaking Clock
  • Achieving Mutual Exclusion Using This Clock
  • Correctness
  • Criticisms
  • Need for Physical Clocks
  • Conditions for Physical Clocks
  • Assumptions About Clocks and Messages
  • How Do We Achieve Physical Clock Goal?

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