Distributed computing


INVARIANTS while first TM active



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distcomp

INVARIANTS while first TM active

  • No server can be in the willing state while any other server (live or failed) is in the committed state
  • No server can be in the aborted state while any other server (live or failed) is in the ready-to-commit state
  • Some may have aborted
  • No one has committed.
  • Active
  • Willing
  • Ready to Commit
  • Committed
  • No one has aborted. Some may have committed.

CONTRAST WITH TWO-PHASE COMMIT

  • Some may have aborted
  • Active
  • Ready to Commit
  • Committed
  • Some may have committed.

RECOVERY IN THREE-PHASE COMMIT after first TM fails or slows down too much

  • What the newly elected TM does:
  • Any
  • Ready-to-Commit
  • or
  • Committed
  • Any
  • Aborted?
  • Send
  • ABORT
  • Some Dead
  • All Willing
  • Send
  • Committed
  • Send
  • ABORT;
  • All Servers Alive
  • Send
  • PREPARE; COMMITTED

ROAD MAP: KNOWLEDGE LOGIC AND CONSENSUS

  • Knowledge Logic: consensus.
  • Fail-stop.
  • Network perhaps.
  • Commitment
  • fail-stop
  • site only

EXAMPLE: COORDINATED ATTACK

  • Forget about computers. Think about a pair of allied generals A and B.
  • They have previously agreed to attack simultaneously or not at all. Now, they can only communicate via carrier pigeon (or some other unreliable medium).
  • Suppose general A sends the message to B
  • “Attack at Dawn”
  • Now, general A won’t attack alone. A doesn’t know whether B has received the message. B understand A’s predicament, so B sends an acknowledgment.
  • “Agreed”
  • Attack
  • Agreed
  • A
  • A
  • B
  • B

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