Paper

• 1-safe: transactions propigated
• 2-safe: backup uses commit protocol to commit T synchronously
• 1-safe has a dependency problem: if T1 depends on T2, but T2 didn't get applied, then T1 is lost, too
• solution: predeclare dependencies (possible because T has already been committed once) on backup

Lecture

• 1-safe backups correctness:
  • internal consistancy on backup site
  • backup will reflect a subset of the committed Ts
• 2-safe correctness:
  • in sync, but possibly applies transaction before primary is able to respond: COMMIT
• dependency reconstruction:
  • use locking on backups, BUT we need to ensure locking occurs in the same order
  • solution: assign tickets (local numbers, monotonically increasing) to T
  • modified lock manager waits for the correct ordering of Ts
• epoch installs:
  • if commit(T) <= X, commit T
  • if prepare(T) <= X, but commit(T) > X
    • if T's primary peer was coordinator, do not commit
    • else check with the backup of T's coordinator B':
      • if B' is commit T in X, then commit
      • else do not
  • otherwise don't commit T (defer to next epoch)
• UNDERSTAND SCENARIOS! especially bumping
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