Two-phase Commit

Presumed Nothing

Protocol

1. coordinator sends PREPARE to notify transaction is terminated
2. Each cohort send COMMIT-VOTE or ABORT-VOTE
3. Coordinator collect response
4. Coordinator send result ABORT or COMMIT

Cohort Activity

Abort?

• Client mark first action of a transaction, cohort mark the transaction active when see first.
  • If terminate and not active -> ABORT
• Client and cohort record number of actions
  • Client send count to coordinator
  • If not match -> ABORT

Force Write

• Before COMMIT-VOTE
  • (because cohort need to ask for outcome if crashed)
• Before ACK
  • (because coordinator might forget outcome after ACK)

Protocol Database

• Transactions
  • cohorts & state:
    • committed, aborted, active
    • ACK, not ACK

Coordinator Activity

• Coordinator force result to disk before sending ``
• Write END after all cohorts ACK (not force)
  • No need to restore

Conclusion

• coordinator
  • 2 log write, 1 forced
  • 2 messages
• cohort
  • 2 log write, all forced
  • 2 messages

Presumed Abort

• No log if transaction aborted
• No ack for ABORT
• Message count same as PrN

Presumed Commit

• Coordinator force write when init
• Remove when complete
• Cohort force prepare, but non-force write before commit
  • because forgetting = commit
• coordinator (when commit)
  • 2 log write, 2 forced
  • 2 messages
• cohort
  • 2 log write, 1 forced
  • 1 messages

Read-only Transaction

• Coordinator sent PREPARE
  • PrA: no log writes
    • because data not forced write, forget = abort
  • PrC: 2 write, 1 force
    • 1 when init
    • 1 to mark it delete
• Cohorts send READ-ONLY-VOTE
  • no log writes

PrC Optimization

• MaintainIN
  • Store permanently
  • Might not have initiated
  • Assume abort
  • IN = REC - (COM & REC)
• COM
  • committed transaction
• REC
  • $REC=tid|ti{d}_l<tid<ti{d}_h$
• tid_stable
  • Highest committed tid
• Recovering IN
  • tid_h
    • Method 1: set tid_h = tid_stable + fixed amount
    • Method 2: dynamic set tid_h but log to disk
  • tid_l
    • if oldest transaction: increment tid_l and log to disk
  • COM & REC
    • Scan from tid_h to tid_l and check if committed
    • store in bit vector or list

nPRC Protocol

Abort

• Coordinator send PREPARE
• Cohort send COMMIT-VOTE, one send ABORT-VOTE
• Coordinator send ABORT
• Cohort send ACK
• Coordinator possibly write increase tid_l
• 4 msgs, 1 soft write

Update

• Coordinator send PREPARE
• Cohort send COMMIT-VOTE
• Coordinator send COMMIT
• Coordinator write increase tld_l & commit
• 3 msgs, 1 force write

Read Only

• Coordinator send PREPARE
• Cohort send READ-ONLY-VOTE
• 2 msgs

Recalcitrant Transaction

• Transaction too long (or cohort crash)
• Fall back to PrC for that transaction
  • Write init info to disk

Garbage Collect

• Maintain all corhort
• Send ABORT to all