Chapter 5 Distributed Transaction Management

Transcription

1 Chapter 5 Distributed Transaction Management Table of Contents Definition of a Transaction Properties of Transactions Supporting Atomicity of Distributed Transactions Chapter5-1 1

2 1. Definition of a Transaction Transaction The unit of execution of database operations SQL command: COMMIT, ROLLBACK partially committed committed active failed aborted Chapter5-2 Example of Transaction Begin_transaction Reservation begin input(flight_no, date, customer_name); EXEC SQL SELECT STSOLD, CAP INTO :temp1, :temp2 FROM FLIGHT WHERE FNO = :flight_no AND DATE = :date; if temp1 = temp2 then { output( no free seats ); ABORT } else { EXEC SQL UPDATE FLIGHT SET STSOLD = STSOLD + 1 WHERE FNO = :flight_no AND DATE = :date; EXEC SQL INSERT INTO FC(FNO, DATE, CNAME, SPECIAL) VALUES(:flight_no, :date, :customer_name, NULL); COMMIT; Output( reservation completed ) } end. Chapter5-3 2

3 2. Properties of Transactions Atomicity: All-or-nothing property Distributed transaction commit protocol Consistency: Correctness of transaction execution If T starts with consistent state + T executes in isolation T leaves consistent state Isolation: Concurrency Control lost update, dirty read, unrepeatable read Durability: Recovery Buffer management policies, Log management, Chapter Supporting Atomicity of Distributed Transactions Distributed Transaction / Root agent: ó Commit/Abort (Coordinator) Local agent: ñ þ, (Participant) Notation: T 1 T 11 T 12 T 1n Chapter5-5 3

4 Commitment Protocol ú (commit or abort) Coordinator participant 1 Phase Communication 2 or more Phase Communication 2PC: de facto standard 1ñ : ú Prepare for commitment Ready to commit or abort 2ñ : ú Commit or Abort Acknowledgement Chapter5-6 2PC Protocol tm/acm U RM/CM AAM/ACM A I -/PM C I PM/RM R CM/- ACM/- C A ua/pm/aam Coordinator Participant Notes I: Initial state U: Uncertain R: Ready to commit A: Abort C: Commit Messages PM: Prepare Message RM: Ready Answer Message AAM: Abort Answer Message ACM: Abort Command Message CM: Commit Message Local Conditions ua: local unilateral abort tm: timeout Chapter5-7 4

5 3.1 2PC Optimizations Performance of 2PC 2PC Variations 2PC Optimizations Chapter5-8 2PC with Intermediate Coordinator Coordinator Intermediate Coordinator Participant *Log Committed Log END Prepare Vote Yes Commit ACK *Log Prepared *Log Committed Log END Prepare Vote Yes Commit ACK *Log Prepared *Log Committed Chapter5-9 5

6 Performance of 2PC N Nodes Number of message passing = 4( N 1) Number of forced log writes = 2N 1 Number of non-forced log writes = N : 2PC Number of message Number of log writes ò Resource ó Lock Chapter5-10 2PC Variation Presumed Nothing (PN) Heuristic Decision Coordinator ò, Always commit/abort Always commit deposit transactions Coordinator Heuristic Damage Presumed Nothing PM Coordinator Participant Recoveryñ heuristic damage Heuristic Damage, Coordinator Chapter5-11 6

7 2PC Variation Presumed Abort (PA) Transaction Commit 2PC ú PM ( PN) 2PC Participant Coordinator Abort Recovery Commit Abort Presumed Abort Transaction Abort, Coordinator No Logging! Transaction ó Recovery Participant Chapter5-12 2PC Optimizations (1) Read Only Transaction 2PC 2ñ, log Message Log write? Read Only Transaction, ñ Serializability Last Agent ó, # of Participating Nodes = 2 Coordinator YES PM Implied ACK with next message to participant ó Chapter5-13 7

8 2PC Optimizations (2) Unsolicited Vote Participant transaction, CM, ò Participant PM Network delay Participant, Flattening Transaction Tree Transaction Tree depth,pm Coordinator Participant, PM : Security Problem Prepare ñ Participant Commit ñ Chapter5-14 2PC Optimizations (3) Log Sharing Coordinator Participant ú Participant Log Write Non-Force Recovery Coordinator: Commit Log, Participant: Prepare Log Participant Commit Log Non-Force Group Commit ú : or Timeout Transaction throughput vs. Transaction response time Chapter5-15 8

9 2PC Optimization (4) Long Locks ACK ñ Piggybacking Network delay, : ó Commit Acknowledgement Intermediate Coordinator ACK Early ACK: Transaction Commit ñ Late ACK: Participant Heuristic Decision, Chapter Dealing with Site Failures Coordinator Timeouts Timeout in the Uncertain state: ACM Timeout in the Commit or Abort states: repeat ACM/CM Participant Timeouts Timeout in the Initial state: unilateral abort Timeout in the Ready state: ñ participant P j P j is in the Initial state: unilateral abort P j is in the Ready state: ÿ P j is in the Abort or Commit states:. Chapter5-17 9

10 Failure of Coordinator Site Non-blocking Condition ú Blocking Condition ú ú ú Chapter5-18 3PC Introduction 2PC 2ñ Commit Prepared-to-commit (PC). Non-blocking Commitment Protocol ú. ( prepared-to-commit ) Chapter

11 3PC Protocol I -/PM PM/RM I PM/AAM ua/- tm/acm A R AAM/ ACM RM/PCM BC R PCM/OK BC ACM/- A tm/acm OK/CM CM/- C C Coordinator Participant Chapter