Distributed Transactions. CS5204 Operating Systems 1

Transcription

1 Distributed Transactions CS5204 Operating Systems 1

2 . Transactions transactions T. TM T Distributed DBMS Model data manager DM T. TM T transaction manager network DM TM DM T T physical database CS 5204 Operating Systems 2

3 Serialization T 1 : T 2 : T 3 : log : concurrent execution DB CS 5204 Operating Systems 3

4 T 1 : T 2 : T 3 : log : Serialization concurrent execution DB OPERATIONS READ(X): WRITE (Z): read any one copy of X R 1 (X 3 ) write all copies of Z W 3 (Z 2 ) and W 3 (Z 3 ) CS 5204 Operating Systems 4

5 T 1 : T 2 : T 3 : log : Serialization concurrent execution DB is acceptable if it is guaranteed to have resulted from any one of: T 1 T 2 T 3 T 2 T 1 T 3 T 2 T 3 T 1 T 1 T 3 T 2 T 3 T 1 T 2 T 3 T 2 T 1 DB CS 5204 Operating Systems 5

6 Serialization Consider two concurrent transactions executed at only one DM LOG: R 1 (X) R 2 (Y) R 1 (Y) W 1 (Z) W 1 (X) W 2 (X) R 2 (Z) CS 5204 Operating Systems 6

7 Serialization Consider two concurrent transactions executed at only one DM LOG: R 1 (X) R 2 (Y) R 1 (Y) W 1 (Z) W 1 (X) W 2 (X) R 2 (Z) Serial Order: R 2 (Y) W 2 (X) R 2 (Z) ; R 1 (X) R 1 (Y) W 1 (Z) W 1 (X) CS 5204 Operating Systems 7

8 Serialization Consider two concurrent transactions executed at only one DM 2 LOG: R 1 (X) R 2 (Y) R 1 (Y) W 1 (Z) W 1 (X) W 2 (X) R 2 (Z) Serial Order: 2 R 2 (Y) W 2 (X) R 2 (Z) ; R 1 (X) R 1 (Y) W 1 (Z) W 1 (X) last write conflict read source conflict CS 5204 Operating Systems 8

9 Serialization Consider two concurrent transactions executed at only one DM LOG: R 1 (X) R 2 (Y) R 1 (Y) W 1 (Z) W 1 (X) W 2 (X) R 2 (Z) Serial Order: R 1 (X) R 1 (Y) W 1 (Z) W 1 (X) ; R 2 (Y) W 2 (X) R 2 (Z) CS 5204 Operating Systems 9

10 Serialization Consider two concurrent transactions executed at only one DM LOG: R 1 (X) R 2 (Y) R 1 (Y) W 1 (Z) W 1 (X) W 2 (X) R 2 (Z) Serial Order: R 1 (X) R 1 (Y) W 1 (Z) W 1 (X) ; R 2 (Y) W 2 (X) R 2 (Z) CS 5204 Operating Systems 10

11 Distributed Transaction Processing Transactions: T 1 : READ(X); WRITE(Y); T 2 : READ(Y); WRITE(Z); DM 1 DM 2 Y 2 Z 2 X 1 Y 1 T 3 : READ(Z); WRITE(X); DM 3 Z 3 X 3 CS 5204 Operating Systems 11

12 Transactions: Distributed Transaction Processing T 1 : READ(X); WRITE(Y); T 2 : READ(Y); WRITE(Z); T 3 : READ(Z); WRITE(X); DM 1 DM 2 DM 3 Y 2 Z 2 X 1 Y 1 Z 3 X 3 LOGS: L 1 : R 2 (Y 1 ) R 1 (X 1 ) W 1 (Y 1 ) W 3 (X 1 ) L 2 : R 3 (Z 2 ) W 2 (Z 2 ) W 1 (Y 2 ) L 3 : W 3 (X 3 ) W 2 (Z 3 ) CS 5204 Operating Systems 12

13 Distributed Transaction Processing Transactions Transactions: T 1 : READ(X); WRITE(Y); T 2 : READ(Y); WRITE(Z); DM 1 DM 2 Y 2 Z 2 X 1 Y 1 T 3 : READ(Z); WRITE(X); DM 3 Z 3 X 3 LOGS: L 1 : R 2 (Y 1 ) R 1 (X 1 ) W 1 (Y 1 ) W 3 (X 1 ) L 2 : R 3 (Z 2 ) W 2 (Z 2 ) W 1 (Y 2 ) L 3 : W 3 (X 3 ) W 2 (Z 3 ) Question: Are these logs equivalent to some serial execution of the transactions? CS 5204 Operating Systems 13

14 Serialization of Distributed Logs Transactions Conflict: P j (A X ) and Q i (B Y ) conflict if (1) P and Q are not both READ, and (2) A = B (same data item), and (3) i j (different transactions), and (4) X = Y (same data manager/log) CS 5204 Operating Systems 14

15 Serialization of Distributed Logs Conflict: P j (A X ) and Q i (B Y ) conflict if (1) P and Q are not both READ, and (2) A = B, and (3) i j, and (4) X = Y LOGS: L 1 : R 2 (Y 1 ) R 1 (X 1 ) W 1 (Y 1 ) W 3 (X 1 ) L 2 : R 3 (Z 2 ) W 2 (Z 2 ) W 1 (Y 2 ) L 3 : W 3 (X 3 ) W 2 (Z 3 ) Transactions CS 5204 Operating Systems 15

16 Serialization of Distributed Logs Conflict: P j (A X ) and Q i (B Y ) conflict if LOGS: (1) P and Q are not both READ, and (2) A = B, and (3) i j, and (4) X = Y L 1 : R 2 (Y 1 ) R 1 (X 1 ) W 1 (Y 1 ) W 3 (X 1 ) L 2 : R 3 (Z 2 ) W 2 (Z 2 ) W 1 (Y 2 ) 1 3 L 3 : W 3 (X 3 ) W 2 (Z 3 ) 2 Transactions CS 5204 Operating Systems 16

17 Serialization of Distributed Logs Conflict: P j (A X ) and Q i (B Y ) conflict if (1) P and Q are not both READ, and (2) A = B, and (3) i j, and (4) X = Y Transactions LOGS: 2 L 1 : R 2 (Y 1 ) R 1 (X 1 ) W 1 (Y 1 ) W 3 (X 1 ) L 2 : R 3 (Z 2 ) W 2 (Z 2 ) W 1 (Y 2 ) 1 3 L 3 : W 3 (X 3 ) W 2 (Z 3 ) => T 1 T 3 => T 2 T 1 => T 3 T 2 Contradictory No total order Not serializable CS 5204 Operating Systems 17

18 Serialization of Distributed Logs Transactions Theorem: Distributed logs are serializable if there exists a total ordering of the transactions such that for conflicting operations P j and Q i a log shows P j Q i only if T j T i LOGS: 2 L 1 : R 2 (Y 1 ) R 1 (X 1 ) W 1 (Y 1 ) W 3 (X 1 ) L 2 : R 3 (Z 2 ) W 2 (Z 2 ) W 1 (Y 2 ) 1 3 L 3 : W 3 (X 3 ) W 2 (Z 3 ) => T 1 T 3 => T 2 T 1 => T 3 T 2 Contradictory No total order Not serializable CS 5204 Operating Systems 18

19 Locking transactions must use Two Phase Locking (2PL) locking phase release phase T i time no locks released no new locks requested only the following lock requests are granted current lock state lock request not locked READ locked WRITE locked READ OK OK DENY WRITE OK DENY DENY CS 5204 Operating Systems 19

20 Locking T i R i (X) W i (Y) lock(x) lock(y) release(x,y) concurrency controller request lock before accessing a data item release all locks at the end of transaction This guarantees serializability [ESWAREN] CS 5204 Operating Systems 20

21 Effects of Locking L 1 : R 2 (Y 1 ) Suppose the transactions have executed to this point: L 2 : R 3 (Z 2 ) L 3 : W 3 (X 3 ) The locks are then: Lock for Lock state Waiting for lock X write-locked by T 3 T 1 Y read-locked by T 2 Z read-locked by T 3 T 2 Only T 3 is able to execute; it will complete its write and release its locks, leading to CS 5204 Operating Systems 21

22 Effects of Locking L 1 : R 2 (Y 1 ), W 3 (X 1 ) these logs, and L 2 : R 3 (Z 2 ) L 3 : W 3 (X 3 ) this lock state: Lock for Lock state Waiting for lock X unlocked T 1 Y read-locked by T 2 Z unlocked T 2 Both T 1 and T 2 can acquire the locks needed to execute their next actions, resulting in CS 5204 Operating Systems 22

23 Effects of Locking L 1 : R 2 (Y 1 ), W 3 (X 1 ), R 1 (X 1 ) these logs, and L 2 : R 3 (Z 2 ), W 2 (Z 2 ) L 3 : W 3 (X 3 ), W 2 (Z 3 ) this lock state: Lock for Lock state Waiting for lock X read-locked by T 1 Y read-locked by T 2 T 1 Z write-locked by T 2 Transaction T 2 completes, releases its lock - resulting in CS 5204 Operating Systems 23

24 Effects of Locking this lock state: Lock for Lock state Waiting for lock X read-locked by T 1 Y unlocked T 1 Z unlocked At this point, T 1 can acquire the write-lock on Y, perform its write operations and complete, leading to the final serializable logs: L 1 : R 2 (Y 1 ), W 3 (X 1 ), R 1 (X1), W 1 (Y 1 ) L 2 : R 3 (Z 2 ), W 2 (Z 2 ), W 1 (Y 2 ) L 3 : W 3 (X 3 ), W 2 (Z 3 ) CS 5204 Operating Systems 24