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Distributed DBMS Model

DM. TM. TM. T. T. …. …. TM. T. T. T. T. …. DM. DM. Distributed DBMS Model. data manager. transactions. network. transaction manager. physical database. Serialization. T 1 : T 2 : T 3 :. concurrent execution. DB. log :. Serialization. T 1 : T 2 : T 3 :.

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Distributed DBMS Model

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  1. DM TM TM T T …. …. TM T T T T …. DM DM Distributed DBMS Model data manager transactions network transaction manager physical database CS 5204 Spring 99

  2. Serialization T1: T2: T3 : concurrent execution DB log : CS 5204 Spring 99

  3. Serialization T1 : T2 : T3 : concurrent execution DB log : OPERATIONS READ(X): read anyone copy of X R1 (X3) WRITE (Z): write all copies of Z W3(Z2) and W3 (Z3) CS 5204 Spring 99

  4. Serialization T1 : T2 : T3 : concurrent execution DB log : • DB is acceptable if it is guaranteed to have resulted from any one of: • T1T2T3 • T2T1T3 • T2T3T1 • T1T3T2 • T3T1T2 • T3T2T1 CS 5204 Spring 99

  5. Serialization Consider two concurrent transactions executed at only one DM LOG: R1(X)R2(Y)R1(Y)W1(Z) W1(X)W2(X) R2(Z) CS 5204 Spring 99

  6. Serialization Consider two concurrent transactions executed at only one DM LOG: R1(X)R2(Y)R1(Y)W1(Z) W1(X)W2(X) R2(Z) Serial Order: R2(Y) W2(X) R2(Z) ; R1(X)R1(Y)W1(Z) W1(X) CS 5204 Spring 99

  7. Serialization Consider two concurrent transactions executed at only one DM 2 LOG: R1(X)R2(Y)R1(Y)W1(Z) W1(X)W2(X) R2(Z) 1 2 Serial Order: R2(Y) W2(X) R2(Z) ; R1(X)R1(Y)W1(Z) W1(X) last write conflict 1 read source conflict 2 CS 5204 Spring 99

  8. Serialization Consider two concurrent transactions executed at only one DM LOG: R1(X)R2(Y)R1(Y)W1(Z) W1(X)W2(X) R2(Z) Serial Order: R1(X)R1(Y)W1(Z) W1(X); R2(Y) W2(X) R2(Z) CS 5204 Spring 99

  9. Serialization Consider two concurrent transactions executed at only one DM LOG: R1(X)R2(Y)R1(Y)W1(Z) W1(X)W2(X) R2(Z) Serial Order: R1(X)R1(Y)W1(Z) W1(X); R2(Y) W2(X) R2(Z) CS 5204 Spring 99

  10. Distributed Transaction Processing Transactions: X1 Y1 T1 : READ(X); WRITE(X); T2 : READ(Y); WRITE(Z); T3 : READ(Z); WRITE(X); DM1 Y2 Z2 DM2 Z3 X3 DM3 CS 5204 Spring 99

  11. Distributed Transaction Processing Transactions: X1 Y1 T1 : READ(X); WRITE(X); T2 : READ(Y); WRITE(Z); T3 : READ(Z); WRITE(X); DM1 Y2 Z2 DM2 Z3 X3 DM3 LOGS: L1 : R2(Y1) R1(X1) W1(Y1) W3(X1) L2 : R3(Z2) W2(Z2) W1(Y2) L3 : W3(X3) W2(Z3) CS 5204 Spring 99

  12. Distributed Transaction Processing Transactions: X1 Y1 T1 : READ(X); WRITE(X); T2 : READ(Y); WRITE(Z); T3 : READ(Z); WRITE(X); DM1 Y2 Z2 DM2 Z3 X3 DM3 LOGS: L1 : R2(Y1) R1(X1) W1(Y1) W3(X1) L2 : R3(Z2) W2(Z2) W1(Y2) L3 : W3(X3) W2(Z3) • Question: • Are these logs equivalent to some serial execution of the transactions? CS 5204 Spring 99

  13. Serialization of Distributed Logs Conflict: Pj(AX) and Qi(BY) conflict if (1) P and Q are not both READ, and (2) A = B (3) i  j (4) X = Y CS 5204 Spring 99

  14. Serialization of Distributed Logs Conflict: Pj(AX) and Qi(BY) conflict if (1) P and Q are not both READ, and (2) A = B (3) i  j (4) X = Y LOGS: L1 : R2(Y1) R1(X1) W1(Y1) W3(X1) L2 : R3(Z2) W2(Z2) W1(Y2) L3 : W3(X3) W2(Z3) CS 5204 Spring 99

  15. LOGS: L1 : R2(Y1) R1(X1) W1(Y1) W3(X1) L2 : R3(Z2) W2(Z2) W1(Y2) L3 : W3(X3) W2(Z3) 2 1 3 => T1 T3 => T2 T1 => T3 T2 1 2 3 Serialization of Distributed Logs Conflict: Pj(AX) and Qi(BY) conflict if (1) P and Q are not both READ, and (2) A = B (3) i  j (4) X = Y Contradictory  No total order  Not serializable CS 5204 Spring 99

  16. LOGS: L1 : R2(Y1) R1(X1) W1(Y1) W3(X1) L2 : R3(Z2) W2(Z2) W1(Y2) L3 : W3(X3) W2(Z3) 2 1 3 => T1 T3 => T2 T1 => T3 T2 1 2 3 Serialization of Distributed Logs • Theorem: Distributed logs are serializable if • there exists a total ordering of the transactions such that for conflicting operations Pj and Qi • Pj Qi in a LOG only if Tj Ti Contradictory  No total order  Not serializable CS 5204 Spring 99

  17. current lock state lock request not locked READ locked WRITE locked READ OK OK DENY WRITE OK DENY DENY Locking • transactions must use Two Phase Locking (2PL) locking phase release phase Ti time no locks released no new locks requested • only the following lock requests are granted CS 5204 Spring 99

  18. Locking Ti lock(X) Ri(X) lock(Y) release(X,Y) concurrency controller Wi(Y) • request lock before accessing a data item • release all locks at the end of transaction • This guarantees serializability [ESWAREN] CS 5204 Spring 99

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