Notes09 - CS245:DatabaseSystem Principles...

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CS 245 Notes 09 1 CS 245: Database System  Principles Notes 09: Concurrency Control Steven Whang
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CS 245 Notes 09 2 Chapter 18 [18]  Concurrency Control    T1 T2 Tn DB (consistency constraints)
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CS 245 Notes 09 3 Example: T1: Read(A) T2: Read(A)  A+100  A × 2 Write(A) Write(A) Read(B) Read(B)  B+100 B × 2 Write(B) Write(B) Constraint:  A=B
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CS 245 Notes 09 4 Schedule A T1 T2 Read(A); A   A+100 Write(A); Read(B); B    B+100; Write(B); Read(A);A    A × 2; Write(A);      Read(B);B    B × 2; Write(B); A B 25 25 125 125 250 250 250 250
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CS 245 Notes 09 5 Schedule B T1 T2 Read(A);A    A × 2; Write(A); Read(B);B    B × 2; Write(B); Read(A); A   A+100 Write(A); Read(B); B    B+100; Write(B); A B 25 25 50 50 150 150 150 150
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CS 245 Notes 09 6 Schedule C T1 T2 Read(A); A   A+100 Write(A); Read(A);A    A × 2; Write(A); Read(B); B    B+100; Write(B);      Read(B);B    B × 2; Write(B); A B 25 25 125 250 125 250 250 250
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CS 245 Notes 09 7 Schedule D T1 T2 Read(A); A   A+100 Write(A); Read(A);A    A × 2; Write(A);      Read(B);B    B × 2; Write(B); Read(B); B    B+100; Write(B); A B 25 25 125 250 50 150 250 150
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CS 245 Notes 09 8 Schedule E T1 T2’ Read(A); A   A+100 Write(A); Read(A);A    A × 1; Write(A);      Read(B);B    B × 1; Write(B); Read(B); B    B+100; Write(B); A B 25 25 125 125 25 125 125 125 Same as Schedule D but with new T2’
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CS 245 Notes 09 9 Want schedules that are “good”, regardless of initial state and transaction semantics Only look at order of read and writes Example:  Sc=r 1 (A)w 1 (A)r 2 (A)w 2 (A)r 1 (B)w 1 (B)r 2 (B)w 2 (B)
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CS 245 Notes 09 10 Sc’=r 1 (A)w 1 (A) r 1 (B)w 1 (B)r 2 (A)w 2 (A)r 2 (B)w 2 (B)      T 1          T 2 Example:  Sc=r 1 (A)w 1 (A)r 2 (A)w 2 (A)r 1 (B)w 1 (B)r 2 (B)w 2 (B)
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CS 245 Notes 09 11 However, for Sd: Sd=r 1 (A)w 1 (A)r 2 (A)w 2 (A) r 2 (B)w 2 (B)r 1 (B)w 1 (B) as a matter of fact,        T must precede T         in any equivalent schedule,         i.e.,  T 2    T 1
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CS 245 Notes 09 12 T 1     T 2 Sd cannot be rearranged into a serial schedule Sd is not “equivalent” to any serial schedule Sd is “bad”   T 2    T   Also, T 1    T 2
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CS 245 Notes 09 13 Returning to Sc Sc=r 1 (A)w 1 (A)r 2 (A)w 2 (A)r 1 (B)w 1 (B)r 2 (B)w 2 (B)  T 1    T 2   T 1    T 2  no cycles   Sc is “equivalent” to a serial schedule (in this case T 1 ,T 2 )
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Notes 09 14 Concepts Transaction:  sequence of r i (x), w i (x) actions Conflicting actions:   r 1(A)     w 2(A)     w 1(A)          w 2(A)    r 1(A)      w 2(A) Schedule:  represents chronological order
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Notes09 - CS245:DatabaseSystem Principles...

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