Notes09 - Chapter 18 [18] Concurrency Control CS 245:...

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1 CS 245 Notes 09 1 CS 245: Database System Principles Notes 09: Concurrency Control Hector Garcia-Molina CS 245 Notes 09 2 Chapter 18 [18] Concurrency Control T1 T2 Tn DB (consistency constraints) CS 245 Notes 09 3 Example: T1: Read(A) T2: Read(A) A A+100 A A 2 Write(A) Write(A) Read(B) Read(B) B B+100 B B 2 Write(B) Write(B) Constraint: A=B 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); CS 245 Notes 09 5 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); AB 25 25 125 125 250 250 250 250 CS 245 Notes 09 6 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);
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2 CS 245 Notes 09 7 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); AB 25 25 50 50 150 150 150 150 CS 245 Notes 09 8 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); CS 245 Notes 09 9 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); 25 25 125 250 125 250 250 250 CS 245 Notes 09 10 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); CS 245 Notes 09 11 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); 25 25 125 250 50 150 250 150 CS 245 Notes 09 12 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); Same as Schedule D but with new T2’
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3 CS 245 Notes 09 13 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); AB 25 25 125 125 25 125 125 125 Same as Schedule D but with new T2’ CS 245 Notes 09 14 • 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) CS 245 Notes 09 15 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) CS 245 Notes 09 16 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 ma t te r o f fac t , T 2 must precede T 1 in any equivalent schedule, i.e., T 2 T 1 CS 245 Notes 09 17 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 1 •A l s o , T 1 T 2 CS 245 Notes 09 18 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
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4 CS 245 Notes 09 19 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 ) CS 245 Notes 09 20 Concepts Transaction: sequence of r i (x), w i (x) actions Conflicting actions: r
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Notes09 - Chapter 18 [18] Concurrency Control CS 245:...

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