ch11-1 - Chapter 11 Distributed Concurrency Control Chapter...

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Chapter 11 Distributed Concurrency Control Chapter 11 Distributed Concurrency Control Concurrency control ensures the consistency and reliability properties of transactions. 111 SERIALIZABILITY THEORY The most widely accepted correctness criterion for concurrency control algorithms. A schedule S (also called history ) is defined over a set of transactions T={T1, T2, …,Tn} and specifies order of execution of these transactions’ operations. Two operations O x ij ( ) and O x kl ( ) accessing the same data item x are in conflict if one of them is a write. Conflicts are divided into r ead-write ( or write-read ) and write-write types. Two transactions are in conflict if an operation of one transaction is in conflict with an operation of the other by above definition. A complete schedule S T C defined over a set of transactions T={T1,T2,…,Tn} is a partial order 1
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Chapter 11 Distributed Concurrency Control S T C T T = { , } h where 1. = ∪ = i n i T 1 2. i n i T c c 1 = 3. For any two conflicting operations O x ij ( ) and O x kl ( ) inΣ T , either O x ij ( ) h T O x kl ( ) , or O x kl ( ) h T O x ij ( ) . The last condition defines the execution order among conflicting operations. Example: find a complete schedule for two transactions T 1 and T 2 T 1 T 2 time Read(x) Read(x) x x+1 x x+1 Write(x) Write(x) Commit Commit = 1 ∪∑ 2 ={R 1 (x),W 1 (x),C 1 ,R 2 (x),W 2 (x),C 2 } DAG representation of one possible complete schedule. 2
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Chapter 11 Distributed Concurrency Control R 1 (x) R 2 (x) W 1 (x) W 2 (x) C 1 C 2 Note the dashed lines are added to make the schedule complete. The complete schedule can be simplified by { R 1 (x),R 2 (x),W 1 (x),C 1 ,W 2 (x),C 2 } that specifies all operations of T1 and T2 and their relative orders. A schedule is a prefix of a complete schedule. A
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This note was uploaded on 12/23/2009 for the course DBST 663 taught by Professor Tba during the Spring '09 term at MD University College.

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ch11-1 - Chapter 11 Distributed Concurrency Control Chapter...

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