Optimistic-Concurrency-Control

Optimistic-Concurrency-Control - Merger of Semi-Committed...

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Optimistic Concurrency Contro Merger of Semi-Committed Transactions From Several Partitions Combine DCG, DCG 2 , --- DCG N (minimize rollback if cycle exists) NP-complete (Davidson, Princeton) (minimum feedback vertex set problem) Consider each DCG as a single transaction Check acyclicity of this N node graph (too optimistic!) Assign a weight to transactions in each partition Consider DCG 1 with maximum weight Select transactions from other DCG’s that do not create cycles
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Optimistic Concurrency Contro Two Choices Abort transactions who create cycles Consider each transaction that creates cycle one at a time. Abort transactions which optimize rollback (complexity O(N 3 )) Minimization not necessarily optimal globally
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Optimistic Concurrency Contro Semantics of Transaction Computation Commutative Give $5000 bonus to every employee Commutativity can be predetermined or recognized dynamically Maintain log (REDO/UNDO) of commutative and noncommutative actions Partially rollback transactions to their first noncommutative action
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