Formal-Concurrency-Control

Formal-Concurrency-Control - General Comments Information...

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Formal-Concurrency-Control General Comments Information needed by Concurrency Controllers Locks on database objects (System-R, Ingres, Rosenkrantz…) Time stamps on database objects (Thomsa, Reed) Time stamps on transactions (Kung, SDD-1, Schlageter, Bhargava…) Observations Time stamps mechanisms more fundamental than locking Time stamps carry more information Checking locks costs less than checking time stamps
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Formal-Concurrency-Control General Comments (cont.) When to synchronize First access to an object (Locking, pessimistic validation) At each access (question of granularity) After all accesses and before commitment (optimistic validation) Fundamental notions Rollback Identification of useless transactions Delaying commit point Semantics of transactions
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Formal-Concurrency-Control Definition : A dynamic conflict graph (DCG) for a history H = <D, T, Σ , Π > is a diagraph <V,E> where V is the set of vertices representing T, the set of transactions; E is the set of edges where <I,J> is an edge if and only if there exist conflicting atomic operations σ j , σ j for which π ( π 1 )< π ( σ j ). Lemma : The DCG of a serial history is acyclic. Theorem : A history is in DCP if and only if the DCG of H is acyclic. Theorem : In a two-step transaction model (all reads for a transaction precede all writes) whenever there is a transaction rollback in the optimistic approach due to failure in validation. There will be a deadlock in the locking approach and will cause a transaction rollback.
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Formal-Concurrency-Control Basic Terms Database Database entity Distributed database Program Transaction, read set, write set Actions atomic Concurrent processing Conflict Consistency Mutual consistency History Serializability Serial history
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Formal-Concurrency-Control Serializable history Concurrency control Centralized control Distributed control Scheduler Locking Read lock, write lock Two phase locking, lock point Live lock Dead lock Conflict graph Timestamp Version number Rollback Validation commit
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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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Formal-Concurrency-Control - General Comments Information...

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