Ch-10-Notes - Ch 10 Notes contd Concurrency Control -...

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Ch 10 Notes cont’d Concurrency Control - Special steps must be taken when two transactions attempt to use the same data at the same time 3 Main Problems: 1. Lost Updates – part of the transaction is committed (i.e. T1 but not T2). This eliminated by locks 2. Uncommitted Data – occurs when 2 transactions are executed concurrently and the first transaction is rolled back after second transaction has already accessed the uncommitted data. (i.e. T1 is rolled back but not T2). This violates Isolation. 3. Inconsistent Retrievals – T1 calculates aggregate function while T2 is simultaneously updating data that T1 is computing ACIDS Atomicity – all or none Consistency – permanence of the database’s consistent state. When a transaction is completed, the database reaches a consistent state (not in slides) Isolation - means the data used during the execution of one transaction cannot be used by a second transaction until the first one is complete. Uncommitted data is a violation of this rule Durability- ensures that once a transaction changes are done (committed), they cannot be undone or lost, even in the event of system failure Serializability- ensures that the concurrent execution of several transactions yields consistent results. More specifically, the concurrent execution of transactions T1, T2, and
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Ch-10-Notes - Ch 10 Notes contd Concurrency Control -...

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