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Chapter 16 Concurrency Control CS157b Tom Mensch
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Motivation: Bank Example zOne bank account with $100 balance zTwo different ATMs both trying to withdraw $100 from the same account at exactly the same time zBoth ATMs check the balance of the account and see it’s $100
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Bank Example Continued zSince the balance is enough, both ATMs give out $100 zAfter both transactions complete the balance is -$100 zOne of the transactions should have been denied
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Concurrent Transactions zProblem: How can data be accessed by multiple transactions without compromising data integrity? zSolution: Only allow a transaction to access data if it holds a “lock” on it.
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Transactions with Locking zBoth ATMs try to “lock” the account zOne succeeds and withdraws $100 zThe second ATM has to wait until the first ATM releases it’s lock before it can proceed z Now when the second ATM tries to withdraw the $100 it cannot because the balance is $0
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Locks zShared - transaction wants read access to data. zExclusive - transaction wants both read and write access to data.
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shared-mode lock zMultiple transactions can have shared- mode locks on the same data. zEx: Transaction T1 has shared-mode lock on data A. Transaction T2 requests a shared-mode lock on data A; the request is granted.
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exclusive-mode lock zOnly one transaction can have an exclusive-mode lock on a data item. zEx: Transaction T1 has exclusive-mode lock on data A. Transaction T2 requests a exclusive-mode lock on data A; the request is denied.
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Lock-compatibility zShared - other shared-mode lock requests granted. zExclusive - all other lock requests denied.
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Unlocking data zTransaction must unlock data when done accessing it. zShared-mode locks wait for exclusive- mode locks to be released zExclusive-mode locks wait for all other locks be be released
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Indefinite postponement? zCan an exclusive-mode lock be indefinitely postponed (starved) by an infinite number of shared-mode lock requests? zNo. Because the concurrency-control manager will make shared-mode requests wait if an exclusive-mode is already waiting for a shared-mode lock to be released.
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Balance Transfer Example zBank account A has a $100 balance zBank account B has a $200 balance zTransaction T1 transfers $50 from account B to account A zT1 locks A, withdraws $50 and unlocks A zT1 then locks B, adds $50 and unlocks B
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Bank Transfer Continued zTransaction T2 wants to find out the sum of the balances of accounts A and B zT2 asks for a lock on both accounts after T1 has released it’s lock on A but before T1 locks B zT1 must now wait for T2 to complete before adding the $50 to account A
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Bank Transfer Continued zThe result of T2 is $250 which is incorrect zWhy? zT1 should have held it’s lock on A longer
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Deadlocks zT1 requests exclusive lock on A. Granted zT2 requests exclusive lock on B. Granted zT1 requests exclusive lock on B. Wait zT2 requests exclusive lock on A. Wait
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Deadlocks Continued zT2 will never release it’s lock on B until it gets a lock on A zT1 will never release it’s lock on A until it gets a lock on B zBoth transactions will wait forever zThis is bad
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Two-Phase Locking Protocol z1. Growing phase - transactions may acquire new locks z2. Shrinking phase - transactions may release locks, but not acquire new ones
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Two-Phase Locking Continued zStrict two-phase locking protocol - requires that all exclusive locks be held until the transaction commits zRigorous two-phase locking protocol - requires that all locks be held until the transaction commits
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Timestamp-Based protocols zEach transaction gets a timestamp when it enters the system zTimestamp can be either the system time, or simply a logical counter zW-timestamp(Q) - largest timestamp of any transaction that did write(Q) successfully zR-timestamp(Q) - largest timestamp of any transaction that did read(Q) successfully
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Timestamp-Ordering zTransaction Ti with timestamp TS(Ti) issues read(Q) zTS(Ti) < W-timestamp(Q) then read(Q) is rejected and Ti is rolled back zTS(Ti) >= W-timestamp(Q) then read(Q) executes successfully
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Timestamp-Ordering Cont. zTransaction Ti with timestamp TS(Ti) issues write(Q) zTS(Ti) < R-timestamp(Q) then write(Q) is rejected and Ti is rolled back zTS(Ti) < W-timestamp(Q) then write(Q) is rejected and Ti is rolled back zOtherwise write(Q) executes successfully
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Thomas’ Write Rule zTransaction Ti with timestamp TS(Ti) issues write(Q) zTS(Ti) < R-timestamp(Q) then write(Q) is rejected and Ti is rolled back zTS(Ti) < W-timestamp(Q) then write(Q) is ignored and Ti continues zOtherwise write(Q) executes successfully
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Validation-Based Protocols zConcurrency-control schemes impose additional overhead on system and can cause delays zSolution: find conflicts before they happen
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Three-phase execution zRead phase - transaction reads data zValidation phase - transaction makes sure none of it’s writes to it’s temporary variables conflict with other operations zWrite phase - apply actual changes to database
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Validation timestamps zValidation requires three timestamps zStart - time the transaction began executing zValidation - time the Read phase competed and Validation phase began zFinish - time the transaction completed
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