PowerPoint PresentationDatabase Concurrency ControlSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43SummaryCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-WesleyChapter 22Concurrency Control TechniquesCopyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency Control1 Purpose of Concurrency ControlTo enforce Isolation (through mutual exclusion) among conflicting transactions. To preserve database consistency through consistency preserving execution of transactions.To resolve read-write and write-write conflicts.Example: In concurrent execution environment if T1 conflicts with T2 over a data item A, then the existing concurrency control decides if T1 or T2 should get the A and if the other transaction is rolled-back or waits.Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsDatabase requires that all transactions should be well-formed. A transaction is well-formed if:It must lock the data item before it reads or writes to it.It must not lock an already locked data item and it must not try to unlock a free data item.Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking TechniquesLocking is an operation which secures (a) permission to Read(b) permission to Write a data item for a transaction. Example: Lock (X). Data item X is locked on behalf of the requesting transaction. Unlocking is an operation which removes these permissions from the data item. Example:Unlock (X): Data item X is made available to all other transactions.Lock and Unlock are Atomic operations.Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential components Two locks modes:(a) shared (read) (b) exclusive (write).Shared mode: shared lock (X)More than one transaction can apply share lock on X for reading its value but no write lock can be applied on X by any other transaction.Exclusive mode: Write lock (X)Only one write lock on X can exist at any time and no shared lock can be applied by any other transaction on X.Conflict matrixRead WriteRead WriteNNNYCopyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsLock Manager: Managing locks on data items.Lock table: Lock manager uses it to store the identity of transaction locking a data item, the data item, lock mode and pointer to the next data item locked. One simple way to implement a lock table is through linked list.T1Transaction ID Data item id lock mode Ptr to next data itemNextX1 ReadCopyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsLock and unlock operations for binary locks:B:if LOCK (X) = 0 (*item is unlocked*)then LOCK (X) 1 (*lock the item*)else beginwait (until lock (X) = 0) andthe lock manager wakes up the transaction);goto Bend;Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsThe following code performs the unlock operation:LOCK (X) 0 (*unlock the item*)if any transactions are waiting thenwake up one of the waiting transactions;Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsLocking operation for two-mode (read/write) locks: Following code performs the read lock operation:B: if LOCK (X) = “unlocked” thenbegin LOCK (X) “read-locked”;no_of_reads (X) 1;endelse if LOCK (X) “read-locked” then no_of_reads (X) no_of_reads (X) +1 else begin wait (until LOCK (X) = “unlocked” and the lock manager wakes up the transaction); go to Bend;Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsThe following code performs the write lock operation:B: if LOCK (X) = “unlocked” thenLOCK (X) “write-locked”;else beginwait (until LOCK (X) = “unlocked” and the lock manager wakes up the transaction); go to Bend;Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: Essential componentsThe following code performs the unlock operation:if LOCK (X) = “write-locked” thenbegin LOCK (X) “unlocked”; wakes up one of the transactions, if anyendelse if LOCK (X) “read-locked” thenbegin no_of_reads (X) no_of_reads (X) -1 if no_of_reads (X) = 0 then begin LOCK (X) = “unlocked”;wake up one of the transactions, if any endend;Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: The algorithmTwo Phases:(a) Locking (Growing)(b) Unlocking (Shrinking).Locking (Growing) Phase:A transaction applies locks (read or write) on desired data items one at a time.Unlocking (Shrinking) Phase:A transaction unlocks its locked data items one at a time.Requirement:For a transaction these two phases must be mutually exclusive, that is, during locking phase unlocking phase must not start and during unlocking phase locking phase must not begin.Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: The algorithmT1 T2 Resultread_lock (Y); read_lock (X); Initial values: X=20; Y=30read_item (Y); read_item (X); Result of serial executionunlock (Y); unlock (X); T1 followed by T2 write_lock (X); Write_lock (Y); X=50, Y=80.read_item (X); read_item (Y); Result of serial executionX:=X+Y; Y:=X+Y; T2 followed by T1 write_item (X); write_item (Y); X=70, Y=50unlock (X); unlock (Y);Copyright © 2011 Ramez Elmasri and Shamkant NavatheDatabase Concurrency ControlTwo-Phase Locking Techniques: The algorithmT1 T2 Resultread_lock (Y); X=50; Y=50read_item (Y); Nonserializable because it.unlock (Y); violated two-phase policy.read_lock (X); read_item (X); unlock (X); write_lock
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