chapter 9_b

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Chapter 9_B

Concurrency Control

Database Systems: Design, Implementation, and Management,

Rob and Coronel

Database Systems: Design, Implementation, & Management, 8th Edition, Rob & Coronel

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In this chapter, you will learn:

• What a database transaction is and what its properties are

• What concurrency control is and what role it plays in maintaining the database’s integrity

• What locking methods are and how they work

• How stamping methods are used for concurrency control

• How optimistic methods are used for concurrency control

• How database recovery management is used to maintain database integrity


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Concurrency Control

• Coordination of simultaneous transaction execution in a multiprocessing database system

• Objective is to ensure serializability of transactions in a multiuser database environment


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Concurrency Control (continued)

• Simultaneous execution of transactions over a shared database can create several data integrity and consistency problems:– Lost updates– Uncommitted data– Inconsistent retrievals


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Lost Updates

Assume that you have a product whose current PROD_QOH value is 35


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Lost Updates (continued)

Suppose that a transaction is able to read a product’s PROD_QOH value from the table before a previous transaction (using the same product) has been committed


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Uncommitted DataThe phenomenon of uncommitted data occurs when two transactions, T1 and T2, are executed concurrently and the first transaction (T1) is rolled back after the second transaction (T2) has already accessed the uncommitted data—thus violating the isolation property of transactions. T1 is forced to roll back due to an error during the update of the invoice total; hence, it rolls back all the way, undoing the inventory update


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Uncommitted Data (continued)


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Sun 23-6 Inconsistent RetrievalsInconsistent retrievals occur when a transaction accesses data before and after another transaction(s) finish working with such data.


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Inconsistent Retrievals (continued)


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Inconsistent Retrievals (continued)


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The Scheduler

• Special DBMS program – Purpose is to establish order of operations

within which concurrent transactions are executed

• Interleaves execution of database operations to ensure serializability and isolation of transactions


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The Scheduler (continued)

• Bases its actions on concurrency control algorithms, like locking and time stamping.

• Ensures computer’s central processing unit (CPU) is used efficiently, not based on FCFS

• Facilitates data isolation to ensure that two transactions do not update same data element at same time


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The Scheduler (continued)


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Concurrency Controlwith Locking Methods

• Lock – Guarantees exclusive use of a data item to a

current transaction– Required to prevent another transaction from

reading inconsistent data

• Lock manager– Responsible for assigning and policing the

locks used by transactions


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Lock Granularity

• Indicates level of lock use

• Locking can take place at following levels: – Database– Table– Page– Row– Field (attribute)


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Lock Granularity (continued)

• Database-level lock– Entire database is locked

• Table-level lock– Entire table is locked

• Page-level lock– Entire diskpage is locked


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• Row-level lock – Allows concurrent transactions to access

different rows of same table, even if rows are located on same page

• Field-level lock – Allows concurrent transactions to access

same row, as long as they require use of different fields (attributes) within that row


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Mon 24-6 Lock Types

• Binary lock– Has only two states: locked (1) or unlocked (0)

• Exclusive lock – Access is specifically reserved for transaction that

locked object– Must be used when potential for conflict exists

• Shared lock – Concurrent transactions are granted Read access

on basis of a common lock


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Lock Types (continued)


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Two-Phase Lockingto Ensure Serializability

• Defines how transactions acquire and relinquish locks

• Guarantees serializability, but it does not prevent deadlocks – Growing phase - Transaction acquires all

required locks without unlocking any data– Shrinking phase - Transaction releases all

locks and cannot obtain any new lock


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Two-Phase Lockingto Ensure Serializability (continued)

• Governed by the following rules:– Two transactions cannot have conflicting locks– No unlock operation can precede a lock

operation in the same transaction– No data are affected until all locks are

obtained—that is, until transaction is in its locked point


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Two-Phase Lockingto Ensure Serializability (continued)


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Deadlocks

• Condition that occurs when two transactions wait for each other to unlock data

• Possible only if one of the transactions wants to obtain an exclusive lock on a data item– No deadlock condition can exist among

shared locks


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Deadlocks (continued)

• Control through:– Prevention :A transaction requesting a new lock is

aborted (rolled back and rescheduled) when there is the possibility that a deadlock can occur.

– Detection :The DBMS periodically tests the database for deadlocks. If a deadlock is found, one of the transactions (the “victim”) is aborted (rolled back and restarted) and the other transaction continues.

– Avoidance :The transaction must obtain all of the locks it needs before it can be executed


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Deadlocks (continued)


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Concurrency Control with Time Stamping Methods

• Assigns global unique time stamp to each transaction

• Produces explicit order in which transactions are submitted to DBMS

• Uniqueness – Ensures that no equal time stamp values can exist

• Monotonicity– Ensures that time stamp values always increase

• All database operations (READ and WRITE) within the same transaction must have the same time stamp


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Tue 25-6 Wait/Die and Wound/Wait Schemes

• Wait/die – Older transaction waits and younger is rolled

back and rescheduled

• Wound/wait – Older transaction rolls back younger

transaction and reschedules it


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Wait/Die and Wound/Wait Schemes (continued)


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Concurrency Controlwith Optimistic Methods

• Optimistic approach – Based on assumption that majority of

database operations do not conflict– Does not require locking or time stamping

techniques– Transaction is executed without restrictions

until it is committed– Phases are read, validation, and write


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Concurrency Controlwith Optimistic Methods (cont.)

• During the read phase, the transaction reads the database, executes the needed computations, and makes the updates to a private copy of the database values. All update operations of the transaction are recorded in a temporary update file, which is not accessed by the remaining transactions.

• During the validation phase, the transaction is validated to ensure that the changes made will not affect the integrity and consistency of the database. If the validation test is positive, the transaction goes to the write phase. If the validation test is negative, the transaction is restarted and the changes are discarded.

• During the write phase, the changes are permanently applied to the database.


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Database Recovery Management • Database recovery (managed through backups)

– Restores database from given state, usually inconsistent, to previously consistent state

– Based on atomic transaction property• All portions of transaction must be treated as

single logical unit of work, so all operations must be applied and completed to produce consistent database

– If transaction operation cannot be completed, transaction must be aborted, and any changes to database must be rolled back (undone)


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Transaction Recovery

• Makes use of deferred-write and write-through techniques

• Deferred write – Transaction operations do not immediately

update physical database– Only transaction log is updated– Database is physically updated only after

transaction reaches its commit point using transaction log information


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Transaction Recovery (continued)

• Write-through – Database is immediately updated by

transaction operations during transaction’s execution, even before transaction reaches its commit point.


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Transaction Recovery (continued)

chapter 9_b

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