1 it420: database management and organization transactions 31 march 2006 adina crăiniceanu adina
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1
IT420: Database Management and Organization
Transactions31 March 2006
Adina Crăiniceanuwww.cs.usna.edu/~adina
Kroenke, Database Processing 2
Project 2
To insert an encrypted password into the database: PHP: Find encrypted value MySQL: Insert into Users table
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Goals
Concurrency control
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DBA Tasks
Managing database structure Controlling concurrent processing Managing processing rights and responsibilities Developing database security Providing for database recovery Managing the DBMS Maintaining the data repository
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Atomic Transactions
Series of actions taken against the database that occur as an atomic unit Either all actions in a transaction occur - COMMIT Or none of them do - ABORT
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Class Exercise
Transaction steps Possible schedule Possible problems
T1: Transfer money from savings to checking
T2: Add interest for savings account
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Resource Locking
Locking: prevents multiple applications from obtaining copies of the same resource when the resource is about to be changed
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Lock Terminology
Implicit locks - placed by the DBMS Explicit locks - issued by the application
program Lock granularity - size of a locked resource
Rows, page, table, and database level Types of lock
Exclusive lock (X)- prohibits other users from reading the locked resource
Shared lock (S) - allows other users to read the locked resource, but they cannot update it
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Explicit Locks
Lock type?
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Serializable Transactions
Serializable transactions: Run concurrently Results like when they run separately
Strict two-phase locking – locking technique to achieve serializability
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Strict Two-Phase Locking
Strict two-phase locking Locks are obtained throughout the transaction All locks are released at the end of
transaction (COMMIT or ROLLBACK)
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Strict 2PL Example Not 2PL X(Sa) R(Sa) W(Sa) Rel(Sa) X(Ch) R(Ch) W(Ch) Rel(Ch)
Strict 2PL X(Sa) R(Sa) W(Sa)
X(Ch) R(Ch) W(Ch) Rel(Ch) Rel(Sa)
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Deadlock
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Deadlock
Deadlock: two transactions are each waiting on a resource that the other transaction holds
Preventing deadlock Allow users to issue all lock requests at one time Require all application programs to lock resources in the same
order Breaking deadlock
Almost every DBMS has algorithms for detecting deadlock When deadlock occurs, DBMS aborts one of the transactions
and rollbacks partially completed work
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Optimistic versus PessimisticLocking Optimistic locking assumes that no transaction
conflict will occur: DBMS processes a transaction; checks whether
conflict occurred: If not, the transaction is finished If yes, the transaction is repeated until there is no conflict
Pessimistic locking assumes that conflict will occur: Locks are issued before a transaction is processed,
and then the locks are released
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Optimistic Locking
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Pessimistic Locking
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Declaring Lock Characteristics
Most application programs do not explicitly declare locks due to its complication
Mark transaction boundaries and declare locking behavior they want the DBMS to use Transaction boundary markers: BEGIN, COMMIT, and
ROLLBACK TRANSACTION
Advantage If the locking behavior needs to be changed, only the lock
declaration need be changed, not the application program
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Marking Transaction Boundaries
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ACID Transactions
Transaction properties: Atomic - all or nothing Consistent Isolated Durable – changes made by commited transactions
are permanent
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Consistency
Consistency means either statement level or transaction level consistency Statement level consistency: each statement
independently processes rows consistently Transaction level consistency: all rows impacted by
either of the SQL statements are protected from changes during the entire transaction With transaction level consistency, a transaction may not see
its own changes
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Statement Level Consistency
UPDATE CUSTOMER
SET AreaCode = ‘410’
WHERE ZipCode = ‘21218’
All qualifying rows updated No concurrent updates allowed
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Transaction Level Consistency
Start transactionUPDATE CUSTOMERSET AreaCode = ‘425’WHERE ZipCode = ‘21666’….other transaction workUPDATE CUSTOMERSET Discount = 0.25WHERE AreaCode = ‘425’End TransactionThe second Update might not see the changes it made on the first Update
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ACID Transactions
Atomic Consistent Isolated Durable
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Inconsistent-Read Problem
Dirty reads – read uncommitted data T1: R(A), W(A), R(B), W(B), Abort T2: R(A), W(A), Commit
Unrepeatable reads T1: R(A), R(A), W(A), Commit T2: R(A), W(A), Commit
Phantom reads Re-read data and find new rows
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Isolation
SQL-92 defines four transaction isolation levels: Read uncommitted Read committed Repeatable read Serializable
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Transaction Isolation Level
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