Database AdministrationDatabase Administration

Part 1Part 1

Chapter SixChapter Six

CSCI260 Database Applications

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Chapter ObjectivesChapter Objectives

• Understand the need for and importance of Understand the need for and importance of database administrationdatabase administration

• Learn different ways of processing a databaseLearn different ways of processing a database

• Understand the need for concurrency control, Understand the need for concurrency control, security, and backup and recoverysecurity, and backup and recovery

• Learn typical problems that can occur when Learn typical problems that can occur when multiple users process a database multiple users process a database concurrentlyconcurrently

• Understand the use of locking and the Understand the use of locking and the problem of deadlock Understand the use of problem of deadlock Understand the use of locking and the problem of deadlocklocking and the problem of deadlock

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Chapter Objectives Chapter Objectives (continued)(continued)• Learn the difference between optimistic and Learn the difference between optimistic and

pessimistic lockingpessimistic locking• Know the meaning of ACID transactionKnow the meaning of ACID transaction• Learn the four 1992 ANSI standard isolation Learn the four 1992 ANSI standard isolation

levelslevels• Understand the need for security and learn a Understand the need for security and learn a

generalized model of database securitygeneralized model of database security• Know the difference between DBMS and Know the difference between DBMS and

application securityapplication security• Know the difference between recovery via Know the difference between recovery via

reprocessing and recovery via reprocessing and recovery via rollback/rollforwardrollback/rollforward

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Chapter Objectives Chapter Objectives (continued)(continued)• Understand the nature of the tasks required Understand the nature of the tasks required

for recovery using rollback/rollforwardfor recovery using rollback/rollforward• Know basic administrative and managerial Know basic administrative and managerial

DBA functionsDBA functions

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Database Processing Database Processing EnvironmentEnvironment

• A database processing environment A database processing environment is complicated and multi-facetedis complicated and multi-faceted– Multiple usersMultiple users– Multiple queriesMultiple queries– Multiple formsMultiple forms– Multiple reportsMultiple reports– Multiple application programsMultiple application programs

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Processing ConstraintsProcessing Constraints

• Enforcing referential integrityEnforcing referential integrity

• Cascading deletionCascading deletion

• Cascading modificationsCascading modifications

• Data type constraintsData type constraints

• Data size constraintsData size constraints

• Data value constraintsData value constraints

• Null constraintsNull constraints

• Uniqueness constraintsUniqueness constraints

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Internet Application Internet Application ProcessingProcessing

• Internet Application Processing is Internet Application Processing is more complicated than traditional more complicated than traditional application processingapplication processing

• Specifically, with Internet Application Specifically, with Internet Application Processing …Processing …– The network becomes an integral part of The network becomes an integral part of

the applicationthe application

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The Database Processing The Database Processing EnvironmentEnvironment

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Stored ProceduresStored Procedures

• A A stored procedurestored procedure is a module is a module similar to subroutine or function that similar to subroutine or function that performs database actionsperforms database actions– Stored in the database itselfStored in the database itself– Can pass it parameters, receive resultsCan pass it parameters, receive results– Written in PL/SQL (oracle), TRANSACT-Written in PL/SQL (oracle), TRANSACT-

SQL (SQL Server)SQL (SQL Server)

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Stored ProceduresStored Procedures

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Stored ProceduresStored Procedures

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TriggersTriggers

• A A triggertrigger is a stored procedure that is is a stored procedure that is automatically invoked by the DBMS automatically invoked by the DBMS when a specified activity occurswhen a specified activity occurs– BEFORE, AFTER and INSTEAD OFBEFORE, AFTER and INSTEAD OF

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Control, Security and ReliabilityControl, Security and Reliability

• Possible to have many database Possible to have many database functions occurring at the same timefunctions occurring at the same time

• Three necessary database Three necessary database administration functionsadministration functions– Concurrency controlConcurrency control– SecuritySecurity– Backup and RecoveryBackup and Recovery

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Concurrency ControlConcurrency Control

• Concurrency controlConcurrency control ensures that one user’s ensures that one user’s actions do not adversely impact another actions do not adversely impact another user’s actionsuser’s actions

• At the core of concurrency is accessibility. At the core of concurrency is accessibility. In one extreme, data becomes inaccessible In one extreme, data becomes inaccessible once a user touches the data. This ensures once a user touches the data. This ensures that data that is being considered for that data that is being considered for update is not shown. In the other extreme, update is not shown. In the other extreme, data is always readable. The data is even data is always readable. The data is even readable when it is locked for update.readable when it is locked for update.

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Concurrency Control Concurrency Control (continued)(continued)• InterdependencyInterdependency

– Changes required by one user may impact Changes required by one user may impact othersothers

• ConcurrencyConcurrency– People or applications may try to update the People or applications may try to update the

same information at the same timesame information at the same time

• Record retentionRecord retention– When information should be discardedWhen information should be discarded

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Need for Atomic Need for Atomic TransactionsTransactions• A database operation typically involves A database operation typically involves

several transactions. These transactions several transactions. These transactions are atomic and are sometimes called are atomic and are sometimes called logical units of work (LUW).logical units of work (LUW).

• Before an operation is Before an operation is committedcommitted to the to the database, all LUWs must successfully database, all LUWs must successfully complete. If one or more LUW is complete. If one or more LUW is unsuccessful, a unsuccessful, a rollbackrollback is performed and is performed and no changes are saved to the database.no changes are saved to the database.

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Need for Atomic Need for Atomic TransactionsTransactions• Example: Sequence required when Example: Sequence required when

recording new order.recording new order.– 1. Change the customer record, increasing 1. Change the customer record, increasing

value of amount owed.value of amount owed.– 2. Change the salesperson record, 2. Change the salesperson record,

increasing the value of Commission Due.increasing the value of Commission Due.– 3. Insert new order record into the 3. Insert new order record into the

database.database.

If last one fails, roll back all threeIf last one fails, roll back all three

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Lost Update ProblemLost Update Problem

• If two or more users are If two or more users are attempting to update the same attempting to update the same piece of data at the same time, it piece of data at the same time, it is possible that one update may is possible that one update may overwrite another update.overwrite another update.

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Concurrent Processing Concurrent Processing ExampleExample

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Concurrent Processing Concurrent Processing ProblemProblem

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Concurrency IssuesConcurrency Issues

• Concurrency issues common – have Concurrency issues common – have standardized names…standardized names…

• Dirty readsDirty reads– The transaction reads a changed record that The transaction reads a changed record that

has not been committed to the databasehas not been committed to the database– Example: One transaction reads row Example: One transaction reads row

changed by a second transaction. Second changed by a second transaction. Second transaction later cancels (rollsback) its transaction later cancels (rollsback) its changeschanges

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Concurrency IssuesConcurrency Issues

• Inconsistent reads/Non-repeatable readInconsistent reads/Non-repeatable read– The transaction re-reads a data set and finds The transaction re-reads a data set and finds

that the data has changedthat the data has changed

• Phantom readsPhantom reads– The transaction re-reads a data set and finds The transaction re-reads a data set and finds

that a new record has been addedthat a new record has been added

Fix some of these issues with Fix some of these issues with Resource Resource LockingLocking

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Resource Locking Resource Locking

• To avoid concurrency issues, resource To avoid concurrency issues, resource locking will disallow transactions from locking will disallow transactions from reading, modifying, and/or writing to a reading, modifying, and/or writing to a data set that has been “data set that has been “lockedlocked””

• Prevent sharing of dataPrevent sharing of data

• Transactions can use a lock commandTransactions can use a lock command

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Implicit versus Explicit Implicit versus Explicit Resource LockingResource Locking

• ImplicitImplicit locks are issued automatically locks are issued automatically by the DBMS based on an activityby the DBMS based on an activity

• ExplicitExplicit locks are issued by users locks are issued by users requesting exclusive rights to the datarequesting exclusive rights to the data

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Lock GranularityLock Granularity

• Size of lock has different impactsSize of lock has different impacts

• Large Granularity lock Large Granularity lock – e.g. lock entire tablee.g. lock entire table– easy for DBMS to administereasy for DBMS to administer– Frequently cause conflictsFrequently cause conflicts

• Smaller Granularity lockSmaller Granularity lock– E.g. lock rowE.g. lock row– Harder for DBMS to administer (lots of details)Harder for DBMS to administer (lots of details)– Fewer conflictsFewer conflicts

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Lock TypesLock Types

• Exclusive LockExclusive Lock– Locks an item from access of any typeLocks an item from access of any type– No other transaction can read or No other transaction can read or

change the datachange the data

• Shared lockShared lock– Locks an item from being changedLocks an item from being changed– Item can still be read, just can’t alter itItem can still be read, just can’t alter it

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Serializable Transactions Serializable Transactions (Two-Phased Locking)(Two-Phased Locking)

• Two-phased locking, whereby Two-phased locking, whereby locks are obtained as they are locks are obtained as they are neededneeded– A A growing growing phase, whereby the phase, whereby the

transaction continues to request transaction continues to request additional locksadditional locks

– A A shrinking shrinking phase, whereby the phase, whereby the transaction begins to release the transaction begins to release the lockslocks

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Serializable Transactions Serializable Transactions ExampleExample

• Order-entry transaction that Order-entry transaction that involves processing data in involves processing data in CUSTOMER, SALESPERSON and CUSTOMER, SALESPERSON and ORDER tables.ORDER tables.– Transaction issues locks on all three Transaction issues locks on all three

tablestables– Makes all the database changesMakes all the database changes– Releases all its locksReleases all its locks

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DeadlockDeadlock

• As a transaction begins to lock As a transaction begins to lock resources, it may have to wait for a resources, it may have to wait for a particular resource to be released by particular resource to be released by another transaction another transaction

• On occasions, two transactions may On occasions, two transactions may indefinitely wait on each another to indefinitely wait on each another to release resources. This condition is release resources. This condition is known as a known as a deadlockdeadlock or a deadly or a deadly embrace embrace

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Deadlock ExampleDeadlock Example

• User A wants to order some paperUser A wants to order some paper– If she can get the paper, she wants to If she can get the paper, she wants to

order pencilsorder pencils

• User B wants to order some pencils. User B wants to order some pencils. – If he can get the pencils, he will order If he can get the pencils, he will order

paperpaper

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Optimistic versus Optimistic versus Pessimistic LockingPessimistic Locking• OptimisticOptimistic

Locking Locking – Read dataRead data– Process transactionProcess transaction– Issue updateIssue update– Look for conflictLook for conflict– If conflict occurred, If conflict occurred,

rollback and repeatrollback and repeat– Else commitElse commit

• PessimisticPessimistic

LockingLocking– Lock required Lock required

resourcesresources– Read dataRead data– Process transactionProcess transaction– Issue commitIssue commit– Release locksRelease locks

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Consistent TransactionsConsistent Transactions

• Consistent transactions are often Consistent transactions are often referred to by the acronym referred to by the acronym ACIDACID – AtomicAtomic– ConsistentConsistent– IsolatedIsolated– DurableDurable

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ACID: AtomicACID: Atomic

• A transaction consists of a series of A transaction consists of a series of steps. Each step must be successful steps. Each step must be successful for the transaction to be savedfor the transaction to be saved

• This ensures that the transaction This ensures that the transaction completes everything it intended to completes everything it intended to do before saving the changesdo before saving the changes

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ACID: ConsistentACID: Consistent

• No other transactions are permitted No other transactions are permitted on the records until the current on the records until the current transaction finishestransaction finishes

• This ensures that the transaction This ensures that the transaction integrity has integrity has statement level statement level consistency consistency among all recordsamong all records

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ACID: ConsistentACID: Consistent

• Example:Example:• UPDATE CUSTOMERUPDATE CUSTOMER

SET SET AreaCode = ‘425’AreaCode = ‘425’WHEREWHERE ZipCode = ‘14048’;ZipCode = ‘14048’;

• CUSTOMER table has 500,000 rows. 500 match CUSTOMER table has 500,000 rows. 500 match above zipcodeabove zipcode

• May take awhile to find them all – can other May take awhile to find them all – can other transactions update during this?transactions update during this?

• Statement level consistency – update will apply to Statement level consistency – update will apply to the set of rows as they existed at the time the the set of rows as they existed at the time the SQL Statement startedSQL Statement started

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ACID: IsolationACID: Isolation

• Within multiuser environments, Within multiuser environments, different transactions may be different transactions may be operating on the same data operating on the same data

• As such, the sequencing of As such, the sequencing of uncommitted updates, rollbacks, and uncommitted updates, rollbacks, and commits continuously change the commits continuously change the data contentdata content

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ACID: DurableACID: Durable

• A durable transaction is one in A durable transaction is one in which all committed changes are which all committed changes are permanentpermanent

• Even in the case of failureEven in the case of failure

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1992 ANSI SQL Isolation levels1992 ANSI SQL Isolation levels