Chapter 12 Managing Multi-user Databases David M. Kroenke Database Processing © 2000 Prentice Hall
Dec 14, 2015
Chapter 12
© 2000 Prentice Hall
Multi-User Issues
• Concurrency Control• Database Reliability• Database Security• Database Administration
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Common Multi-User DBMS
• Windows 2000– Access 2000– SQL Server– ORACLE
• UNIX– ORACLE– Sybase– Informix
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Concurrency Control
“making sure that one user’s work does not inappropriately influence another’s”
The need for atomic transactions “logical work performed as a unit”
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Concurrent Processing Problems
• Lost update problem• Inconsistent read problem
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Remedy for inconsistencies caused by concurrent
processing• Resource Locking“disallow sharing by locking data
that are retrieved for update”
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Lock Terminology
• Implicit locks placed by the DBMS• Explicit locks placed by command• Lock granularity the size of the lock• Exclusive lock from access of any type• Shared lock from change but not read
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Serializable Transactions
“a scheme for processing concurrent transactions”
Strategies– two-phased locking– COMMIT and ROLLBACK
commands
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Deadlock
“deadly embrace”; each transaction waiting for a resource that the other person has locked
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Lock Styles
Optimistic assumption is made that no conflict will occur
Pessimistic assumption is made that conflict will occur
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Database Recovery
• Via Reprocessing• Via Rollback/Rollforward
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Recovery Terminology
Log records of the data changes in chronological order
Before-images/After-images copy of every record before / after it was changed
Checkpoint a point of synchronization between the database and the transaction log
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Database Administration
• DBA database administrator– manages the database structure– manages data activity– manages the DBMS– manages the data repository
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Managing the Database Structure
• Configuration Control• Documentation
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Managing Data Activity
Data dictionary names and formats of the data items, and their relationships
Data proponents key database users
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