CSC-313 Advanced Database Systems Lecture 4 PDF
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The University of Haripur
Farah Shaheen
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Summary
Lecture notes from CSC-313 Advanced Database Systems, covering concurrency control, resource locking, and locking terminology. Authored by Farah Shaheen at The University of Haripur, the notes discuss implicit, explicit, exclusive, and shared locks.
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CSC-313 Advanced Database Systems Lecture 4 Concurrency Control By Farah Shaheen Acknowledgement: D...
CSC-313 Advanced Database Systems Lecture 4 Concurrency Control By Farah Shaheen Acknowledgement: Department of Information Technology – The UniversityLecture slides material from of Haripur Stuart Russell Resource Locking  One way to prevent concurrency problems is to lock the shared data.  Locking ensure that the shared data can be used by one user at one time.  When a user accesses the data, the second user to wait until the first user finishes its work.  For example: There is an item in the database with a value of 100.  If two users try to access the item to update it. Department of Information Technology – The University of Haripur 15 Example Department of Information Technology – The University of Haripur 16 Locking Terminology  Some important terms: Implicit Lock: It is a type of lock placed automatically by DBMS. Explicit Lock: It is a type of lock placed by application program by issuing a command. Exclusive Lock: A type of lock that locks an item from any type of access is known as exclusive lock. If a transaction locks an item using exclusive lock, no other transaction can read or change data. Shared Lock: A type of lock that locks an item from change but not from read is known as shared lock. If a transaction has locked an item using shared lock, the other transaction can only read the item but cannot modify it. Department of Information Technology – The University of Haripur 16 Locking Terminology Lock Granularity: The level of the lock applied on an item is known as lock granularity. Different levels of locks are as follows: Row level (Smallest lock granularity) Page level Table Level Database level (Largest lock granularity) A lock with large granularity is easy to maintain for DBMS but may cause problems. A lock with small granularity is difficult to maintain for DBMS but causes less problems. Department of Information Technology – The University of Haripur 16
