1. Database Management System
Lecture - 42
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Transaction
Generally reflects the activities in the real world system
One transaction in our exam system may be computing gpa of a student, Operations involved…..
More complex transactions
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Consistent State of DB
Consistent state of the database
Duplicated data does not conflict
DB properly reflect the real-world system, i.e. follows the business rules
A transaction should transform database from one consistent state to another consistent state, may be inconsistent temp
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4. Transaction Termination
Two ways a transaction terminates or ends
It may Commit or Abort
If executed successfully, brings the database in a new consistent state, said to be committed
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5. Transaction Termination
If otherwise, database has to be brought in prior consistent state
Such transaction is rolled back or undone
A committed transaction cannot be undone; compensating transaction
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6. Transaction Boundaries
Means being and end of Tr
Generally responsibility of programmer
Otherwise approach varies from DBMS to DBMS
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ACID Properties
Atomicity: all or none
Consistency: maintains consistency of the DB
Isolation: Tr should be protected form the effect of other Trs
Durability: changes by Tr are permanent
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Database Updates
Updates are first made in the buffers in RAM, then transferred to database
There may be some delay between the two
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Database Recovery
If a crash occurs during this delay then the database is in in consistent state
On restart DBMS has to identify it and recover DB into a consistent state
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Reasons of Failure
Natural disaster
Sabotage
Carelessness
Disk crash
System software errors
Generally, improper shut down of DBMS
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Recovery Techniques
System crashes, buffers are lost but the disk copy is safe
Need to determine the transactions that need to redone or undone
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Log File
Tool used for database recovery
Transaction Record
<T, starts>
<T, commits> or <T, aborts>
Data entries?
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13. Transaction Operations
Read (X)
X = X + 5
Write (X)
Y = Y * 3
Write (Y)
Commit
Operations of DBMS concern
Concern of Recovery Manager
So log file contains entries only for write operations
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Deferred Updates
Called incremental log with deferred updates
Updates are not performed until commit is encountered
Data entries in the log file of the form <T, X, c>; simplified
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Log File Entries
Read (X)
X = X + 5
Write (X)
Y = Y * 15
Write (Y)
Commit
<T, starts>
<T, X, 55>
<T, Y, 30>
<T, commit>
Supposing
X = 50
Y = 10
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16. Database Management System
Lecture - 42
© Virtual University of Pakistan