Lesson No. 42

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DatabaseManagement 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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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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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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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
• ltT, startsgt
• ltT, commitsgt or ltT, abortsgt
• Data entries?

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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 ltT, X,
  cgt simplified

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Log File Entries
Read (X) X X 5 Write (X) Y Y 15 Write
(Y) Commit

• ltT, startsgt
• ltT, X, 55gt
• ltT, Y, 30gt
• ltT, commitgt

Supposing X 50 Y 10
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DatabaseManagement System
Lecture - 42