8.5- Immediate Database Modification Recovery System | Recovery System In DBMS In Hindi

Описание: 8.5- Immediate Database Modification Recovery System | Recovery System In DBMS In Hindi

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Recovery System in dbms, Log based recovery in dbms, shadow paging in dbms, Deferred Database Modification in DBMS,

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Recovery Based on Deferred Database Modification :
In the deferred database modification technique, deferred (stops) all the write operations of any Transaction Ti until it partially commits. It means modifying the real database after Ti partially commits. All the activities are recorded in log. Log records are used to modify the actual database.
Suppose a transaction Ti wants to write on data item Aj, then a log record [Ti, Aj, V1, V2] is saved in log and it is used to modify the database. After actual modification, Ti enters in a committed state. In this technique, the old value field is not needed.
Consider the example of the Banking system. Suppose you want to transfer ` 200 from Account A to B in Transaction T1 and deposit ` 200 to Account C in T2.

The crash will happen at any time of execution of transactions. Suppose crash happened (i) After write (A) of T1:
There is no need to redo the operation because no commit record appears in the log. Log records of T1 can be deleted.
(ii) After write (C) of T2 :
In this situation, you have to redo T1 because both [T1 start] and [T1 commit] appears in log. After redo operation, value of A and B are 300 and 1200 respectively. Values remain the same because redo is idempotent.
(iii) During recovery: If the system is crashed at the time of recovery, simply start the recovery again.

Recovery Based on Immediate Database Modification:
In the immediate database modification technique, the database is modified by any transaction Ti during its active state. It means, real database is modified just after the write operation but after log record is written to stable storage. This is because log records are used during recovery. Use both Undo and Redo operations in this method.

Checkpoints: Both the techniques discussed earlier ensures recovery from failure state, but they have some disadvantages such as :
(i) They are time consuming because successfully completed transactions have to be redone.
(ii) Searching procedure is also time-consuming because the whole log has to be searched. So, use checkpoints to reduce overhead. Any of the previous recovery techniques can be used with checkpoints. All the transactions completed successfully or having [Ti commit] record before [checkpoint] record need not to be redone.
During the time of failure, search the most recent checkpoint. All the transactions completed successfully after the checkpoint need to be redone. After searching checkpoint, search the most
recent transaction Ti that started execution before that checkpoint but not completed. After searching that transaction, redo/undo transaction as required in applied method.
Advantages: The major advantages of checkpoints are
1. No need to redo successfully completed transactions before most recent checkpoints. 2. Less searching required. 3. Old records can be deleted.