Question

Explain deferred database modification technique with example

Answer 1

It ensures transaction atomicity by recording all database modifications in the log but deferring the execution of all write operations of a transaction until the transaction partially commits.
A transaction is said to be partially committed once the final action of the transaction has been executed. When a transaction has performed all the actions, then the information in the log associated with the transaction is used in executing the deferred writes. In other words, at partial commits, time logged updates are “replayed” into database item.

The recovery procedure of deferred database modification is based on Redo operation 

Redo(Ti)
It sets the value of all data items updated by transaction Ti to the new values from the log of records.
After a failure has occurred the recovery subsystem consults the log to determine which transaction need to be redone. Transaction Ti needs to be redone if an only if the log contains both the record <Ti, start> and the record  <Ti, commit>. Thus, if the system crashes after the transaction completes its execution, then the information in the log is used in restoring the system to a previous consistence state.


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