Recovery and Atomicity
Log Records: The ‘Log File’ is used to record the database modifications. The log file is a sequence of the ‘log records’, recording all the update activities in the database.
The components of Log Record :
(a) Transaction Identifier: It is the unique identifier of the transaction that performed the ‘write’ operation.
(b) Data-item Identifier: It is the unique identifier of the data item written.
(c) Old–Value: It is the value of the data item prior to the ‘write’.
(d) New-Value: It is the value that the data item will have after
‘write’ operation.
There are three types of log records :
< Ti start> : Transaction Ti has started.
< Ti commt> : Transaction Ti has committed.
< Ti abort > : Transaction Ti has aborted.
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