Discuss how the log file (or journal) is a fundamental feature in any recovery mechanism. Explain what is meant by forward and backward recovery and describe how the log file is used in forward and backward recovery. What is the significance of the write-ahead log protocol? How do checkpoints affect the recovery protocol?
What will be an ideal response?
Log file contains before and after-images of updates to the database. Before images can be used to
undo changes to the database (i.e. perform backward recovery); after-images can be used to redo
changes (i.e. perform forward recovery).
Write-ahead log protocol ensures that log records are written before the corresponding write to the
database. Thus, if the system fails before the write occurs after the log record has been written but
before the write to the database, the system can recover using the information in the log file.
Log file also contains a checkpoint record, which can speed up the time for recovery following a
failure.
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