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<p>
An important part of DB's transactional guarantees is durability.
<span class="emphasis"><em>Durability</em></span> means that once a
transaction has been committed, the database modifications performed
under its protection will not be lost due to system failure.
</p>
<p>
In order to provide the transactional durability guarantee,
DB uses a write-ahead logging system. Every operation performed on
your databases is described in a log before it is performed on
your databases. This is done in order to ensure that an operation can be
recovered in the event of an untimely application
or system failure.
</p>
<p>
<span>
Beyond logging, another important aspect of durability is
recoverability. That is, backup and restore. </span>
DB supports a normal recovery that runs against a subset of
your log files. This is a routine procedure used whenever your
environment is first opened upon application startup, and it is intended to
ensure that your database is in a consistent state. DB also
supports archival backup and recovery in the case of
catastrophic failure, such as the loss of a physical disk
drive.
</p>
<p>
This book describes several different backup procedures
you can use to protect your on-disk data. These procedures
range from simple offline backup strategies to hot failovers. Hot failovers
provide not only a backup mechanism, but
also a way to recover from a fatal hardware failure.
</p>
<p>
This book also describes the recovery procedures you should use
for each of the backup strategies that you might employ.
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For a detailed description of backup and restore procedures, see
<span>
<a href="filemanagement.html">Managing DB Files</a>.
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