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<p>
From time to time this manual mentions that transactions
protect your data against 'system or application failure.' This
is true up to a certain extent. However, not all failures are
created equal and no data protection mechanism can protect you
against every conceivable way a computing system can find to
die.
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<p>
Generally, when this book talks about protection against
failures, it means that transactions offer protection against
the likeliest culprits for system and application crashes. So
long as your data modifications have been committed to disk,
those modifications should persist even if your application or
OS subsequently fails. And, even if the application or OS
fails in the middle of a transaction commit (or abort), the
data on disk should be either in a consistent state, or there
should be enough data available to bring your databases into a
consistent state (via a recovery procedure, for example). You
may, however, lose whatever data you were committing at the
time of the failure, but your databases will be otherwise
unaffected.
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<p>
Be aware that many disks have a disk write cache and on
some systems it is enabled by default. This means that
a transaction can have committed, and to your
application the data may appear to reside on disk, but
the data may in fact reside only in the write cache at
that time. This means that if the disk write cache is
enabled and there is no battery backup for it, data can
be lost after an OS crash even when maximum durability
mode is in use. For maximum durability, disable the
disk write cache or use a disk write cache with a
battery backup.
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<p>
Of course, if your <span class="emphasis"><em>disk</em></span> fails, then the transactional benefits described in this book
are only as good as the backups you have taken.
<span>
By spreading your data and log files across separate disks,
you can minimize the risk of data loss due to a disk
failure, but even in this case it is possible to conjure a
scenario where even this protection is insufficient (a fire
in the machine room, for example) and you must go to your
backups for protection.
</span>
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<p>
Finally, by following the programming examples shown in this book, you can write your code so as to protect
your data in the event that your code crashes. However, no programming API can protect you against logic
failures in your own code; transactions cannot protect you from simply writing the wrong thing to your
databases.
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