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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Transaction Protected Applications</dl></h3></td>
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<p>
<h1 align=center>Why transactions?</h1>
<p>Perhaps the first question to answer is "Why transactions?" There are
a number of reasons for including transactional support in your
applications. The most common ones are:
<p><dl compact>
<p><dt>Recoverability<dd>Applications often need to ensure that, no matter how the system or
application fails, previously saved data is available the next time the
application runs.
<p><dt>Deadlock avoidance<dd>When multiple threads of control change the database at the same time,
there is usually the possibility of deadlock, that is, where each of
the threads of control owns a resource another thread wants, and so no
thread is able to make forward progress, all waiting for a resource.
Deadlocks are resolved by having one of the operations involved release
the resources it controls so the other operations can proceed. (The
operation releasing its resources usually just tries again later.)
Transactions are necessary so that any changes that were already made
to the database can be undone as part of releasing the held resources.
<p><dt>Atomicity<dd>Applications often need to make multiple changes to one or more
databases, but want to ensure that either all of the changes happen, or
none of them happen. Transactions guarantee that a group of changes
are atomic, that is, if the application or system fails, either all of
the changes to the databases will appear when the application next runs,
or none of them.
<p><dt>Repeatable reads<dd>Applications sometimes need to ensure that, while doing a group of
operations on a database, the value returned as a result of a database
retrieval doesn't change, that is, if you retrieve the same key more
than once, the data item will be the same each time. Transactions
guarantee this behavior.
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