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<h2 class="title"><a id="introduction"></a>Chapter 1. Introduction</h2>
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<b>Table of Contents</b>
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<dt>
<span class="sect1">
<a href="introduction.html#txnintro">Transaction Benefits</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="sysfailure.html">A Note on System Failure</a>
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<dt>
<span class="sect1">
<a href="apireq.html">Application Requirements</a>
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<dt>
<span class="sect1">
<a href="multithread-intro.html">Multi-threaded
<span>and Multi-process</span>
Applications</a>
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<dt>
<span class="sect1">
<a href="recovery-intro.html">Recoverability</a>
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<dt>
<span class="sect1">
<a href="perftune-intro.html">Performance Tuning</a>
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<p>
This book provides a thorough introduction and discussion on transactions as
used with Berkeley DB (DB).
It begins by offering a general overview to
transactions, the guarantees they provide, and the general application
infrastructure required to obtain full transactional protection for your
data.
</p>
<p>
This book also provides detailed examples on how to write a
transactional application. Both single threaded and multi-threaded <span>(as well as multi-process
applications)</span> are discussed. A detailed description of various
backup and recovery strategies is included in this manual, as is a
discussion on performance considerations for your transactional application.
</p>
<p>
You should understand the concepts from the
<span>
<em class="citetitle">Getting Started with Berkeley DB</em>
</span>
guide before reading this book.
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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="txnintro"></a>Transaction Benefits</h2>
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<p>
Transactions offer your application's data protection from
application or system failures. That is, DB transactions offer
your application full ACID support:
</p>
<div class="itemizedlist">
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<li>
<p>
<span class="bold"><strong>A</strong></span>tomicity
</p>
<p>
Multiple database operations are treated as a single unit of
work. Once committed, all write operations performed under
the protection of the transaction are saved to your databases.
Further, in the event that you abort a transaction, all write
operations performed during the transaction are discarded.
In this event, your database is left in the state it was in
before the transaction began, regardless of the number or
type of write operations you may have performed during the
course of the transaction.
</p>
<p>
Note that DB transactions can span one or more
database handles.
</p>
</li>
<li>
<p>
<span class="bold"><strong>C</strong></span>onsistency
</p>
<p>
Your databases will never see a partially completed
transaction. This is true even if your application fails while there are
in-progress transactions. If the application or system fails,
then either all of the database changes appear when the
application next runs, or none of them appear.
</p>
<p>
In other words, whatever consistency requirements your application has will never be violated by DB.
If, for example, your application requires every record to include an employee ID, and your
code faithfully adds that ID to its database records, then DB will never
violate that consistency requirement. The ID will remain in the database records until such a time as your
application chooses to delete it.
</p>
</li>
<li>
<p>
<span class="bold"><strong>I</strong></span>solation
</p>
<p>
While a transaction is in progress, your databases will appear
to the transaction as if there are no other operations
occurring outside of the transaction. That is, operations
wrapped inside a transaction will always have a clean and
consistent view of your databases. They never have to see
updates currently in progress under the protection of another transaction.
Note, however, that isolation guarantees can be
relaxed from the default setting. See
<a class="xref" href="isolation.html" title="Isolation">Isolation</a>
for more information.
</p>
</li>
<li>
<p>
<span class="bold"><strong>D</strong></span>urability
</p>
<p>
Once committed to your databases, your modifications will
persist even in the event of an application or system failure.
Note that like isolation, your durability guarantee can be
relaxed. See <a class="xref" href="nodurabletxn.html" title="Non-Durable Transactions">Non-Durable Transactions</a>
for more information.
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