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<h2 class="title"><a id="txn"></a>Chapter 20. The Transaction Subsystem </h2>
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<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="txn.html#txn_intro">Introduction to the transaction subsystem</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="txn_config.html">Configuring transactions</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="txn_limits.html">Transaction limits</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="txn_limits.html#idm140654535804464">Transaction IDs</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="txn_limits.html#idm140654535926768">Cursors</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="txn_limits.html#idm140654535792896">Multiple Threads of Control</a>
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<div>
<h2 class="title" style="clear: both"><a id="txn_intro"></a>Introduction to the transaction subsystem</h2>
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<p>
The Transaction subsystem makes operations atomic,
consistent, isolated, and durable in the face of system and
application failures. The subsystem requires that the data be
properly logged and locked in order to attain these
properties. Berkeley DB contains all the components necessary
to transaction-protect the Berkeley DB access methods, and
other forms of data may be protected if they are logged and
locked appropriately.
</p>
<p>
The Transaction subsystem is created, initialized, and
opened by calls to <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> with the <a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a> flag
specified. Note that enabling transactions automatically
enables logging, but does not enable locking because a single
thread of control that needed atomicity and recoverability
would not require it.
</p>
<p>
The <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> function starts a transaction, returning an
opaque handle to a transaction. If the parent parameter to
<a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> is non-NULL, the new transaction is a child of the
designated parent transaction.
</p>
<p>
The <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> function ends the designated transaction and
causes all updates performed by the transaction to be undone.
The end result is that the database is left in a state
identical to the state that existed prior to the <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>.
If the aborting transaction has any child transactions
associated with it (even ones that have already been
committed), they are also aborted. Any transactions that are
unresolved (neither committed nor aborted) when the
application or system fails are aborted during
recovery.
</p>
<p>
The <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a> function ends the designated transaction and
makes all the updates performed by the transaction permanent,
even in the face of application or system failure. If this is
a parent transaction committing, all child transactions that
individually committed or had not been resolved are also
committed.
</p>
<p>
Transactions are identified by 32-bit unsigned integers. The
ID associated with any transaction can be obtained using the
<a href="../api_reference/C/txnid.html" class="olink">DB_TXN->id()</a> function. If an application is maintaining information
outside of Berkeley DB it wants to transaction-protect, it
should use this transaction ID as the locking ID.
</p>
<p>
The <a href="../api_reference/C/txncheckpoint.html" class="olink">DB_ENV->txn_checkpoint()</a> function causes a transaction
checkpoint. A checkpoint is performed using to a specific log
sequence number (LSN), referred to as the checkpoint LSN. When
a checkpoint completes successfully, it means that all data
buffers whose updates are described by LSNs less than the
checkpoint LSN have been written to disk. This, in turn, means
that the log records less than the checkpoint LSN are no
longer necessary for normal recovery (although they would be
required for catastrophic recovery if the database files were
lost), and all log files containing only records prior to the
checkpoint LSN may be safely archived and removed.
</p>
<p>
The time required to run normal recovery is proportional to
the amount of work done between checkpoints. If a large number
of modifications happen between checkpoints, many updates
recorded in the log may not have been written to disk when
failure occurred, and recovery may take longer to run.
Generally, if the interval between checkpoints is short, data
may be being written to disk more frequently, but the recovery
time will be shorter. Often, the checkpoint interval is tuned
for each specific application.
</p>
<p>
The <a href="../api_reference/C/txnstat.html" class="olink">DB_TXN->stat()</a> method returns information about the status of
the transaction subsystem. It is the programmatic interface
used by the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility.
</p>
<p>
The transaction system is closed by a call to
<a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a>.
</p>
<p>
Finally, the entire transaction system may be removed using
the <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> method.
</p>
<p>
For more information on the transaction subsystem methods,
see the <a href="../api_reference/C/txn.html#txnlist" class="olink">
Transaction Subsystem and Related Methods</a> section
in the <em class="citetitle">Berkeley DB C API Reference Guide.</em>
</p>
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