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170 lines
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 19. The Transaction Subsystem</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="prev" href="mp_warm.html" title="Warming the memory pool" />
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<p>Library Version 11.2.5.3</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Chapter 19.
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The Transaction Subsystem
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="mp_warm.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="txn_config.html">Next</a></td>
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<div class="chapter" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title"><a id="txn"></a>Chapter 19.
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The Transaction Subsystem
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</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="txn.html#txn_intro">Introduction to the transaction subsystem</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="txn_config.html">Configuring transactions</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="txn_limits.html">Transaction limits</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="txn_limits.html#idp3173248">Transaction IDs</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="txn_limits.html#idp3049928">Cursors</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="txn_limits.html#idp3082656">Multiple Threads of Control</a>
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</span>
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</dt>
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</dl>
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</dd>
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</dl>
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="txn_intro"></a>Introduction to the transaction subsystem</h2>
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</div>
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</div>
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</div>
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<p>The Transaction subsystem makes operations atomic, consistent, isolated,
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and durable in the face of system and application failures. The subsystem
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requires that the data be properly logged and locked in order to attain
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these properties. Berkeley DB contains all the components necessary to
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transaction-protect the Berkeley DB access methods, and other forms of data may
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be protected if they are logged and locked appropriately.</p>
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<p>The Transaction subsystem is created, initialized, and opened by calls to
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<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
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that enabling transactions automatically enables logging, but does not
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enable locking because a single thread of control that needed atomicity
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and recoverability would not require it.</p>
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<p>The <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> function starts a transaction, returning an opaque
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handle to a transaction. If the parent parameter to <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> is
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non-NULL, the new transaction is a child of the designated parent
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transaction.</p>
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<p>The <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> function ends the designated transaction and causes
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all updates performed by the transaction to be undone. The end result is
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that the database is left in a state identical to the state that existed
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prior to the <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>. If the aborting transaction has any child
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transactions associated with it (even ones that have already been
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committed), they are also aborted. Any transactions that are unresolved
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(neither committed nor aborted) when the application or system fails
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are aborted during recovery.</p>
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<p>The <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a> function ends the designated transaction and makes
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all the updates performed by the transaction permanent, even in the face
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of application or system failure. If this is a parent transaction
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committing, all child transactions that individually committed or
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had not been resolved are also committed.</p>
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<p>Transactions are identified by 32-bit unsigned integers. The ID
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associated with any transaction can be obtained using the <a href="../api_reference/C/txnid.html" class="olink">DB_TXN->id()</a>
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function. If an application is maintaining information outside of Berkeley DB
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it wants to transaction-protect, it should use this transaction ID as
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the locking ID.</p>
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<p>The <a href="../api_reference/C/txncheckpoint.html" class="olink">DB_ENV->txn_checkpoint()</a> function causes a transaction checkpoint. A
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checkpoint is performed using to a specific log sequence number (LSN),
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referred to as the checkpoint LSN. When a checkpoint completes
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successfully, it means that all data buffers whose updates are described
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by LSNs less than the checkpoint LSN have been written to disk. This, in
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turn, means that the log records less than the checkpoint LSN are no
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longer necessary for normal recovery (although they would be required for
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catastrophic recovery if the database files were lost), and all log files
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containing only records prior to the checkpoint LSN may be safely archived
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and removed.</p>
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<p>The time required to run normal recovery is proportional to the amount
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of work done between checkpoints. If a large number of modifications
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happen between checkpoints, many updates recorded in the log may
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not have been written to disk when failure occurred, and recovery may
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take longer to run. Generally, if the interval between checkpoints is
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short, data may be being written to disk more frequently, but the
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recovery time will be shorter. Often, the checkpoint interval is tuned
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for each specific application.</p>
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<p>The <a href="../api_reference/C/txnstat.html" class="olink">DB_TXN->stat()</a> method returns information about the status of the
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transaction subsystem. It is the programmatic interface used by the
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<a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility.</p>
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<p>The transaction system is closed by a call to <a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a>.</p>
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<p>Finally, the entire transaction system may be removed using the
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<a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> method.</p>
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<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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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="mp_warm.html">Prev</a> </td>
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<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="txn_config.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Warming the memory pool </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Configuring transactions</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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