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Yasuhiro KIMURA
203 lines
9.2 KiB
HTML
203 lines
9.2 KiB
HTML
<?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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<html xmlns="http://www.w3.org/1999/xhtml">
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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 20. The Transaction Subsystem</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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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="up" 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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<link rel="next" href="txn_config.html" title="Configuring transactions" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</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 20. The Transaction Subsystem </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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</tr>
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</table>
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<hr />
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</div>
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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 20. The Transaction Subsystem </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#idm140654535804464">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#idm140654535926768">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#idm140654535792896">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>
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The Transaction subsystem makes operations atomic,
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consistent, isolated, and durable in the face of system and
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application failures. The subsystem requires that the data be
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properly logged and locked in order to attain these
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properties. Berkeley DB contains all the components necessary
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to transaction-protect the Berkeley DB access methods, and
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other forms of data may be protected if they are logged and
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locked appropriately.
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</p>
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<p>
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The Transaction subsystem is created, initialized, and
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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
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specified. Note that enabling transactions automatically
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enables logging, but does not enable locking because a single
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thread of control that needed atomicity and recoverability
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would not require it.
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</p>
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<p>
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The <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a> function starts a transaction, returning an
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opaque handle to a transaction. If the parent parameter to
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<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
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designated parent transaction.
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</p>
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<p>
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The <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> function ends the designated transaction and
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causes all updates performed by the transaction to be undone.
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The end result is that the database is left in a state
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identical to the state that existed prior to the <a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>.
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If the aborting transaction has any child transactions
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associated with it (even ones that have already been
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committed), they are also aborted. Any transactions that are
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unresolved (neither committed nor aborted) when the
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application or system fails are aborted during
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recovery.
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</p>
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<p>
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The <a href="../api_reference/C/txncommit.html" class="olink">DB_TXN->commit()</a> function ends the designated transaction and
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makes all the updates performed by the transaction permanent,
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even in the face of application or system failure. If this is
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a parent transaction committing, all child transactions that
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individually committed or had not been resolved are also
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committed.
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</p>
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<p>
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Transactions are identified by 32-bit unsigned integers. The
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ID associated with any transaction can be obtained using the
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<a href="../api_reference/C/txnid.html" class="olink">DB_TXN->id()</a> function. If an application is maintaining information
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outside of Berkeley DB it wants to transaction-protect, it
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should use this transaction ID as the locking ID.
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</p>
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<p>
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The <a href="../api_reference/C/txncheckpoint.html" class="olink">DB_ENV->txn_checkpoint()</a> function causes a transaction
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checkpoint. A checkpoint is performed using to a specific log
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sequence number (LSN), referred to as the checkpoint LSN. When
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a checkpoint completes successfully, it means that all data
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buffers whose updates are described by LSNs less than the
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checkpoint LSN have been written to disk. This, in turn, means
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that the log records less than the checkpoint LSN are no
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longer necessary for normal recovery (although they would be
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required for catastrophic recovery if the database files were
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lost), and all log files containing only records prior to the
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checkpoint LSN may be safely archived and removed.
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</p>
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<p>
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The time required to run normal recovery is proportional to
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the amount of work done between checkpoints. If a large number
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of modifications happen between checkpoints, many updates
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recorded in the log may not have been written to disk when
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failure occurred, and recovery may take longer to run.
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Generally, if the interval between checkpoints is short, data
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may be being written to disk more frequently, but the recovery
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time will be shorter. Often, the checkpoint interval is tuned
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for each specific application.
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</p>
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<p>
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The <a href="../api_reference/C/txnstat.html" class="olink">DB_TXN->stat()</a> method returns information about the status of
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the transaction subsystem. It is the programmatic interface
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used by the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility.
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</p>
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<p>
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The transaction system is closed by a call to
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<a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a>.
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</p>
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<p>
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Finally, the entire transaction system may be removed using
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the <a href="../api_reference/C/envremove.html" class="olink">DB_ENV->remove()</a> method.
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</p>
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<p>
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For more information on the transaction subsystem methods,
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see the <a href="../api_reference/C/txn.html#txnlist" class="olink">
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Transaction Subsystem and Related Methods</a> section
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in the <em class="citetitle">Berkeley DB C API Reference Guide.</em>
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</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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