mirror of
https://github.com/yasuhirokimura/db18
synced 2026-06-08 18:29:23 +00:00
Yasuhiro KIMURA
150 lines
6.2 KiB
HTML
150 lines
6.2 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>Transaction limits</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="txn.html" title="Chapter 20. The Transaction Subsystem" />
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<link rel="prev" href="txn_config.html" title="Configuring transactions" />
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<link rel="next" href="sequence.html" title="Chapter 21. Sequences" />
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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">Transaction limits</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="txn_config.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 20. The Transaction Subsystem </th>
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<td width="20%" align="right"> <a accesskey="n" href="sequence.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="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_limits"></a>Transaction limits</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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<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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</div>
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<div class="sect2" 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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<h3 class="title"><a id="idm140654535804464"></a>Transaction IDs</h3>
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</div>
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</div>
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</div>
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<p>
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Transactions are identified by 31-bit unsigned integers,
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which means there are just over two billion unique
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transaction IDs. When a database environment is initially
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created or recovery is run, the transaction ID name space
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is reset, and new transactions are numbered starting from
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0x80000000 (2,147,483,648). The IDs will wrap if the
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maximum transaction ID is reached, starting again from
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0x80000000. The most recently allocated transaction ID is
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the <span class="bold"><strong>st_last_txnid</strong></span> value
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in the transaction statistics information, and can be
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displayed 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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</div>
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<div class="sect2" 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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<h3 class="title"><a id="idm140654535926768"></a>Cursors</h3>
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</div>
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</div>
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</div>
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<p>
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When using transactions, cursors are localized to a
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single transaction. That is, a cursor may not span
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transactions, and must be opened and closed within a
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single transaction. In addition, intermingling
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transaction-protected cursor operations and
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non-transaction-protected cursor operations on the same
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database in a single thread of control is practically
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guaranteed to deadlock because the locks obtained for
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transactions and non-transactions can conflict.
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</p>
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</div>
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<div class="sect2" 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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<h3 class="title"><a id="idm140654535792896"></a>Multiple Threads of Control</h3>
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</div>
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</div>
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</div>
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<p>
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Because transactions must hold all their locks until
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commit, a single transaction may accumulate a large number
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of long-term locks during its lifetime. As a result, when
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two concurrently running transactions access the same
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database, there is strong potential for conflict. Although
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Berkeley DB allows an application to have multiple
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outstanding transactions active within a single thread of
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control, great care must be taken to ensure that the
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transactions do not block each other (for example, attempt
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to obtain conflicting locks on the same data). If two
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concurrently active transactions in the same thread of
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control do encounter a lock conflict, the thread of
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control will deadlock so that the deadlock detector cannot
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detect the problem. In this case, there is no true
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deadlock, but because the transaction on which a
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transaction is waiting is in the same thread of control,
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no forward progress can be made.
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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="txn_config.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="txn.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="sequence.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">Configuring transactions </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"> Chapter 21. Sequences </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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