2011-09-13 17:44:24 +00:00
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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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<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 19. 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 20. Sequences" />
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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 11.2.5.2</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 19.
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The Transaction Subsystem
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</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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2011-12-20 00:07:10 +00:00
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<a href="txn_limits.html#id3975315">Transaction IDs</a>
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2011-09-13 17:44:24 +00:00
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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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2011-12-20 00:07:10 +00:00
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<a href="txn_limits.html#id3974049">Cursors</a>
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2011-09-13 17:44:24 +00:00
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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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2011-12-20 00:07:10 +00:00
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<a href="txn_limits.html#id3974034">Multiple Threads of Control</a>
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2011-09-13 17:44:24 +00:00
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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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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3975315"></a>Transaction IDs</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>Transactions are identified by 31-bit unsigned integers, which means
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there are just over two billion unique transaction IDs. When a database
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environment is initially created or recovery is run, the transaction ID
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name space is reset, and new transactions are numbered starting from
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0x80000000 (2,147,483,648). The IDs will wrap if the maximum
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transaction ID is reached, starting again from 0x80000000. The most
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recently allocated transaction ID is the <span class="bold"><strong>st_last_txnid</strong></span> value in
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the transaction statistics information, and can be displayed 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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</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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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3974049"></a>Cursors</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>When using transactions, cursors are localized to a single transaction.
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That is, a cursor may not span transactions, and must be opened and
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closed within a single transaction. In addition, intermingling
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transaction-protected cursor operations and non-transaction-protected
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cursor operations on the same database in a single thread of control is
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practically guaranteed to deadlock because the locks obtained for
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transactions and non-transactions can conflict.</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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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3974034"></a>Multiple Threads of Control</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>Because transactions must hold all their locks until commit, a single
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transaction may accumulate a large number of long-term locks during its
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lifetime. As a result, when two concurrently running transactions
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access the same database, there is strong potential for conflict.
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Although Berkeley DB allows an application to have multiple outstanding
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transactions active within a single thread of control, great care must
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be taken to ensure that the transactions do not block each other (for
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example, attempt to obtain conflicting locks on the same data). If two
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concurrently active transactions in the same thread of control do
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encounter a lock conflict, the thread of control will deadlock so that
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the deadlock detector cannot detect the problem. In this case, there
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is no true deadlock, but because the transaction on which a transaction
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is waiting is in the same thread of control, no forward progress can be
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made.</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 20.
|
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|
|
|
Sequences
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</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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