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128 lines
4.9 KiB
HTML
128 lines
4.9 KiB
HTML
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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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<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>Read/Modify/Write</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="Getting Started with Berkeley DB Transaction Processing" />
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<link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
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<link rel="prev" href="txn_ccursor.html" title="Transactional Cursors and Concurrent Applications" />
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<link rel="next" href="txnnowait.html" title="No Wait on Blocks" />
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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 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">Read/Modify/Write</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_ccursor.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Concurrency</th>
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<td width="20%" align="right"> <a accesskey="n" href="txnnowait.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="readmodifywrite"></a>Read/Modify/Write</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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If you are retrieving
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<span>a record from the database</span>
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for the purpose of modifying or deleting it, you should declare
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a read-modify-write cycle at the time that you read the
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<span>record.</span>
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Doing so causes DB to obtain write locks (instead of a read
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locks) at the time of the read. This helps to prevent deadlocks by
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preventing another transaction from acquiring a read lock on the same
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record while the read-modify-write cycle is in progress.
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</p>
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<p>
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Note that declaring a read-modify-write cycle may actually increase the amount of blocking that your
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application sees, because readers immediately obtain write locks and write locks cannot be shared. For this
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reason, you should use read-modify-write cycles only if you are seeing a large amount of deadlocking
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occurring in your application.
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</p>
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<p>
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In order to declare a read/modify/write cycle when you perform a
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read operation,
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<span>
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pass the <code class="literal">DB_RMW</code> flag
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<span>
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to the database or cursor get method.
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</span>
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</span>
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</p>
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<p>
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For example:
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</p>
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<pre class="programlisting">// Begin the deadlock retry loop as is normal.
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while (retry_count < MAX_DEADLOCK_RETRIES) {
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try {
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envp->txn_begin(NULL, txn, 0);
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...
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// key and data are DBTs. Their usage is omitted for brevity.
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...
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// Read the data. Declare the read/modify/write cycle here
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dbp->get(txn, &key, &data, DB_RMW);
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...
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// Modify the data as is required. (not shown here)
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...
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// Put the data. Note that you do not have to provide any
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// additional flags here due to the read/modify/write
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// cycle. Simply put the data and perform your deadlock
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// detection as normal.
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dbp->put(txn, &key, &data, 0);
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txn->commit(0);
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return (EXIT_SUCCESS);
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} catch (DbDeadlockException &de) {
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// Deadlock detection and exception handling omitted
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// for brevity
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...</pre>
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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_ccursor.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="txnconcurrency.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="txnnowait.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">Transactional Cursors and Concurrent Applications </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"> No Wait on Blocks</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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