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160 lines
6.2 KiB
HTML
160 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>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, Java Edition 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="jebackuprestore.html" title="Chapter 5. Backing up and Restoring Berkeley DB, Java Edition Applications" />
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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 12.2.7.5</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="jebackuprestore.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 or a class from the store</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 JE 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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specify
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<code class="classname">com.sleepycat.je.LockMode.RMW</code>
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to the database, cursor,
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<code class="classname">PrimaryIndex</code>, or
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<code class="classname">SecondaryIndex</code> get method.
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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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txn = myEnv.beginTransaction(null, null);
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...
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// key and data are DatabaseEntry objects.
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// 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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myDatabase.get(txn, key, data, LockMode.RMW);
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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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myDatabase.put(txn, key, data);
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txn.commit();
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return 0;
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} catch (DeadlockException 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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<p>
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Or, with the DPL:
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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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txn = myEnv.beginTransaction(null, null);
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...
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// 'store' is an EntityStore and 'Inventory' is an entity class
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// Their usage and implementation is omitted for brevity.
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...
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// Read the data, using the PrimaryIndex for the entity object
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PrimaryIndex<String,Inventory> pi =
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store.getPrimaryIndex(String.class, Inventory.class);
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Inventory iv = pi.get(txn, "somekey", LockMode.RMW);
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// Do something to the retreived object
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// Put the object. 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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pi.put(txn, iv);
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txn.commit();
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return 0;
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} catch (DeadlockException 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="jebackuprestore.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"> Chapter 5. Backing up and Restoring Berkeley DB, Java Edition Applications</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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