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https://github.com/berkeleydb/je.git
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271 lines
11 KiB
HTML
271 lines
11 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>Non-Durable Transactions</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="usingtxns.html" title="Chapter 3. Transaction Basics" />
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<link rel="prev" href="usingtxns.html" title="Chapter 3. Transaction Basics" />
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<link rel="next" href="abortresults.html" title="Aborting a Transaction" />
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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">Non-Durable Transactions</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="usingtxns.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Transaction Basics</th>
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<td width="20%" align="right"> <a accesskey="n" href="abortresults.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="nodurabletxn"></a>Non-Durable Transactions</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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As previously noted, by default transaction commits are
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durable because they cause the modifications performed
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under the transaction to be synchronously recorded in
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your on-disk log files. However, it is possible to use
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non-durable transactions.
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</p>
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<p>
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You may want non-durable transactions for performance
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reasons. For example, you might be using transactions
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simply for the isolation guarantee.
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<span>
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In this case, you might want to relax the synchronized write to disk that JE normally performs
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as part of a transaction commit. Doing so means that your data will still make it to disk; however,
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your application will not necessarily have to wait for the disk I/O to complete before it can
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perform another database operation. This can greatly improve throughput for some workloads.
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</span>
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</p>
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<p>
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To relax the durability guarantee for your transactions,
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you use the <code class="classname">Durability</code> class to
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define the durability policy that you want to use. The
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<code class="classname">Durability</code> class constructor takes
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three arguments, only one of which is interesting for a
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standalone transactional application:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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The synchronization policy for the local machine.
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</p>
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</li>
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<li>
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<p>
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The synchronization policy for Replicas. Used only for
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JE HA applications.
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</p>
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</li>
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<li>
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<p>
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The acknowledgement policy. Again, this is required
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only for JE HA applications.
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</p>
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</li>
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</ul>
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</div>
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<p>
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We describe JE High Availability Applications in the
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<em class="citetitle">Berkeley DB, Java Edition Getting Started with High Availability Applications</em> guide.
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</p>
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<p>
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The synchronization policy that you give the
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<code class="classname">Durability</code> class constructor can be
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one of the following:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="literal">Durability.SyncPolicy.SYNC</code>
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</p>
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<p>
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Write and synchronously flush the log to disk upon
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transaction commit. This offers the most durable
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transaction configuration because the commit
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operation will not return until all of the disk I/O
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is complete. But, conversely, this offers the worse
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possible write performance because disk I/O is an
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expensive and time-consuming operation.
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</p>
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<p>
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This is the default synchronization policy. A
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transaction that uses this policy is considered to
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be <span class="emphasis"><em>durable</em></span>.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">Durability.SyncPolicy.NO_SYNC</code>
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</p>
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<p>
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This causes JE to not synchronously force any data
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to disk upon transaction commit. That is, the
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modifications are held entirely inside the JVM and
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the modifications are not forced to the file system
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for long-term storage. Note, however, that the
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data will eventually make it to the filesystem
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(assuming no application or OS crashes) as a part
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of JE's management of its logging buffers and/or
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cache.
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</p>
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<p>
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This form of a commit provides a weak durability
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guarantee because data loss can occur due to an
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application, JVM, or OS crash. In fact, this
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represents the least durable configuration that you
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can provide for your transactions. But it also
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offers much better write performance than the other
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options.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">Durability.SyncPolicy.WRITE_NO_SYNC</code>
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</p>
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<p>
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This causes data to be synchronously written to the
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OS's file system buffers upon transaction commit.
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The data will eventually be written to disk, but
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this occurs when the operating system chooses to
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schedule the activity; the transaction commit can
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complete successfully before this disk I/O is
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performed by the OS.
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</p>
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</li>
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<li>
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<p>
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This form of commit protects you against
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application and JVM crashes, but not against OS
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crashes. This method offers less room for the
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possibility of data loss than does NO_SYNC.
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</p>
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</li>
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</ul>
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</div>
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<p>
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You can specify your durability policy on an
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environment-wide basis by creating a
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<code class="classname">Durability</code> class and then giving it
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to <code class="methodname">EnvironmentConfig.setDurability()</code>.
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You can also override the environment default durability
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policy on a transaction-by-transaction basis by providing a
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<code class="classname">Durability</code> class to the
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<code class="classname">TransactionConfig</code> object you use to
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configure your transaction using the
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<code class="methodname">TransactionConfig.setDurability()</code>
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method.
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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">package je.txn;
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import com.sleepycat.je.Database;
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import com.sleepycat.je.DatabaseConfig;
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import com.sleepycat.je.DatabaseEntry;
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import com.sleepycat.je.DatabaseException;
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<strong class="userinput"><code>import com.sleepycat.je.Durability;</code></strong>
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import com.sleepycat.je.Environment;
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import com.sleepycat.je.EnvironmentConfig;
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import com.sleepycat.je.Transaction;
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<strong class="userinput"><code>import com.sleepycat.je.TransactionConfig;</code></strong>
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import java.io.File;
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...
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Database myDatabase = null;
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Environment myEnv = null;
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try {
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<strong class="userinput"><code>Durability defaultDurability =
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new Durability(Durability.SyncPolicy.NO_SYNC,
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null, // unused by non-HA applications.
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null); // unused by non-HA applications. </code></strong>
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EnvironmentConfig myEnvConfig = new EnvironmentConfig();
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myEnvConfig.setTransactional(true);
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<strong class="userinput"><code>myEnvConfig.setDurability(defaultDurability);</code></strong>
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myEnv = new Environment(new File("/my/env/home"),
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myEnvConfig);
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// Open the database. Create it if it does not already exist.
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DatabaseConfig dbConfig = new DatabaseConfig();
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dbConfig.setTransactional(true);
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myDatabase = myEnv.openDatabase(null,
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"sampleDatabase",
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dbConfig);
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String keyString = "thekey";
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String dataString = "thedata";
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DatabaseEntry key =
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new DatabaseEntry(keyString.getBytes("UTF-8"));
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DatabaseEntry data =
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new DatabaseEntry(dataString.getBytes("UTF-8"));
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<strong class="userinput"><code>Durability newDurability =
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new Durability(Durability.SyncPolicy.WRITE_NO_SYNC,
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null, // unused by non-HA applications.
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null); // unused by non-HA applications.
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TransactionConfig tc = new TransactionConfig();
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tc.setDurability(newDurability);
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Transaction txn = myEnv.beginTransaction(null, tc);</code></strong>
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try {
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myDatabase.put(txn, key, data);
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txn.commit();
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} catch (Exception e) {
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if (txn != null) {
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txn.abort();
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txn = null;
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}
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}
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} catch (DatabaseException de) {
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// Exception handling goes here
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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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|||
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<tr>
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|||
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<td width="40%" align="left"><a accesskey="p" href="usingtxns.html">Prev</a> </td>
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|||
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<td width="20%" align="center">
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|||
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<a accesskey="u" href="usingtxns.html">Up</a>
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|||
|
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</td>
|
|||
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<td width="40%" align="right"> <a accesskey="n" href="abortresults.html">Next</a></td>
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|||
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Chapter 3. Transaction Basics </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"> Aborting a Transaction</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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