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120 lines
5.6 KiB
HTML
120 lines
5.6 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>Managing Transaction Rollbacks</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 High Availability Applications" />
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<link rel="up" href="txn-management.html" title="Chapter 3. Transaction Management" />
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<link rel="prev" href="cons_and_dur.html" title="Consistency and Durability Use Cases" />
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<link rel="next" href="runtransaction.html" title="Example Run Transaction Class" />
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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">Managing Transaction Rollbacks</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="cons_and_dur.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Transaction Management</th>
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<td width="20%" align="right"> <a accesskey="n" href="runtransaction.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="txnrollback"></a>Managing Transaction Rollbacks</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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In the event that a new Master is elected, it is possible for a
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Replica to find that some of its logs are ahead of the logs
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held by the Master. While this is unlikely to occur, your code
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must still be ready to deal with the situation. When it
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happens, you must roll back the transactions represented by the
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logs that are ahead of the Master.
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</p>
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<p>
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You do this by simply closing all your <a class="ulink" href="../java/com/sleepycat/je/rep/ReplicatedEnvironment.html" target="_top">ReplicatedEnvironment</a>
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handles, and then reopening. During the handshaking process
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that occurs when the Replica joins the replication group, the
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discrepancy in log files is resolved for you.
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</p>
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<p>
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Note that the problem of logs on replicas being ahead of the
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log on the master is unlikely to occur because the election
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mechanism favors nodes with the most recent logs. When
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selecting a master, a simple majority of nodes are required to
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vote on the choice of master, and they will vote for the node
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with the most recent log files. When the problem does occur,
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though, it results in the updates reflected in the Replica's
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log being discarded when the log is rolled back.
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</p>
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<p>
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Logs on a Replica can be ahead of the logs on the Master if
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network or node failures result in transactions becoming
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durable on fewer than a majority of the nodes in the
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replication group. This reduced durability is more likely in
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cases where one or more Replicas show large replication lags
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relative to the Master. Administrators should monitor
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replication lags and evaluate whether they are caused by issues
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with network or host performance. Applications can reduce the
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chance of transaction rollbacks by avoiding the use of weak
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durability requirements like
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<code class="literal">ReplicaAckPolicy.NONE</code> or a
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<code class="literal">ReplicationMutableConfig.NODE_PRIORITY</code> of
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zero.
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</p>
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<p>
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JE HA lets your application know that a transaction must be
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rolled back by throwing <a class="ulink" href="../java/com/sleepycat/je/rep/RollbackException.html" target="_top">RollbackException</a>. This exception can
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by thrown by any operation that is performing routine database
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access.
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</p>
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<pre class="programlisting"> ReplicatedEnvironment repEnv = new ReplicatedEnvironment(...);
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boolean doWork = true;
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while doWork {
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try {
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// performSomeDBWork is the method that
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// performs your database access.
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doWork = performSomeDBWork();
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} catch (RollbackException rb) {
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if (repEnv != null) {
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repEnv.close();
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repEnv = new ReplicatedEnvironment(...);
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}
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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="cons_and_dur.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="txn-management.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="runtransaction.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">Consistency and Durability Use Cases </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"> Example Run Transaction Class</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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