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158 lines
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HTML
158 lines
7 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>Opening a Replicated Environment</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="progoverview.html" title="Chapter 2. Replication API First Steps" />
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<link rel="prev" href="exceptions.html" title="HA Exceptions" />
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<link rel="next" href="replicawrites.html" title="Managing Write Requests at a Replica" />
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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">Opening a Replicated Environment</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="exceptions.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2. Replication API First Steps</th>
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<td width="20%" align="right"> <a accesskey="n" href="replicawrites.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="repenvironmentopen"></a>Opening a Replicated Environment</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 previous two sections we looked at the basics of how to create a
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replicated environment, and what exceptions you can expect to see in a JE HA
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application. Now we need to combine these two topics in order to examine how you should
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open a <a class="ulink" href="../java/com/sleepycat/je/rep/ReplicatedEnvironment.html" target="_top">ReplicatedEnvironment</a> handle to an existing replicated
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environment.
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</p>
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<p>
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When you open the handle, the underlying HA code will attempt to open a TCP/IP
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connection to other nodes in the replication group, based on the node's stored
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replication group metadata or the helper host information that you provide. In doing so,
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the node will attempt to locate a Master or, failing that, will hold an election in order
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to select a new Master, if it is an electable node.
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</p>
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<p>
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Due to issues of timing and network performance, the node may or may not be able to:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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locate the master; and
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</p>
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</li>
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<li>
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<p>
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hold an election.
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</p>
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</li>
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</ol>
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</div>
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<p>
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This can happen if there simply are not enough electable nodes available in order for the current
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node to start up, find the current master, or hold an election. Remember that a majority
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of the electable nodes registered in the replication group must be available in order to hold an
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election.
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</p>
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<p>
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If this situation occurs, the <a class="ulink" href="../java/com/sleepycat/je/rep/ReplicatedEnvironment.html" target="_top">ReplicatedEnvironment</a> constructor will throw an
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<a class="ulink" href="../java/com/sleepycat/je/rep/UnknownMasterException.html" target="_top">UnknownMasterException</a>. Therefore, typically, it is best that you prepare for this
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situation by performing the handle creation in a retry loop, as shown in the following
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code snippet.
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</p>
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<p>
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In addition, if the Replica has been down for a long enough
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period of time, it might be so far out of date that it cannot
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be brought up to date using the normal replication stream. In
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this case, the <a class="ulink" href="../java/com/sleepycat/je/rep/ReplicatedEnvironment.html" target="_top">ReplicatedEnvironment</a> constructor will throw
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an <a class="ulink" href="../java/com/sleepycat/je/rep/InsufficientLogException.html" target="_top">InsufficientLogException</a>. See <a class="xref" href="logfile-restore.html" title="Restoring Log Files">Restoring Log Files</a> for information on how to handle this
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exception.
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</p>
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<pre class="programlisting">private static int REP_HANDLE_RETRY_MAX = 100;
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...
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ReplicatedEnvironment getEnvironment(File envHome, String groupName,
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String nodeName, String nodeHost,
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String helperHosts)
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throws IllegalStateException, InterruptedException {
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EnvironmentConfig envConfig = new EnvironmentConfig();
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envConfig.setAllowCreate(true);
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envConfig.setTransactional(true);
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// Identify the node
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ReplicationConfig repConfig =
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new ReplicationConfig();
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repConfig.setGroupName(groupName);
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repConfig.setNodeName(nodeName);
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repConfig.setNodeHostPort(nodeHost);
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repConfig.setHelperHosts(helperHosts);
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for (int i = 0; i < REP_HANDLE_RETRY_MAX; i++) {
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try {
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return new
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ReplicatedEnvironment(envHome, repConfig, envConfig);
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} catch (UnknownMasterException ume) {
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/*
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* Insert application specific code here to indicate that
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* this problem was encountered, such as writing the
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* condition to a log file.
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*/
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Thread.sleep(5 * 1000);
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continue;
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} catch (InsufficientLogException ile) {
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/* A network restore is required, make the necessary calls */
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}
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}
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throw new
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IllegalStateException("getEnvironment: reached max retries");
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} </pre>
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<p>
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Note that for production code, you may want to retry the handle open without any maximum
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retry limit.
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</p>
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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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<td width="40%" align="left"><a accesskey="p" href="exceptions.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="progoverview.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="replicawrites.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">HA Exceptions </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"> Managing Write Requests at a Replica</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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