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180 lines
7.8 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>Listening for Events</title>
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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="monitors.html" title="Chapter 5. Writing Monitor Nodes" />
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<link rel="prev" href="monitors.html" title="Chapter 5. Writing Monitor Nodes" />
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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">Listening for Events</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="monitors.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 5. Writing Monitor Nodes</th>
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<td width="20%" align="right"> <a accesskey="n" href="repexample.html">Next</a></td>
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</tr>
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</table>
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<hr />
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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="events"></a>Listening for Events</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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One of the things the <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/Monitor.html" target="_top">Monitor</a> class allows you to do is to
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listen for certain events that occur in the composition of the
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replication group. Your Monitor can be notified of these events
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by running an event listener using <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/Monitor.html#startListener(com.sleepycat.je.rep.monitor.MonitorChangeListener)" target="_top">Monitor.startListener()</a>.
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For example:
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</p>
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<p>
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<code class="methodname">Monitor.startListener()</code> takes a single
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argument, and that is an instance of <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/MonitorChangeListener.html" target="_top">MonitorChangeListener</a>.
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<code class="classname">MonitorChangeListener</code> is an interface
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that you implement for the purpose of handling replication
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group events.
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</p>
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<p>
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There are four events that the change listener can be notified
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of. Each of these are represented by a unique class:
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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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<a class="ulink" href="../java/com/sleepycat/je/rep/monitor/GroupChangeEvent.html" target="_top">GroupChangeEvent</a>
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</p>
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<p>
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A new instance of this event is generated each time
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an electable or monitor node, but not a secondary
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node, is added or removed from the replication group.
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</p>
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</li>
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<li>
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<p>
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<a class="ulink" href="../java/com/sleepycat/je/rep/monitor/NewMasterEvent.html" target="_top">NewMasterEvent</a>
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</p>
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<p>
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A new instance of this event is generated each time a
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new Master is elected.
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</p>
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</li>
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<li>
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<p>
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<a class="ulink" href="../java/com/sleepycat/je/rep/monitor/JoinGroupEvent.html" target="_top">JoinGroupEvent</a>
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</p>
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<p>
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A new instance of this event is generated each time an
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electable or secondary node, but not a monitor node,
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joins a group. The event is generated on a "best
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effort" basis. It may not be generated, for example,
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if the joining node was unable to communicate with the
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monitor due to a network problem. The application must
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be resilient in the face of such missing events.
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</p>
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</li>
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<li>
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<p>
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<a class="ulink" href="../java/com/sleepycat/je/rep/monitor/LeaveGroupEvent.html" target="_top">LeaveGroupEvent</a>
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</p>
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<p>
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A new instance of this event is generated each time an
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electable or secondary node, but not a monitor node,
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node leaves the group. The event is generated on a
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"best effort" basis. It may not be generated if the
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node leaving the group dies (for example, it was
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killed) before it has a chance to generate the event,
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or if the node was unable to communicate with the
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monitor due to a network problem. The application must
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be resilient in the face of such missing events.
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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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For example, an implementation of the <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/MonitorChangeListener.html" target="_top">MonitorChangeListener</a>
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interface might be:
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</p>
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<pre class="programlisting">class MyChangeListener implements MonitorChangeListener {
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public void notify(NewMasterEvent newMasterEvent) {
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String newNodeName = newMasterEvent.getNodeName();
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InetSocketAddress newMasterAddr =
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newMasterEvent.getSocketAddress();
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String newMasterHostName = newMasterAddr.getHostName();
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int newMasterPort = newMasterAddr.getPort();
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// Do something with this information here.
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}
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public void notify(GroupChangeEvent groupChangeEvent) {
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ReplicationGroup repGroup = groupChangeEvent.getRepGroup();
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// Do something with the new ReplicationGroup composition here.
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}
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...
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} </pre>
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<p>
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You can then start the Monitor listener as follows:
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</p>
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<pre class="programlisting"> // Initialize the monitor node config
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ReplicationConfig config =
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new ReplicationConfig("MyRepGroupName",
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"mon1",
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"monhost1.acme.com:7000");
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config.setNodeType(NodeType.MONITOR);
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config.setHelperHosts("node1.acme.com:5000,node2.acme.com:5000");
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Monitor monitor = new Monitor(config);
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// If the monitor has not been registered as a member of the
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// group, register it now. register() returns the current node
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// that is the master.
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ReplicationNode currentMaster = monitor.register();
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<strong class="userinput"><code>// Start up the listener, so that it can be used to track changes
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// in the master node, or group composition.
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monitor.startListener(new MyChangeListener());</code></strong> </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="monitors.html">Prev</a> </td>
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<a accesskey="u" href="monitors.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="repexample.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">Chapter 5. Writing Monitor Nodes </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 6. Replication Examples</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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