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HTML
165 lines
7.7 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>Chapter 5. Writing Monitor Nodes</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="index.html" title="Getting Started with Berkeley DB, Java Edition High Availability Applications" />
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<link rel="prev" href="enablerep.html" title="Converting Existing Environments for Replication" />
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<link rel="next" href="events.html" title="Listening for Events" />
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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">Chapter 5. Writing Monitor Nodes</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="enablerep.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="events.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="chapter" 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"><a id="monitors"></a>Chapter 5. Writing Monitor Nodes</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="monitors.html#monitorclass">Monitor Class</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="events.html">Listening for Events</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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So far in this book we have mostly discussed electable and
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secondary nodes, which are by definition nodes that have access
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to a JE <a class="ulink" href="../java/com/sleepycat/je/rep/ReplicatedEnvironment.html" target="_top">ReplicatedEnvironment</a>. However, replication groups
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can include any number of nodes that have no access to the JE
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replicated environment in use by the replication group.
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</p>
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<p>
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These type of nodes without environments are called
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<span class="emphasis"><em>monitor nodes</em></span>. The point of a monitor node
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is to allow a process to have some understanding of the
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replication group's structure such as which node is the Master
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and what nodes belong to the group as Replicas. Monitor nodes
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also have the ability to know when certain events have happened
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in the replication group, such as when a new Master is elected or
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when new nodes are added to, or removed from, the group.
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</p>
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<p>
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There are many uses for Monitor nodes, starting with the ability to
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write processes that monitor the current status of your HA
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application. But another, arguably more interesting, use for
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Monitor nodes is for request routing. As we have explained earlier
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in this book, Replicas can only service read-only requests; all
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write requests must occur on the Master. However, Replicas are only
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capable of noticing that they have been asked to process a write
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request. At most, out of the box, they can complain about it by
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throwing a <a class="ulink" href="../java/com/sleepycat/je/rep/ReplicaWriteException.html" target="_top">ReplicaWriteException</a>, and then completely rejecting
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the request.
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</p>
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<p>
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One way to handle this problem is by writing an request router that
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sits on your network between the data nodes and your clients.
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This router can send write requests to the Master, and read
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requests to the Replicas. A robust example of this sort of thing
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could also perform load balancing across the various Replicas, so
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that no one Replica becomes swamped by too many read requests.
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</p>
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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="monitorclass"></a>Monitor Class</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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You implement Monitor nodes using the <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/Monitor.html" target="_top">Monitor</a> class. The
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<code class="classname">Monitor</code> class allows you to obtain
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information about the replication group, such as its name,
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where the Master is, and other such information. The
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<code class="classname">Monitor</code> class also allows you to run an
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event listener that can alert you to changes in the composition
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of the replication group.
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</p>
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<p>
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You instantiate a <code class="classname">Monitor</code> class object
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in much the same way as you instantiate a
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<a class="ulink" href="../java/com/sleepycat/je/rep/ReplicatedEnvironment.html" target="_top">ReplicatedEnvironment</a> class object. It is necessary to give
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the node a name, to indicate that it is a Monitor node, to
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identify the node's host and port information, and to identify
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helper hosts. You use a <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/MonitorConfig.html" target="_top">MonitorConfig</a> object to do these
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things.
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</p>
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<p>
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Once the <code class="classname">Monitor</code> object has been
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instantiated, it must be registered at least once with the
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Master so that the replication group will know to keep the node
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informed about changes in the group composition. (Subsequent
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attempts to register the node are simply ignored by the
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Master.) You use the <a class="ulink" href="../java/com/sleepycat/je/rep/monitor/Monitor.html#register()" target="_top">Monitor.register()</a> method to register a
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Monitor node with a Master.
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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"> // Initialize the monitor node config
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MonitorConfig config = new MonitorConfig();
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config.setGroupName("MyRepGroupName");
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config.setNodeName("mon1");
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config.setNodeHostPort("monhost1.acme.com:7000");
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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(); </pre>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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|||
|
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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="enablerep.html">Prev</a> </td>
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|||
|
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<td width="20%" align="center"> </td>
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|||
|
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<td width="40%" align="right"> <a accesskey="n" href="events.html">Next</a></td>
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|||
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</tr>
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|||
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<tr>
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<td width="40%" align="left" valign="top">Converting Existing Environments for Replication </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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|||
|
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</td>
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|||
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<td width="40%" align="right" valign="top"> Listening for Events</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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