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350 lines
15 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>Permanent Message Handling</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 Replicated Berkeley DB Applications" />
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<link rel="up" href="repapp.html" title="Chapter 3. The DB Replication Manager" />
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<link rel="prev" href="repmgr_init_example_c.html" title="Adding the Replication Manager to ex_rep_gsg_simple" />
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<link rel="next" href="electiontimes.html" title="Managing Election Times" />
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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 11.2.5.3</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">Permanent Message Handling</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="repmgr_init_example_c.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. The DB Replication Manager</th>
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<td width="20%" align="right"> <a accesskey="n" href="electiontimes.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="fwrkpermmessage"></a>Permanent Message Handling</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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<dl>
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<dt>
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<span class="sect2">
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<a href="fwrkpermmessage.html#fmwrkpermpolicy">Identifying Permanent Message Policies</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="fwrkpermmessage.html#fmwrkpermtimeout">Setting the Permanent Message Timeout</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="fwrkpermmessage.html#perm2fmwrkexample">Adding a Permanent Message Policy to
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<span>ex_rep_gsg_repmgr</span>
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</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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As described in <a class="xref" href="permmessages.html" title="Permanent Message Handling">Permanent Message Handling</a>,
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messages are marked permanent if they contain database
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modifications that should be committed at the replica.
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DB's replication code decides if it must flush its
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transaction logs to disk depending on whether it receives
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sufficient permanent message acknowledgments from the
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participating replicas. More importantly, the thread
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performing the transaction commit blocks
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until it either receives enough acknowledgments, or the
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acknowledgment timeout expires.
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</p>
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<p>
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The Replication Manager is fully capable of managing permanent messages
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for you if your application requires it (most do).
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Almost all of the details of this are handled by the
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Replication Manager for you. However, you do have to set some policies
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that tell the Replication Manager how to handle permanent messages.
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</p>
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<p>
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There are two things that you have to do:
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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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Determine how many acknowledgments
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must be received by the master.
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</p>
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</li>
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<li>
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<p>
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Identify the amount of time that
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replicas have to send their
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acknowledgments.
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</p>
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</li>
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</ul>
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</div>
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<div class="sect2" 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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<h3 class="title"><a id="fmwrkpermpolicy"></a>Identifying Permanent Message Policies</h3>
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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 identify permanent message policies using the
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<span>
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<code class="methodname">DB_ENV->repmgr_set_ack_policy()</code>
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method.
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</span>
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Note that you can set permanent message
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policies at any time during the life of the
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application.
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</p>
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<p>
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The following permanent message policies are available when you use
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the Replication Manager:
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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The following list mentions <span class="emphasis"><em>electable peer</em></span>
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several times. This is simply another environment that can be
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elected to be a master (that is, it has a priority greater than 0).
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Do not confuse this with the concept of a peer as used for client
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to client transfers. See <a class="xref" href="c2ctransfer.html" title="Client to Client Transfer">Client to Client Transfer</a> for more information on client
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to client transfers.
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</p>
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</div>
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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">DB_REPMGR_ACKS_NONE</code>
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</p>
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<p>
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No permanent message acknowledgments are required. If
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this policy is selected, permanent message handling is
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essentially "turned off." That is, the master will
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never wait for replica acknowledgments. In this case,
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transaction log data is either flushed or not strictly
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depending on the type of commit that is being performed
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(synchronous or asynchronous).
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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">DB_REPMGR_ACKS_ONE</code>
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</p>
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<p>
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At least one replica must acknowledge the permanent
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message within the timeout period.
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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">DB_REPMGR_ACKS_ONE_PEER</code>
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</p>
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<p>
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At least one electable peer must acknowledge the permanent
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message within the timeout period.
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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">DB_REPMGR_ACKS_ALL</code>
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</p>
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<p>
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All replicas must acknowledge the message within
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the timeout period. This
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policy should be selected only if your replication
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group has a small number of replicas, and those replicas
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are on extremely reliable networks and servers.
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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">DB_REPMGR_ACKS_ALL_AVAILABLE</code>
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</p>
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<p>
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All currently connected replication clients must
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acknowledge the message. This policy will invoke the
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<code class="literal">DB_EVENT_REP_PERM_FAILED</code> event if
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fewer than a quorum of clients acknowledged during that
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time.
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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">DB_REPMGR_ACKS_ALL_PEERS</code>
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</p>
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<p>
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All electable peers must acknowledge the message within the
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timeout period. This
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policy should be selected only if your replication
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group is small, and its various environments
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are on extremely reliable networks and servers.
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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">DB_REPMGR_ACKS_QUORUM</code>
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</p>
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<p>
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A quorum of electable peers must acknowledge the message within the timeout period.
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A quorum is reached when acknowledgments are received from the minimum number
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of environments needed to ensure that the record remains durable
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if an election is held. That is, the master wants to hear from enough
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electable replicas that they have committed the record so that if an election
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is held, the master knows the record will exist even if a new master is selected.
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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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By default, a quorum of electable peers must must acknowledge a permanent
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message in order for it considered to have been successfully
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transmitted.
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</p>
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</div>
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<div class="sect2" 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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<h3 class="title"><a id="fmwrkpermtimeout"></a>Setting the Permanent Message Timeout</h3>
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</div>
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</div>
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</div>
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<p>
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The permanent message timeout represents the
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maximum amount of time the committing thread
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will block waiting for message
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acknowledgments. If sufficient
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acknowledgments arrive before this timeout has
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expired, the thread continues operations as
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normal. However, if this timeout expires, the
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committing thread flushes its transaction log
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buffer before continuing with normal
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operations.
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</p>
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<p>
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You set the timeout value using the
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<code class="methodname">DB_ENV->rep_set_timeout()</code>
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method. When you do this, you provide the
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<code class="literal">DB_REP_ACK_TIMEOUT</code> value to
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the <code class="literal">which</code> parameter, and the
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timeout value in microseconds to the
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<code class="literal">timeout</code> parameter.
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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"> dbenv->rep_set_timeout(dbenv, DB_REP_ACK_TIMEOUT, 100); </pre>
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<p>
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This timeout value can be set at anytime during the
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life of the application.
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</p>
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</div>
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<div class="sect2" 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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<h3 class="title"><a id="perm2fmwrkexample"></a>Adding a Permanent Message Policy to
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<span>ex_rep_gsg_repmgr</span>
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</h3>
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</div>
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</div>
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</div>
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<p>
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For illustration purposes, we will now update
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<code class="literal">ex_rep_gsg_repmgr</code>
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such that it requires only one acknowledgment from
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a replica on transactional commits. Also, we will give
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this acknowledgment a 500 microsecond timeout
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value. This means that our application's main
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thread will block for up to 500 microseconds waiting
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for an acknowledgment. If it does not receive at
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least one acknowledgment in that amount of time,
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DB will flush the transaction logs to disk
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before continuing on.
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</p>
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<p>
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This is a very simple update. We can perform the
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entire thing immediately before we parse our
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command line options. This is where we configure
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our environment handle anyway, so it is a good
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place to put it.
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</p>
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<pre class="programlisting"> if ((ret = create_env(progname, &dbenv)) != 0)
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goto err;
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/* Default priority is 100 */
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dbenv->rep_set_priority(dbenv, 100);
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<strong class="userinput"><code>/* Permanent messages require at least one ack */
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dbenv->repmgr_set_ack_policy(dbenv, DB_REPMGR_ACKS_ONE);
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/* Give 500 microseconds to receive the ack */
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dbenv->rep_set_timeout(dbenv, DB_REP_ACK_TIMEOUT, 500);</code></strong>
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/* Collect the command line options */
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while ((ch = getopt(argc, argv, "h:<strong class="userinput"><code>l:n:p:r:</code></strong>")) != EOF)
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... </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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||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="repmgr_init_example_c.html">Prev</a> </td>
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||
<td width="20%" align="center">
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||
<a accesskey="u" href="repapp.html">Up</a>
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||
</td>
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||
<td width="40%" align="right"> <a accesskey="n" href="electiontimes.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">Adding the Replication Manager to
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<span>ex_rep_gsg_simple</span>
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</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 Election Times</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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