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Yasuhiro KIMURA
170 lines
9.2 KiB
HTML
170 lines
9.2 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>Replication views</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="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="rep.html" title="Chapter 12. Berkeley DB Replication" />
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<link rel="prev" href="group_membership.html" title="Managing Replication Manager group membership" />
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<link rel="next" href="rep_filename.html" title="Managing replication directories and files" />
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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 18.1.40</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">Replication views</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="group_membership.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 12. Berkeley DB Replication </th>
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<td width="20%" align="right"> <a accesskey="n" href="rep_filename.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="rep_partview"></a>Replication views</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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A <span class="bold"><strong>view</strong></span> is a replication
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site that maintains a copy of the replication group's data
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without incurring other overhead from participating in the
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replication group. Views are useful for applications that
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require read scalability, are geographically distributed, have
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slow network connections to some sites, or require only a
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subset of replicated databases at some sites.
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</p>
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<p>
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A client is a <span class="bold"><strong>participant</strong></span>
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in the replication group because it helps to determine the
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master and contributes to transaction durability. A view
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receives its copy of the replicated data without participating
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in the replication group in these other ways. Specifically, a
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view cannot ever become master, a view does not vote in
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elections, and a view does not contribute to transaction
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durability. A view is similar to an unelectable client because
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neither can become master. They differ because an unelectable
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client participates in the replication group by voting and
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contributing to transaction durability.
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</p>
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<p>
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A <span class="bold"><strong>full view</strong></span> contains a
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copy of all replicated databases. You define a full view by
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calling the <a href="../api_reference/C/repset_view.html" class="olink">DB_ENV->rep_set_view()</a> method before opening the
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environment and supplying a NULL value for the <span class="bold"><strong>partial_func</strong></span> parameter. You can
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then complete any other environment configuration, open the
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environment, and eventually start replication using the
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<a href="../api_reference/C/repmgrstart.html#repmgrstart_DB_REP_CLIENT" class="olink">DB_REP_CLIENT</a> flags value.
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</p>
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<p>
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A <span class="bold"><strong>partial view</strong></span> contains a
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subset of the replicated databases. You define a partial view
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using the same steps used to define a full view, except that
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you supply a <span class="bold"><strong>partial_func</strong></span>
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callback to the <a href="../api_reference/C/repset_view.html" class="olink">DB_ENV->rep_set_view()</a> method. The partial function
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uses application-specific logic to determine the names of the
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database files to replicate. The partial function should set
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its <span class="bold"><strong>result</strong></span> output parameter
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to 0 to reject a database file or to a non-zero value to
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replicate it. Note that named in-memory databases are always
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replicated to partial views regardless of partial function
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logic.
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</p>
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<p>
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The decision about whether to replicate a particular
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database file to a partial view is made at the time the
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database file is first created. It is possible to change the
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partial function to replicate a different set of database
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files. However, any database file that was already being
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replicated will continue to be replicated regardless of the
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new partial function logic because it already exists at that
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site.
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</p>
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<p>
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Recovery on a partial view should always be performed using
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the <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> flag to the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method after calling
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the <a href="../api_reference/C/repset_view.html" class="olink">DB_ENV->rep_set_view()</a> method to supply the partial function. You
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should not use the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility to recover a partial view
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because it cannot use a partial function.
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</p>
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<p>
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Defining a site as a view is a permanent decision; once a
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site is defined as a view it can never be transformed into a
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participant. This means that on a view site, the <a href="../api_reference/C/repset_view.html" class="olink">DB_ENV->rep_set_view()</a>
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method must always be called before opening the environment.
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</p>
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<p>
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An existing participant can be permanently demoted to a
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view at any time by calling the <a href="../api_reference/C/repset_view.html" class="olink">DB_ENV->rep_set_view()</a> method before
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opening its environment and starting replication. Demoting a
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participant to a view can fail when starting replication and
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return <code class="literal">DB_REP_UNAVAIL</code> if the master is
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unavailable or there are insufficient participants to
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acknowledge this operation. You can pause and retry starting
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replication until it succeeds.
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</p>
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<p>
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A view cannot become master and does not vote in elections,
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but elections among the participant sites need to know the
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number of participants in the replication group. Replication
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Manager automatically manages elections and tracks the number
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of participants. In Base API applications, the number of sites
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in the replication group set by the <a href="../api_reference/C/repnsites.html" class="olink">DB_ENV->rep_set_nsites()</a> method should
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only count participant sites. Similarly, when a participant
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site calls the <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a> method, the <span class="bold"><strong>nsites</strong></span>
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value should only count participant
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sites. A view should never call the <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a> method.
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</p>
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<p>
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A transaction's durability within a replication group is
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based on its replication to other sites that can potentially
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take over as master if the current master fails. A view cannot
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become master, so it cannot contribute to transaction
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durabilty. As a result, Replication Manager views do not send
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acknowledgements. Base API applications should not count
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messages returning <a href="../api_reference/C/repmessage.html#repmsg_DB_REP_ISPERM" class="olink">DB_REP_ISPERM</a> from a view as contributing
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to transaction durability.
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</p>
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<p>
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Replication Manager views participate in replication
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group-aware log archiving. A view can also be configured as a
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client-to-client peer of another site in the replication
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group. If a partial view does not contain the information
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requested, Replication Manager will redirect the request to
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the master.
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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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<tr>
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<td width="40%" align="left"><a accesskey="p" href="group_membership.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="rep.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="rep_filename.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">Managing Replication Manager
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group membership </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 replication directories and files</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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