mirror of
https://github.com/berkeleydb/libdb.git
synced 2024-11-16 17:16:25 +00:00
307 lines
15 KiB
HTML
307 lines
15 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
||
<html xmlns="http://www.w3.org/1999/xhtml">
|
||
<head>
|
||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
|
||
<title>DB_ENV->rep_elect()</title>
|
||
<link rel="stylesheet" href="apiReference.css" type="text/css" />
|
||
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
|
||
<link rel="start" href="index.html" title="Berkeley DB C API Reference" />
|
||
<link rel="up" href="rep.html" title="Chapter 10. Replication Methods" />
|
||
<link rel="prev" href="dbsite_set_config.html" title="DB_SITE->set_config()" />
|
||
<link rel="next" href="repget_clockskew.html" title="DB_ENV->rep_get_clockskew()" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 11.2.5.3</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">DB_ENV->rep_elect()</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="dbsite_set_config.html">Prev</a> </td>
|
||
<th width="60%" align="center">Chapter 10.
|
||
Replication Methods
|
||
</th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="repget_clockskew.html">Next</a></td>
|
||
</tr>
|
||
</table>
|
||
<hr />
|
||
</div>
|
||
<div class="sect1" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h2 class="title" style="clear: both"><a id="repelect"></a>DB_ENV->rep_elect()</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<pre class="programlisting">#include <db.h>
|
||
|
||
int
|
||
DB_ENV->rep_elect(DB_ENV *env,
|
||
u_int32_t nsites, u_int32_t nvotes, u_int32_t flags); </pre>
|
||
<p>
|
||
The <code class="methodname">DB_ENV->rep_elect()</code> method holds an election for the master of a
|
||
replication group.
|
||
</p>
|
||
<p>
|
||
The <code class="methodname">DB_ENV->rep_elect()</code> method is not called by most replication
|
||
applications. It should only be called by Base API applications implementing
|
||
their own network transport layer, explicitly holding replication
|
||
group elections and handling replication messages outside of the
|
||
Replication Manager framework.
|
||
</p>
|
||
<p>
|
||
If the election is successful, Berkeley DB will notify the application
|
||
of the results of the election by means of either the
|
||
<a class="link" href="envevent_notify.html#event_notify_DB_EVENT_REP_ELECTED">DB_EVENT_REP_ELECTED</a>
|
||
or <a class="link" href="envevent_notify.html#event_notify_DB_EVENT_REP_NEWMASTER">DB_EVENT_REP_NEWMASTER</a>
|
||
events (see <a class="xref" href="envevent_notify.html" title="DB_ENV->set_event_notify()">DB_ENV->set_event_notify()</a>
|
||
method for more information). The application is responsible for
|
||
adjusting its relationship to the other database environments in the
|
||
replication group, including directing all database updates to the
|
||
newly selected master, in accordance with the results of the election.
|
||
</p>
|
||
<p>
|
||
The thread of control that calls the <code class="methodname">DB_ENV->rep_elect()</code> method must
|
||
not be the thread of control that processes incoming messages;
|
||
processing the incoming messages is necessary to successfully complete
|
||
an election.
|
||
</p>
|
||
<p>
|
||
Before calling this method do the following:
|
||
</p>
|
||
<div class="itemizedlist">
|
||
<ul type="disc">
|
||
<li>
|
||
<p>
|
||
open the database environment by calling the
|
||
<a class="xref" href="envopen.html" title="DB_ENV->open()">DB_ENV->open()</a> method.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
configure the database environment to send replication messages by calling the
|
||
<a class="xref" href="reptransport.html" title="DB_ENV->rep_set_transport()">DB_ENV->rep_set_transport()</a> method.
|
||
</p>
|
||
</li>
|
||
<li>
|
||
<p>
|
||
configure the database environment as a client or a master
|
||
by calling the <a class="xref" href="repstart.html" title="DB_ENV->rep_start()">DB_ENV->rep_start()</a> method.
|
||
</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idp61907368"></a>How Elections are Held</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
Elections are done in two parts: first, replication sites collect information from the other replication
|
||
sites they know about, and second, replication sites cast their votes for a new master. The second phase
|
||
is triggered by one of two things: either the replication site gets election information from <span class="bold"><strong>nsites</strong></span> sites, or the election timeout
|
||
expires. Once the second phase is triggered, the replication site will cast a vote for the new master of
|
||
its choice if, and only if, the site has election information from at least <span class="bold"><strong>nvotes</strong></span> sites. If a site receives <span class="bold"><strong>nvotes</strong></span>
|
||
votes for it to become the new master, then it will become the new master.
|
||
</p>
|
||
<p>
|
||
We recommend <span class="bold"><strong>nvotes</strong></span> be set to at least:
|
||
</p>
|
||
<pre class="programlisting"> (sites participating in the election / 2) + 1</pre>
|
||
<p>
|
||
to ensure there are never more than two masters active at the same time even in the case of a
|
||
network partition. When a network partitions, the side of the partition with more than half the
|
||
environments will elect a new master and continue, while the environments communicating with fewer
|
||
than half of the environments will fail to find a new master, as no site can get
|
||
<span class="bold"><strong>nvotes</strong></span> votes.
|
||
</p>
|
||
<p>
|
||
We recommend <span class="bold"><strong>nsites</strong></span> be set to:
|
||
</p>
|
||
<pre class="programlisting"> number of sites in the replication group - 1</pre>
|
||
<p>
|
||
when choosing a new master after a current master fails. This allows the group to reach a consensus
|
||
without having to wait for the timeout to expire.
|
||
</p>
|
||
<p>
|
||
When choosing a master from among a group of client sites all restarting at the same time, it makes
|
||
more sense to set <span class="bold"><strong>nsites</strong></span> to the total number of sites in the group,
|
||
since there is no known missing site. Furthermore, in order to ensure the best choice from among
|
||
sites that may take longer to boot than the local site, setting <span class="bold"><strong>nvotes</strong></span> also to this same total number of sites will guarantee that
|
||
every site in the group is considered. Alternatively, using the special timeout for full elections
|
||
allows full participation on restart but allows election of a master if one site does not reboot and
|
||
rejoin the group in a reasonable amount of time. (See the
|
||
<a href="../../programmer_reference/rep_elect.html" class="olink">Elections</a> section in the
|
||
<em class="citetitle">Berkeley DB Programmer's Reference Guide</em> for more information.)
|
||
</p>
|
||
<p>
|
||
Setting <span class="bold"><strong>nsites</strong></span> to lower values can increase the speed of an
|
||
election, but can also result in election failure, and is usually not recommended.
|
||
</p>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idp61898656"></a>Parameters</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="sect3" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h4 class="title"><a id="idp61911480"></a>nsites</h4>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
The <span class="bold"><strong>nsites</strong></span> parameter specifies the
|
||
number of replication sites expected to participate in the election.
|
||
Once the current site has election information from that many sites,
|
||
it will short-circuit the election and immediately cast its vote for a
|
||
new master. The <span class="bold"><strong>nsites</strong></span> parameter must
|
||
be no less than <span class="bold"><strong>nvotes</strong></span>, or 0 if the
|
||
election should use the value previously set using the
|
||
<a class="xref" href="repnsites.html" title="DB_ENV->rep_set_nsites()">DB_ENV->rep_set_nsites()</a>
|
||
method. If an application is using master leases, then the value
|
||
<span class="bold"><strong>must</strong></span> be 0 and the value from
|
||
<a class="xref" href="repnsites.html" title="DB_ENV->rep_set_nsites()">DB_ENV->rep_set_nsites()</a>
|
||
method must be used.
|
||
</p>
|
||
</div>
|
||
<div class="sect3" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h4 class="title"><a id="idp61941528"></a>nvotes</h4>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
The <span class="bold"><strong>nvotes</strong></span> parameter specifies the
|
||
minimum number of replication sites from which the current site must
|
||
have election information, before the current site will cast a vote
|
||
for a new master. The <span class="bold"><strong>nvotes</strong></span>
|
||
parameter must be no greater than <span class="bold"><strong>nsites</strong></span>, or 0 if the election should use the
|
||
value ((<span class="bold"><strong>nsites</strong></span> / 2) + 1) as the
|
||
<span class="bold"><strong>nvotes</strong></span> argument.
|
||
</p>
|
||
</div>
|
||
<div class="sect3" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h4 class="title"><a id="idp61932856"></a>flags</h4>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
The <span class="bold"><strong>flags</strong></span> parameter is currently
|
||
unused, and must be set to 0.
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idp61920768"></a>Errors</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
The <code class="methodname">DB_ENV->rep_elect()</code> <span>
|
||
<span>
|
||
method may fail and return one of the following non-zero errors:
|
||
</span>
|
||
|
||
</span>
|
||
</p>
|
||
<div class="sect3" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h4 class="title"><a id="idp61942544"></a>DB_REP_UNAVAIL</h4>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
The replication group was unable to elect a master, or was unable to
|
||
complete the election in the election timeout period (see
|
||
<a class="xref" href="repset_timeout.html" title="DB_ENV->rep_set_timeout()">DB_ENV->rep_set_timeout()</a>
|
||
method for more information).
|
||
</p>
|
||
</div>
|
||
<div class="sect3" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h4 class="title"><a id="idp61943880"></a>EINVAL</h4>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
If the database environment was not already configured to
|
||
communicate with a replication group by a call to
|
||
<a class="xref" href="reptransport.html" title="DB_ENV->rep_set_transport()">DB_ENV->rep_set_transport()</a>;
|
||
if the database environment was not already opened; if this
|
||
method is called from a Replication Manager application; or if
|
||
an invalid flag value or parameter was specified.
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idp61944736"></a>Class</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
<a class="link" href="env.html" title="Chapter 5. The DB_ENV Handle">DB_ENV</a>
|
||
</p>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="idp61947952"></a>See Also</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>
|
||
<a class="xref" href="rep.html#replist" title="Replication and Related Methods">Replication and Related Methods</a>
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="dbsite_set_config.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="rep.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="repget_clockskew.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">DB_SITE->set_config() </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> DB_ENV->rep_get_clockskew()</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|