Import BSDDB 4.7.25 (as of svn r89086)
This commit is contained in:
Zachary Ware
227
docs/gsg_db_rep/JAVA/elections.html
Normal file
227
docs/gsg_db_rep/JAVA/elections.html
Normal file
@@ -0,0 +1,227 @@
|
||||
<?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>Holding Elections</title>
|
||||
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.62.4" />
|
||||
<link rel="home" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
|
||||
<link rel="up" href="introduction.html" title="Chapter 1. Introduction" />
|
||||
<link rel="previous" href="apioverview.html" title="The Replication APIs" />
|
||||
<link rel="next" href="permmessages.html" title="Permanent Message Handling" />
|
||||
</head>
|
||||
<body>
|
||||
<div class="navheader">
|
||||
<table width="100%" summary="Navigation header">
|
||||
<tr>
|
||||
<th colspan="3" align="center">Holding Elections</th>
|
||||
</tr>
|
||||
<tr>
|
||||
<td width="20%" align="left"><a accesskey="p" href="apioverview.html">Prev</a> </td>
|
||||
<th width="60%" align="center">Chapter 1. Introduction</th>
|
||||
<td width="20%" align="right"> <a accesskey="n" href="permmessages.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="elections"></a>Holding Elections</h2>
|
||||
</div>
|
||||
</div>
|
||||
<div></div>
|
||||
</div>
|
||||
<p>
|
||||
Finding a master environment is one of the fundamental activities that
|
||||
every replication replica must perform. Upon startup, the
|
||||
underlying DB replication code will attempt to
|
||||
locate a master. If a master cannot be found, then the
|
||||
environment should initiate an election.
|
||||
</p>
|
||||
<p>
|
||||
How elections are held depends upon the API that you use to
|
||||
implement replication. For example, if you are using the
|
||||
replication framework elections are held transparently without any
|
||||
input from your application's code. In this case,
|
||||
DB will determine which environment is the master and which
|
||||
are replicas.
|
||||
</p>
|
||||
<div class="sect2" lang="en" xml:lang="en">
|
||||
<div class="titlepage">
|
||||
<div>
|
||||
<div>
|
||||
<h3 class="title"><a id="influencingelections"></a>Influencing Elections</h3>
|
||||
</div>
|
||||
</div>
|
||||
<div></div>
|
||||
</div>
|
||||
<p>
|
||||
If you want to control the election process, you can declare
|
||||
a specific environment to be the master. Note that for the replication framework,
|
||||
it is only possible to do this at application startup.
|
||||
Should the master become unavailable during run-time for any
|
||||
reason, an election is held. The environment that receives
|
||||
the most number of votes, wins the election and becomes the
|
||||
master. A machine receives a vote because it has the most
|
||||
number of log records.
|
||||
</p>
|
||||
<p>
|
||||
Because ties are possible when elections are held,
|
||||
it is possible to influence which environment will win
|
||||
the election. How you do this depends on which API you
|
||||
are using. In particular, if you are writing a custom replication
|
||||
layer, then there are a great many ways to manually influence
|
||||
elections.
|
||||
</p>
|
||||
<p>
|
||||
One such mechanism is priorities. When votes are
|
||||
cast during an election, both the number of log records
|
||||
contained in the environment <span class="emphasis"><em>and</em></span>
|
||||
the environment's priority are considered. So given two
|
||||
environments with the same number of log records, votes
|
||||
are cast for the environment with the higher priority.
|
||||
</p>
|
||||
<p>
|
||||
Therefore, if you have a machine that you prefer to
|
||||
become a master in the event of an election, assign it
|
||||
a high priority. Assuming that the election is held at
|
||||
a time when the preferred machine has up-to-date log
|
||||
records, that machine will win the election.
|
||||
</p>
|
||||
</div>
|
||||
<div class="sect2" lang="en" xml:lang="en">
|
||||
<div class="titlepage">
|
||||
<div>
|
||||
<div>
|
||||
<h3 class="title"><a id="winningelections"></a>Winning Elections</h3>
|
||||
</div>
|
||||
</div>
|
||||
<div></div>
|
||||
</div>
|
||||
<p>
|
||||
To win an election:
|
||||
</p>
|
||||
<div class="orderedlist">
|
||||
<ol type="1">
|
||||
<li>
|
||||
<p>
|
||||
There cannot currently be a
|
||||
master environment.
|
||||
</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>
|
||||
The environment must have the most
|
||||
recent log records. Part of
|
||||
holding the election is
|
||||
determining which environments have
|
||||
the most recent log records.
|
||||
This process happens
|
||||
automatically; your code does
|
||||
not need to involve itself in
|
||||
this process.
|
||||
</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>
|
||||
The environment must receive the most
|
||||
number of votes from the
|
||||
replication environments that are
|
||||
participating in the election.
|
||||
</p>
|
||||
</li>
|
||||
</ol>
|
||||
</div>
|
||||
<p>
|
||||
If you are using the replication framework, then in the event of a
|
||||
tie vote the environment with the highest priority wins
|
||||
the election. If two or more environments receive the same
|
||||
number of votes and have the same priority, then
|
||||
the underlying replication code picks one of the
|
||||
environments to
|
||||
be the winner. Which winner will be picked by the
|
||||
replication code is unpredictable from the
|
||||
perspective of your application code.
|
||||
</p>
|
||||
</div>
|
||||
<div class="sect2" lang="en" xml:lang="en">
|
||||
<div class="titlepage">
|
||||
<div>
|
||||
<div>
|
||||
<h3 class="title"><a id="switchingmasters"></a>Switching Masters</h3>
|
||||
</div>
|
||||
</div>
|
||||
<div></div>
|
||||
</div>
|
||||
<p>
|
||||
To switch masters:
|
||||
</p>
|
||||
<div class="orderedlist">
|
||||
<ol type="1">
|
||||
<li>
|
||||
<p>
|
||||
Start up the environment that you want
|
||||
to be master as normal. At this
|
||||
time it is a replica. Make
|
||||
sure this environment has a higher
|
||||
priority than all the other
|
||||
environments.
|
||||
</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>
|
||||
Allow the new environment to run for a
|
||||
time as a replica. This allows
|
||||
it to obtain the most recent
|
||||
copies of the log files.
|
||||
</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>
|
||||
Shut down the current master.
|
||||
This should force an election.
|
||||
Because the new environment has the
|
||||
highest priority, it will win
|
||||
the election, provided it has
|
||||
had enough time to obtain all
|
||||
the log records.
|
||||
</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>
|
||||
Optionally restart the old
|
||||
master environment. Because there is
|
||||
currently a master environment, an
|
||||
election will not be held and
|
||||
the old master will now run as
|
||||
a replica environment.
|
||||
</p>
|
||||
</li>
|
||||
</ol>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
<div class="navfooter">
|
||||
<hr />
|
||||
<table width="100%" summary="Navigation footer">
|
||||
<tr>
|
||||
<td width="40%" align="left"><a accesskey="p" href="apioverview.html">Prev</a> </td>
|
||||
<td width="20%" align="center">
|
||||
<a accesskey="u" href="introduction.html">Up</a>
|
||||
</td>
|
||||
<td width="40%" align="right"> <a accesskey="n" href="permmessages.html">Next</a></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td width="40%" align="left" valign="top">The Replication APIs </td>
|
||||
<td width="20%" align="center">
|
||||
<a accesskey="h" href="index.html">Home</a>
|
||||
</td>
|
||||
<td width="40%" align="right" valign="top"> Permanent Message Handling</td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user