Sindbad~EG File Manager
<?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>Building replicated applications</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
<link rel="up" href="rep.html" title="Chapter 12. Berkeley DB Replication" />
<link rel="prev" href="rep_pri.html" title="Replication environment priorities" />
<link rel="next" href="rep_mgr_meth.html" title="Replication Manager methods" />
</head>
<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 18.1.40</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Building replicated applications</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="rep_pri.html">Prev</a> </td>
<th width="60%" align="center">Chapter 12. Berkeley DB Replication </th>
<td width="20%" align="right"> <a accesskey="n" href="rep_mgr_meth.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="rep_app"></a>Building replicated applications</h2>
</div>
</div>
</div>
<p>
The simplest way to build a replicated Berkeley DB
application is to first build (and debug!) the transactional
version of the same application. Then, add a thin replication
layer: application initialization must be changed and the
application's communication infrastructure must be
added.
</p>
<p>
The application initialization changes are relatively
simple. Replication Manager provides a communication
infrastructure, but in order to use the replication Base APIs
you must provide your own.
</p>
<p>
For implementation reasons, all replicated databases must
reside in the data directories set from <a href="../api_reference/C/envadd_data_dir.html" class="olink">DB_ENV->add_data_dir()</a> (or
in the default environment home directory, if not using
<a href="../api_reference/C/envadd_data_dir.html" class="olink">DB_ENV->add_data_dir()</a>), rather than in a subdirectory below the
specified directory. Care must be taken in applications using
relative pathnames and changing working directories after
opening the environment. In such applications the replication
initialization code may not be able to locate the databases,
and applications that change their working directories may
need to use absolute pathnames.
</p>
<p>
During application initialization, the application performs
three additional tasks: first, it must specify the
<a href="../api_reference/C/envopen.html#envopen_DB_INIT_REP" class="olink">DB_INIT_REP</a> flag when opening its database environment and
additionally, a Replication Manager application must also
specify the <a href="../api_reference/C/dbopen.html#open_DB_THREAD" class="olink">DB_THREAD</a> flag; second, it must provide Berkeley
DB information about its communications infrastructure; and
third, it must start the Berkeley DB replication system.
Generally, a replicated application will do normal Berkeley DB
recovery and configuration, exactly like any other
transactional application.
</p>
<p>
Replication Manager applications configure the built-in
communications infrastructure by calling obtaining a <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a>
handle, and then using it to configure the local site. It can
optionally obtain one or more <a href="../api_reference/C/db_site.html" class="olink">DB_SITE</a> handles to configure
remote sites. Once the environment has been opened, the
application starts the replication system by calling the
<a href="../api_reference/C/repmgrstart.html" class="olink">DB_ENV->repmgr_start()</a> method.
</p>
<p>
A Base API application calls the <a href="../api_reference/C/reptransport.html" class="olink">DB_ENV->rep_set_transport()</a> method to
configure the entry point to its own communications
infrastructure, and then calls the <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a> method to join
or create the replication group.
</p>
<p>
When starting the replication system, an application has two
choices: it may choose the group master site explicitly, or
alternatively it may configure all group members as clients
and then call for an election, letting the clients select the
master from among themselves. Either is correct, and the
choice is entirely up to the application.
</p>
<p>
Replication Manager applications make this choice simply by
setting the flags parameter to the <a href="../api_reference/C/repmgrstart.html" class="olink">DB_ENV->repmgr_start()</a>
method.
</p>
<p>
For a Base API application, the result of calling <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a>
is usually the discovery of a master, or the declaration of
the local environment as the master. If a master has not been
discovered after a reasonable amount of time, the application
should call <a href="../api_reference/C/repelect.html" class="olink">DB_ENV->rep_elect()</a> to call for an election.
</p>
<p>
Consider a Base API application with multiple processes or
multiple environment handles that modify databases in the
replicated environment. All modifications must be done on the
master environment. The first process to join or create the
master environment must call both the <a href="../api_reference/C/reptransport.html" class="olink">DB_ENV->rep_set_transport()</a> and the
<a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a> method. Subsequent replication processes must at
least call the <a href="../api_reference/C/reptransport.html" class="olink">DB_ENV->rep_set_transport()</a> method. Those processes may call
the <a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a> method (as long as they use the same master or
client argument). If multiple processes are modifying the
master environment there must be a unified communication
infrastructure such that messages arriving at clients have a
single master ID. Additionally the application must be
structured so that all incoming messages are able to be
processed by a single <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle.
</p>
<p>
Note that not all processes running in replicated
environments need to call <a href="../api_reference/C/repmgrstart.html" class="olink">DB_ENV->repmgr_start()</a>, <a href="../api_reference/C/reptransport.html" class="olink">DB_ENV->rep_set_transport()</a> or
<a href="../api_reference/C/repstart.html" class="olink">DB_ENV->rep_start()</a>. Read-only processes running in a master
environment do not need to be configured for replication in
any way. Processes running in a client environment are
read-only by definition, and so do not need to be configured
for replication either (although, in the case of clients that
may become masters, it is usually simplest to configure for
replication on process startup rather than trying to
reconfigure when the client becomes a master). Obviously, at
least one thread of control on each client must be configured
for replication as messages must be passed between the master
and the client.
</p>
<p>
Any site in a replication group may have its own private
transactional databases in the environment as well. A site may
create a local database by specifying the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_TXN_NOT_DURABLE" class="olink">DB_TXN_NOT_DURABLE</a>
flag to the <a href="../api_reference/C/dbset_flags.html" class="olink">DB->set_flags()</a> method. The application must never
create a private database with the same name as a database
replicated across the entire environment as data corruption
can result.
</p>
<p>
If encryption is desired in a replicated application, all sites
in the replication group must use encryption. The use of a secure
transport is strongly recommended because some log and database
information is transmitted between sites in unencrypted form.
</p>
<p>
For implementation reasons, Base API applications must
process all incoming replication messages using the same <a href="../api_reference/C/env.html" class="olink">DB_ENV</a>
handle. It is not required that a single thread of control
process all messages, only that all threads of control
processing messages use the same handle.
</p>
<p>
No additional calls are required to shut down a database
environment participating in a replication group. The
application should shut down the environment in the usual
manner, by calling the <a href="../api_reference/C/envclose.html" class="olink">DB_ENV->close()</a> method. For Replication
Manager applications, this also terminates all network
connections and background processing threads.
</p>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="rep_pri.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="rep_mgr_meth.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">Replication environment priorities </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> Replication Manager methods</td>
</tr>
</table>
</div>
</body>
</html>
Sindbad File Manager Version 1.0, Coded By Sindbad EG ~ The Terrorists