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<h2 class="title" style="clear: both"><a id="apioverview"></a>The Replication APIs</h2>
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<p>
There are two ways that you can choose to implement
replication in your transactional application. The first,
and preferred, mechanism is to use the pre-packaged
replication framework that comes with the
DB distribution. This framework should be sufficient
for most customers.
</p>
<p>
If for some reason the Replication Framework
does not meet your application's technical requirements,
you will have to use the replication APIs available
through the Berkeley DB library to write your own custom
replication framework.
</p>
<p>
Both of these approaches are described in slightly greater
detail in this section. The bulk of the chapters later in
this book are dedicated to these two replication
implementation mechanisms.
</p>
<div class="sect2" lang="en" xml:lang="en">
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<div>
<h3 class="title"><a id="repframeworkoverview"></a>Replication Framework Overview</h3>
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<div></div>
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<p>
DB's pre-packaged replication framework exists
as a layer on top of the DB library. The replication framework is a
multi-threaded implementation that allows you to easily add
replication to your existing transactional application.
<span>
You access and manage the replication framework using methods that are
available off the
<tt class="classname">DB_ENV</tt>
class.
</span>
</p>
<p>
The replication framework:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Provides a multi-threaded communications layer
using pthreads (on Unix-style systems and
similar derivatives such as Mac OS X), or
Windows threads on Microsoft Windows systems.
</p>
</li>
<li>
<p>
Uses TCP/IP sockets. Network traffic is
handled via threads that handle inbound and
outbound messages. However, each
process uses a single socket
that is shared using <tt class="function">select()</tt>.
</p>
<p>
Note that for this reason, the replication framework is
limited to a maximum of 60 replicas (on
Windows) and approximately 1000 replicas (on
Unix and related systems), depending on how
your system is configured.
</p>
</li>
<li>
<p>
Requires a single process for the master replica.
</p>
</li>
<li>
<p>
Requires that only one instance of the
environment handle be used.
</p>
</li>
<li>
<p>
Upon application startup, a master can be
selected either manually or via elections.
After startup time, however, during the course of
normal operations it is possible for the
replication group to need to locate a new master (due
to network or other hardware related problems,
for example) and in this scenario elections are
always used to select the new master.
</p>
</li>
</ul>
</div>
<p>
If your application has technical requirements that do
not conform to the implementation provided by the
replication framework, you must write a custom
replication framework using the DB replication APIs
directly. See the next section for introductory
details.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h3 class="title"><a id="repapioverview"></a>Replication API Overview</h3>
</div>
</div>
<div></div>
</div>
<p>
The replication API is a series of Berkeley DB library
classes and methods that you can use to build your own
replication infrastructure. You should use the
replication API only if the replication framework does not meet your
application's technical requirements.
</p>
<p>
To make use of the replication API, you must write your
own networking code. This frees you from the technical
constraints imposed by the replication framework. For example, by
writing your own framework, you can:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Use a threading package other than
pthreads (Unix) or Windows threads
(Microsoft Windows). This might be interesting
to you if you are using a platform
whose preferred threading package
is something other than (for
example) pthreads, such as is the case for
Sun Microsystem's Solaris operating systems.
</p>
</li>
<li>
<p>
Implement your own sockets. The
replication framework uses TCP/IP sockets. While
this should be acceptable for the
majority of applications, sometimes
UDP or even raw sockets might be
desired.
</p>
</li>
<li>
<p>
Write a multi-process master
replica.
</p>
</li>
</ul>
</div>
<p>
For information on writing a replicated application
using the Berkeley DB replication APIs, see the
<i class="citetitle">Berkeley DB Programmer's Reference Guide</i>.
</p>
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