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<h2 class="title"><a id="fwrkmasterreplica"></a>Chapter 4. Replica versus Master Processes</h2>
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<b>Table of Contents</b>
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<dt>
<span class="sect1">
<a href="fwrkmasterreplica.html#determinestate">Determining State</a>
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<span class="sect1">
<a href="processingloop.html">Processing Loop</a>
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<p>
Every environment participating in a replicated application
must know whether it is a <span class="emphasis"><em>master</em></span> or
<span class="emphasis"><em>replica</em></span>. The reason for this is
because, simply, the master can modify the database while
replicas cannot. As a result, not only will you open
databases differently depended on whether the environment is
running as a master, but the environment will frequently
behave quit a bit differently depending on whether it
thinks it is operating as the read/write interface for
your database.
</p>
<p>
Moreover, an environment must also be capable of
gracefully switching between master and replica states.
This means that the environment must be able to detect when
it has switched states.
</p>
<p>
Not surprisingly, a large part of your application's code
will be tied up in knowing which state a given
environment is in and then in the logic of how to behave depending on
its state.
</p>
<p>
This chapter shows you how to determine your environment's
state, and it then shows you some sample code on how
an application might behave depending on whether it is a
master or a replica in a replicated application.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h2 class="title" style="clear: both"><a id="determinestate"></a>Determining State</h2>
</div>
</div>
<div></div>
</div>
<p>
In order to determine whether your code is running
as a master or a replica, you must write your
application as an implementation of
<tt class="classname">com.sleepycat.db.EventHandler</tt>.
This class gives you a single method,
<tt class="methodname">handleEvent()</tt>, within
which you can detect and respond to various events
that occur in your DB code.
</p>
<p>
Note that <tt class="methodname">EventHandler</tt>
responds to a number of
<tt class="classname">com.sleepycat.db.EventType</tt>
events, only some of which are related to
replication. For replication, the events that we
care about are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<tt class="literal">EventType.REP_MASTER</tt>
</p>
<p>
The local environment is now a master.
</p>
</li>
<li>
<p>
<tt class="literal">EventType.REP_CLIENT</tt>
</p>
<p>
The local environment is now a replica.
</p>
</li>
<li>
<p>
<tt class="literal">EventType.REP_STARTUPDONE</tt>
</p>
<p>
The replica has completed startup
synchronization and is now
processing log records received
from the master.
</p>
</li>
<li>
<p>
<tt class="literal">EventType.REP_NEWMASTER</tt>
</p>
<p>
An election was held and a new
environment was made a master. However,
the current environment <span class="emphasis"><em>is
not</em></span> the master. This
event exists so that you can
cause your code to take some
unique action in the event that the
replication groups switches masters.
</p>
</li>
</ul>
</div>
<p>
Note that these events are raised whenever the
state is established. That is, when the current
environment becomes a client, and that includes
at application startup, the event is raised.
Also, when an election is held and a client is elected to be a
master, then the event occurs.
</p>
<p>
The <tt class="classname">EventHandler</tt>
implementation is fairly simple. First you detect
the event, and then you record the state change
in some data member maintained in a location that
is convenient to you.
</p>
<p>
For example:
</p>
<pre class="programlisting">
package db.repquote;
// We make our main class an EventHandler implementation
...
import com.sleepycat.db.EventHandler;
...
public class MyReplicationClass implements EventHandler
{
...
// Somewhere we provide a data member that is used to track
// whether we are a master server. This could be in our main
// class, or it could be part of a supporting class.
private boolean isMaster;
...
isMaster = false;
...
// In the code where we open our environment and start replication,
// we must identify the class that is the event handler. In this
// example, we are performing this from within the class that
// implements com.sleepycat.db.EventHandler so we identify
// "this" class as the event handler
envConfig.setEventHandler(this); </pre>
<p>
That done, we still need to implement the <tt class="methodname">handleEvent</tt>
method. This implementation can be fairly trivial.
</p>
<pre class="programlisting"> public int handleEvent(EventType event)
{
int ret = 0;
if (event == EventType.REP_MASTER)
dbenv.setIsMaster(true);
else if (event == EventType.REP_CLIENT)
dbenv.setIsMaster(false);
else if (event == EventType.REP_NEW_MASTER) {
// ignored for now.
} else {
System.err.println("Unknown event callback received.\n");
ret = 1;
}
return ret;
} </pre>
<p>
Of course, this only gives us the current state of the environment. We
still need the code that determines what to do when the environment
changes state and how to behave depending on the state (described
in the next section).
</p>
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