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<h2 class="title"><a id="fwrkmasterreplica"></a>Chapter 4. Replica versus Master Processes</h2>
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<p>
<b>Table of Contents</b>
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<dl>
<dt>
<span class="sect1">
<a href="fwrkmasterreplica.html#determinestate">Determining State</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="processingloop.html">Processing Loop</a>
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<span class="sect1">
<a href="exampledoloop.html">Example Processing Loop</a>
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<span class="sect2">
<a href="exampledoloop.html#runningit">Running It</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 quite 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">
<div>
<div>
<h2 class="title" style="clear: both"><a id="determinestate"></a>Determining State</h2>
</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
<code class="classname">com.sleepycat.db.EventHandler</code>.
This class gives you a series of methods within
which you can detect and respond to various events
that occur in your DB code. Some, but not all, of these methods have to
do with elections:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="methodname">EventHandler.handleRepMasterEvent()</code>
</p>
<p>
The local environment is now a master.
</p>
</li>
<li>
<p>
<code class="methodname">EventHandler.handleRepClientEvent()</code>
</p>
<p>
The local environment is now a replica.
</p>
</li>
<li>
<p>
<code class="methodname">EventHandler.handleRepStartupDoneEvent()</code>
</p>
<p>
The replica has completed startup
synchronization and is now
processing log records received
from the master.
</p>
</li>
<li>
<p>
<code class="methodname">EventHandler.handleRepElectedEvent()</code>
</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>
<li>
<p>
<code class="methodname">EventHandler.handleRepPanicEvent()</code>
</p>
<p>
An error has occured in the Berkeley DB library requiring your
application to shut down and then run recovery.
</p>
</li>
<li>
<p>
<code class="methodname">EventHandler.handleRepPermFailedFailedEvent()</code>
</p>
<p>
The Replication Manager did not receive enough acknowledgements to
ensure the transaction's durability within the replicationg group.
The Replication Manager has therefore flushed the transaction to
the master's local disk for storage.
</p>
<p>
How the Replication Manager knows whether the acknowledgements it
has received is determined by the ack policy you have set for your
applicaton. See <a class="xref" href="fwrkpermmessage.html#fmwrkpermpolicy" title="Identifying Permanent Message Policies">Identifying Permanent Message Policies</a>
for more information.
</p>
</li>
<li>
<p>
<code class="methodname">EventHandler.handleWriteFailedEvent()</code>
</p>
<p>
A Berkeley DB write to stable storage failed.
</p>
</li>
</ul>
</div>
<p>
Note that these events are raised whenever the
state is established. That is, when the current
environment becomes a replica, and that includes
at application startup, the event is raised.
Also, when an election is held and a replica is elected to be a
master, then the event occurs.
</p>
<p>
The <code class="classname">EventHandler</code>
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 methods required for handling replication events.
For a simple application like this one, these implementations can be trivial.
</p>
<pre class="programlisting"> public void handleRepClientEvent()
{
dbenv.setIsMaster(false);
}
public void handleRepMasterEvent()
{
dbenv.setIsMaster(true);
}
public void handleRepNewMasterEvent(int envId)
{
// Ignored for now
}
public void handleWriteFailedEvent(int errorCode)
{
System.err.println("Write to stable storage failed!" +
"Operating system error code:" + errorCode);
System.err.println("Continuing....");
}
public void handleRepStartupDoneEvent()
{
System.out.println("Replication startup is completed.");
}
public void handleRepPermFailedEvent()
{
System.out.println("This application failed to receive enough" +
"acks for a permanent message. The transaction is flushed" +
"to disk on this master host.");
}
public void handleRepElectedEvent()
{
// Safely ignored for Replication Manager applications.
}
public void handlePanicEvent()
{
System.err.println("Panic encountered!");
System.err.println("Shutting down.");
System.err.println("You should restart, running recovery.");
try {
terminate();
} catch (DatabaseException dbe) {
System.err.println("Caught an exception during " +
"termination in handlePanicEvent: " + dbe.toString());
}
System.exit(-1);
}</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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