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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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<dt>
<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>
As an alternative, write forwarding simplifies adding replication
to an application. Once configured, there is no need to confine
write operations to the master or even to know which site is the master.
For more information, see
<a class="xref" href="writeforwarding.html" title="Chapter 5. Configuring for Write Forwarding">Configuring for Write Forwarding</a>.
</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>
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<p>
In order to determine whether your code is running as a
master or a replica, you implement an event handling
callback, which we initially describe in
<a class="xref" href="eventhandling.html" title="Event Handling">Event Handling</a>.
When the current environment becomes a client —
including at application startup — the
<code class="literal">DB_EVENT_REP_CLIENT</code> event is raised.
When an election is held and a replica is elected to be a
master, the <code class="literal">DB_EVENT_REP_MASTER</code> event is
raised on the newly elected master and the
<code class="literal">DB_EVENT_REP_NEWMASTER</code> is raised on the
other replicas.
</p>
<p>
The implementation of the event handling callback is fairly
simple. First you pass a structure to the environment
handle that you can use to record the environment's state,
and then you implement a switch statement within the
callback that you use to record the current state,
depending on the arriving event.
</p>
<p>
For example:
</p>
<pre class="programlisting">#include <db.h>
/* Forward declaration */
void *event_callback(DB_ENV *, u_int32_t, void *);
...
/* The structure we use to track our environment's state */
typedef struct {
int is_master;
} APP_DATA;
...
/*
* Inside our main() function, we declare an APP_DATA variable.
*/
APP_DATA my_app_data;
my_app_data.is_master = 0; /* Assume we start as a replica */
...
/*
* Now we create our environment handle and set the APP_DATA structure
* to it's app_private member.
*/
if ((ret = db_env_create(&dbenv, 0)) != 0 ) {
fprintf(stderr, "Error creating handles: %s\n",
db_strerror(ret));
goto err;
}
dbenv->app_private = &my_app_data;
/* Having done that, register the callback with the
* Berkeley DB library
*/
dbenv->set_event_notify(dbenv, event_callback); </pre>
<p>
That done, we still need to implement the callback itself. This
implementation can be fairly trivial.
</p>
<pre class="programlisting">/*
* A callback used to determine whether the local environment is a
* replica or a master. This is called by the Replication Manager
* when the local environment changes state.
*/
void *
event_callback(DB_ENV *dbenv, u_int32_t which, void *info)
{
APP_DATA *app = dbenv->app_private;
info = NULL; /* Currently unused. */
switch (which) {
case DB_EVENT_REP_MASTER:
app->is_master = 1;
break;
case DB_EVENT_REP_CLIENT:
app->is_master = 0;
break;
case DB_EVENT_REP_STARTUPDONE: /* fallthrough */
case DB_EVENT_REP_NEWMASTER:
/* Ignore. */
break;
default:
dbenv->errx(dbenv, "ignoring event %d", which);
}
} </pre>
<p>
Notice how we access the <code class="literal">APP_DATA</code>
information using the environment handle's
<code class="literal">app_private</code> data member. We also ignore the
<code class="literal">DB_EVENT_REP_NEWMASTER</code> and
<code class="literal">DB_EVENT_REP_STARTUPDONE</code> cases since these
are not relevant for simple replicated applications.
</p>
<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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