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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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<dl>
<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">
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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 a
callback whose function it is to respond to
events that happen within the DB library.
</p>
<p>
Note that this callback is usable for events beyond
those required for replication purposes. In this
section, however, we only discuss the
replication-specific events.
</p>
<p>
The callback is required to determine
which event has been passed to it, and then take
action depending on the event. For replication,
the events that we care about are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<tt class="literal">DB_EVENT_REP_MASTER</tt>
</p>
<p>
The local environment is now a master.
</p>
</li>
<li>
<p>
<tt class="literal">DB_EVENT_REP_CLIENT</tt>
</p>
<p>
The local environment is now a replica.
</p>
</li>
<li>
<p>
<tt class="literal">DB_EVENT_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">DB_EVENT_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 implementation of this 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_cxx.h>
/* Forward declaration */
void *event_callback(DbEnv *, 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 open our environment handle and set the APP_DATA structure
* to it's app_private member.
*/
DbEnv *dbenv = new DbEnv(0);
dbenv->set_app_private(&my_app_data);
/* Having done that, register the callback with the
* Berkeley DB library
*/
dbenv->set_event_notify(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 framework
* when the local environment changes state.
*/
void *
event_callback(DbEnv *dbenv, u_int32_t which, void *info)
{
APP_DATA *app = dbenv->get_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
<tt class="literal">APP_DATA</tt> information using the environment
handle's <tt class="literal">app_private</tt> data member. We also ignore
the <tt class="literal">DB_EVENT_REP_NEWMASTER</tt> and
<tt class="literal">DB_EVENT_REP_STARTUPDONE</tt> 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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