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. Note that these events are raised whenever the state is established. For example, when the current environment becomes a client — including at application startup — the DB_EVENT_REP_CLIENT event is raised. Also, when an election is held and a replica is elected to be a master, the DB_EVENT_REP_MASTER event is raised on the newly elected master and the DB_EVENT_REP_NEWMASTER is raised on the other replicas.

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.

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:

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.

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.

For example:

#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); 

That done, we still need to implement the callback itself. This implementation can be fairly trivial.

/*
 * 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(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);
    }
} 

Notice how we access the APP_DATA information using the environment handle's app_private data member. We also ignore the DB_EVENT_REP_NEWMASTER and DB_EVENT_REP_STARTUPDONE cases since these are not relevant for simple replicated applications.

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).