Chapter 4. Replica versus Master Processes
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## Chapter 4. Replica versus Master Processes

Every environment participating in a replicated application must know whether it is a master or replica. 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.

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.

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.

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.

## Determining State

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 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:

- `DB_EVENT_REP_MASTER`

  The local environment is now a master.

- `DB_EVENT_REP_CLIENT`

  The local environment is now a replica.

- `DB_EVENT_REP_STARTUPDONE`

  The replica has completed startup synchronization and is now processing log records received from the master.

- `DB_EVENT_REP_NEWMASTER`

  An election was held and a new environment was made a master. However, the current environment is not 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.

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.

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 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);
        }
    }

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).

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