keepalived which time exec track script , notify script when vrrp transition

    track_interface {                   # Interfaces state we monitor
      <STRING>
      <STRING>
      <STRING> weight <INTEGER:-254..254>
      ...
    }
    track_script {                     # Scripts state we monitor
      <STRING>
      <STRING> weight <INTEGER:-254..254>
      ...
    }
    dont_track_primary                  # (default unset) ignore VRRP interface faults.
                                        #  useful for cross-connect VRRP config.

When a weight is specified in track_interface, instead of setting the vrrp
instance to the FAULT state in case of failure, its priority will be
increased by the weight when the interface is up (for positive weights),
or decreased by the weight's absolute value when the interface is down
(for negative weights). The weight must be comprised between -254 and +254
inclusive. 0 is the default behaviour which means that a failure implies a
FAULT state. The common practise is to use positive weights to count a
limited number of good services so that the server with the highest count
becomes master. Negative weights are better to count unexpected failures
among a high number of interfaces, as it will not saturate even with high
number of interfaces.

The same principle can be applied to track_script entries, except that an
unspecified weight means that the default weight declared in the script
will be used.

vrrp_sync_group <STRING> {      # VRRP sync group declaration
    group {                     # group of instance to sync together
      <STRING>                  #   a
      <STRING>                  #       set
      ...                       #             of VRRP_Instance string
    }
    notify_master <STRING>|<QUOTED-STRING> # Script to run during MASTER transit
    notify_backup <STRING>|<QUOTED-STRING> # Script to run during BACKUP transit
    notify_fault <STRING>|<QUOTED-STRING>  # Script to run during FAULT transit
    notify <STRING>|<QUOTED-STRING>        # Script to run during ANY state transit (1)
    smtp_alert           # Send email notif during state transit
}

(1) The "notify" script is called AFTER the corresponding notify_* script has
    been called, and is given exactly 4 arguments (the whole string is interpreted
    as a litteral filename so don't add parameters!):

    $1 = A string indicating whether it's a "GROUP" or an "INSTANCE"
    $2 = The name of said group or instance
    $3 = The state it's transitioning to ("MASTER", "BACKUP" or "FAULT")
    $4 = The priority value

    $1 and $3 are ALWAYS sent in uppercase, and the possible strings sent are the
    same ones listed above ("GROUP"/"INSTANCE", "MASTER"/"BACKUP"/"FAULT").

Important: for a SYNC group to run reliably, it is vital that all instances in
           the group are MASTER or that they are all either BACKUP or FAULT. A
           situation with half instances having higher priority on machine A
           half others with higher priority on machine B will lead to constant
           re-elections. For this reason, when instances are grouped, their
           tracking weights are automatically set to zero, in order to avoid
           inconsistent priorities across instances.

    notify_master <STRING>|<QUOTED-STRING>      # Same as vrrp_sync_group
    notify_backup <STRING>|<QUOTED-STRING>      # Same as vrrp_sync_group
    notify_fault <STRING>|<QUOTED-STRING>       # Same as vrrp_sync_group
    notify_stop <STRING>|<QUOTED-STRING>        # Script to launch when stopping vrrp
    notify <STRING>|<QUOTED-STRING>             # Same as vrrp_sync_group

vrrp_scheduler.c

                        log_message(LOG_INFO, "VRRP_Instance(%s) Entering BACKUP STATE",
                               vrrp->iname);

                        /* Set BACKUP state */
                        vrrp_restore_interface(vrrp, 0);
                        vrrp->state = VRRP_STATE_BACK;
                        vrrp_smtp_notifier(vrrp);
                        notify_instance_exec(vrrp, VRRP_STATE_BACK);
............
                                log_message(LOG_INFO, "VRRP_Instance(%s) Entering BACKUP STATE",
                                       vrrp->iname);
                                vrrp->state = VRRP_STATE_BACK;
                                vrrp_smtp_notifier(vrrp);
                                notify_instance_exec(vrrp, VRRP_STATE_BACK);
................
        if (!VRRP_ISUP(vrrp)) {
                vrrp_log_int_down(vrrp);
                log_message(LOG_INFO, "VRRP_Instance(%s) Now in FAULT state",
                       vrrp->iname);
                if (vrrp->state != VRRP_STATE_FAULT)
                        notify_instance_exec(vrrp, VRRP_STATE_FAULT);
                vrrp->state = VRRP_STATE_FAULT;
                vrrp->ms_down_timer = 3 * vrrp->adver_int + VRRP_TIMER_SKEW(vrrp);
                notify_instance_exec(vrrp, VRRP_STATE_FAULT);