code: private void startAppendTimer() {
      // Set a timer that will be used to periodically synchronize with other nodes
      // in the cluster. This timer acts as a heartbeat to ensure this node remains
      // the leader.
      LOGGER.trace("{} - Starting append timer", context.getCluster().member().address());
      appendTimer = context.getThreadContext().schedule(Duration.ZERO, context.getHeartbeatInterval(), this::appendMembers);
    }
docs: Starts sending AppendEntries requests to all cluster members.