@Override
  public String toString() {
    String s = getSender() == null ? getRecipients().toString() : "" + getSender();
    return "ViewAckMessage(" + s + "; " + viewId + "; preparing=" + preparing + "; altview="
        + alternateView + ")";
  }
Members receiving an InstallViewMessage must respond by sending a ViewAckMessage.
geode-membership/src/main/java/org/apache/geode/distributed/internal/membership/gms/messages/ViewAckMessage.java