@Override
  public void addGatewayEventFilter(GatewayEventFilter filter) {
    if (filter == null) {
      throw new IllegalStateException(
          "null value can not be added to gateway-event-filters list");
    }
    List<GatewayEventFilter> templist = new ArrayList<>(eventFilters);
    templist.add(filter);
    eventFilters = Collections.unmodifiableList(templist);
  }
A <code>Region</code> used for storing <code>GatewaySender</code> event id indexes. This
<code>Region</code> along with a <code>DistributedLock</code> facilitates creation of unique
indexes across members.
geode-core/src/main/java/org/apache/geode/internal/cache/wan/AbstractGatewaySender.java