private OptionalLong getExpireBefore() {
    ChildData data = cache.getCurrentData();
    if (data == null) {
      return OptionalLong.empty();
    }
    byte[] bytes = data.getData();
    if (bytes == null || bytes.length != Long.BYTES) {
      return OptionalLong.empty();
    }
    return OptionalLong.of(Bytes.toLong(bytes));
  }
Internal watcher that keep "data" up to date asynchronously.
hbase-examples/src/main/java/org/apache/hadoop/hbase/coprocessor/example/ZooKeeperScanPolicyObserver.java