private void close() {
      if (cache != null) {
        try {
          cache.close();
        } catch (IOException e) {
          // should not happen
          throw new AssertionError(e);
        }
        cache = null;
      }
      if (client != null) {
        client.close();
        client = null;
      }
    }
Internal watcher that keep "data" up to date asynchronously.
hbase-examples/src/main/java/org/apache/hadoop/hbase/coprocessor/example/ZooKeeperScanPolicyObserver.java