public synchronized NodeCache acquire() throws Exception {
      if (ref == 0) {
        try {
          create();
        } catch (Exception e) {
          close();
          throw e;
        }
      }
      ref++;
      return cache;
    }
Internal watcher that keep "data" up to date asynchronously.
hbase-examples/src/main/java/org/apache/hadoop/hbase/coprocessor/example/ZooKeeperScanPolicyObserver.java