public String getId() {
    try {
      if (id == null) {
        id = readClusterIdZNode(watcher);
      }
    } catch (KeeperException ke) {
      abortable.abort("Unexpected exception from ZooKeeper reading cluster ID", ke);
    }
    return id;
  }
Publishes and synchronizes a unique identifier specific to a given HBase cluster. The stored
identifier is read from the file system by the active master on startup, and is subsequently
available to all watchers (including clients).
hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/ZKClusterId.java