CompletableFuture<String> fetchClusterId() {
    getClusterId(0);
    // close the rpc client after we finish the request
    FutureUtils.addListener(future, (r, e) -> rpcClient.close());
    return future;
  }
Try get cluster id from the server with the given {@code index} in {@link #bootstrapServers}.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/ClusterIdFetcher.java