code: public CoronaTaskTrackerProtocol getClient(
        String host, int port) throws IOException {
      String key = makeKey(host, port);
      Node ttNode = topologyCache.getNode(host);
      CoronaTaskTrackerProtocol client = null;
      synchronized (ttNode) {
        client = trackerClients.get(key);
        if (client == null) {
          client = createClient(host, port);
          trackerClients.put(key, client);
        }
      }
      return client;
    }
docs: API to get the RPC client.
  @param host The host
  @param port The port
  @return The RPC client
  @throws IOException