private boolean doConnectionRegistryResponse() throws IOException {
    if (!(rpcServer.server instanceof ConnectionRegistryEndpoint)) {
      // should be in tests or some scenarios where we should not reach here
      return false;
    }
    // on backup masters, this request may be blocked since we need to fetch it from filesystem,
    // but since it is just backup master, it is not a critical problem
    String clusterId = ((ConnectionRegistryEndpoint) rpcServer.server).getClusterId();
    RpcServer.LOG.debug("Response connection registry, clusterId = '{}'", clusterId);
    if (clusterId == null) {
      // should be in tests or some scenarios where we should not reach here
      return false;
    }
    GetConnectionRegistryResponse resp =
      GetConnectionRegistryResponse.newBuilder().setClusterId(clusterId).build();
    doPreambleResponse(resp);
    return true;
  }
Has the request header and the request param and optionally encoded data buffer all in this one
array.
<p/>
Will be overridden in tests.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/ServerRpcConnection.java