private Stream<RaftServerImpl> getServerStream(RaftGroupId groupId) { final Stream<RaftServerProxy> stream = getRaftServerProxyStream(groupId); return groupId != null? stream.filter(s -> s.containsGroup(groupId)).map(s -> RaftServerTestUtil.getRaftServerImpl(s, groupId)) : stream.flatMap(s -> RaftServerTestUtil.getRaftServerImpls(s).stream()); }