code: private void failRequests() {
      final Set<OngoingRequest> requests = ImmutableSet.copyOf(outstanding);
      for (OngoingRequest id : requests) {
        final boolean removed = outstanding.remove(id);
        if (removed) {
          id.reply(Response.forStatus(Status.SERVICE_UNAVAILABLE));
        }
      }
    }
docs: Fail all outstanding requests.