Struct MaxOutstandingMessagesOption (2.19.0)

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::future;
  using ::google::cloud::Options;
  using ::google::cloud::StatusOr;
  auto sample = [](std::string project_id, std::string subscription_id) {
    // Change the flow control watermarks, by default the client library uses
    // 0 and 1,000 for the message count watermarks, and 0 and 10MiB for the
    // size watermarks. Recall that the library stops requesting messages if
    // any of the high watermarks are reached, and the library resumes
    // requesting messages when *both* low watermarks are reached.
    auto constexpr kMiB = 1024 * 1024L;
    auto subscriber = pubsub::Subscriber(pubsub::MakeSubscriberConnection(
        pubsub::Subscription(std::move(project_id), std::move(subscription_id)),
        Options{}
            .set<pubsub::MaxOutstandingMessagesOption>(1000)
            .set<pubsub::MaxOutstandingBytesOption>(8 * kMiB)));

    auto session = subscriber.Subscribe(
        [](pubsub::Message const& m, pubsub::AckHandler h) {
          std::move(h).ack();
          std::cout << "Received message " << m << "\n";
          PleaseIgnoreThisSimplifiesTestingTheSamples();
        });
    return std::make_pair(subscriber, std::move(session));
  };