在 C++ 框架中实施分布式系统时，如何确保可扩展性和维护性？

// 微服务之间的数据传输
using protobuf::Message;
class distributed_service {
 public:
  distributed_service(const size_t num_threads)
      : service_threads_{num_threads} {}

  void start() {
    thread_service_loop();
  }

 private:
  std::vector<std::thread> service_threads_;
  void thread_service_loop() {
    while (running_) {
      GrpcCall call;
      Message request;
      if (!server_->ReceiveMessage(&call, &request)) {
        continue;
      }
      Message response;
      HandleMessage(call, request, &response);
      server_->SendMessage(response);
    }
  }
};

// 事件驱动的服务端实现
using boost::asio;
class async_tcp_server {
 public:
  async_tcp_server(io_service& io, const size_t port)
      : io_(io), acceptor_(io, tcp::endpoint(tcp::v4(), port)) {}

  void start() {
    start_accept();
  }

 private:
  io_service& io_;
  tcp::acceptor acceptor_;
  void start_accept() {
    tcp::socket sock(io_);
    acceptor_.async_accept(sock,
                           boost::bind(&async_tcp_server::on_accept, this,
                                       boost::asio::placeholders::error, sock));
  }

  void on_accept(const boost::system::error_code& err, tcp::socket& sock) {
    if (!err) {
      std::make_shared<session>(io_, std::move(sock))->start();
    }
    start_accept();
  }
};

// 依赖注入容器
struct ioc { ioc() = default; ioc(ioc const&) = delete; ioc(ioc&&) = delete; };
using forwarding<T> = std::enable_shared_from_this<T>;
template <class T> using provider = std::function<forwarding<T>(ioc&)>;
template <class T> provider<T> construct(ioc& ioc) {
  return [ioc](ioc&) -> forwarding<T> { return forwarding<T>(ioc); };
};