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A Concurrent Server Using Boost.Asio

Learn how to write a concurrent single-threaded TCP server with Boost.Asio, using asynchronous application programming and coroutines.

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This section will demonstrate how to write concurrent programs with multiple threads of execution but only use a single OS thread. We are about to implement a rudimentary concurrent single-threaded TCP server that can handle multiple clients. There are no networking capabilities in the C++ standard library but fortunately, Boost.Asio provides us with a platform-agnostic interface for handling socket communication.

Instead of wrapping the callback-based Boost.Asio API, we will demonstrate how to use the boost::asio::awaitable class to show a more realistic example of how asynchronous application programming using coroutines can look. The class template boost::asio::awaitable corresponds to the Task template we created earlier; it’s used as a return type for coroutines that represent asynchronous computations.

Implementing the server

The server is very simple; once a client connects, it starts updating a numeric counter and writes back the value whenever it is updated. This time we will follow the code from top to bottom, starting with the main() function:

#include <boost/asio.hpp>
#include <boost/asio/awaitable.hpp>
#include <boost/asio/use_awaitable.hpp>
using namespace std::chrono;
namespace asio = boost::asio;
using boost::asio::ip::tcp;
asio::awaitable<void> listen(tcp::endpoint endpoint) {
  auto acceptor = tcp::acceptor{co_await asio::this_coro::executor};
  co_await acceptor.async_listen(endpoint, asio::use_awaitable);
}
int main() {
  auto server = [] {
    auto endpoint = tcp::endpoint{tcp::v4(), 37259};
    co_await listen(endpoint);
  };
  auto ctx = asio::io_context{};
  boost::asio::co_spawn(ctx, server, asio::detached);
  ctx.run();
}

The mandatory io_context runs the event processing loop. It’s possible to invoke run() from multiple threads as well if we want our server to execute ...