A Concurrent Server Using Boost.Asio

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: