In recent years, the use of coroutines has gained significant popularity in the world of C++ development. Coroutines provide a convenient way to write asynchronous code that is more readable and maintainable compared to traditional callback-based or thread-based approaches. In this article, we will explore how to build web servers using C++ coroutines, taking advantage of their benefits.

What are Coroutines?

Coroutines are a type of generalization of subroutines. They allow suspending and resuming execution at certain points, which makes them ideal for writing asynchronous code. C++20 introduced standardized coroutines, making them accessible to developers without the need for external libraries.

Building a Simple Web Server

Let’s start by building a simple web server using coroutines. We will be using the Boost.Beast library, which provides HTTP and WebSocket functionality. First, we need to set up the necessary dependencies. Assuming you have CMake installed, you can add the following to your project’s CMakeLists.txt file:

find_package(Boost 1.76 REQUIRED COMPONENTS coroutine beast)
target_link_libraries(your_project Boost::coroutine Boost::beast)

Now, let’s define a coroutine-based handler to process incoming HTTP requests. Here’s an example:

#include <boost/beast.hpp>
#include <boost/asio.hpp>
#include <cppcoro/generator.hpp>

cppcoro::generator<boost::beast::http::response<boost::beast::http::string_body>> handle_request(
    boost::beast::http::request<boost::beast::http::string_body> request)
{
    // Your handling logic here

    // Simulate some asynchronous work
    co_await boost::asio::steady_timer{co_await boost::asio::this_coro::executor} 
        .async_wait(boost::asio::use_awaitable);

    // Generate a response
    boost::beast::http::response<boost::beast::http::string_body> response{
        boost::beast::http::status::ok, request.version()};
    response.body() = "Hello, World!";
    response.prepare_payload();

    co_yield response;
}

Here, we are using the CppCoro library to define our coroutine as a generator. The coroutine takes an HTTP request as input and returns a generator of HTTP responses.

Next, let’s create a function to handle incoming connections and dispatch the requests to our coroutine-based handler:

void handle_connection(boost::asio::ip::tcp::socket socket)
{
    try
    {
        boost::beast::flat_buffer buffer;
        boost::beast::http::request<boost::beast::http::string_body> request;
        boost::beast::http::read(socket, buffer, request);

        for (auto response : handle_request(std::move(request)))
        {
            boost::beast::http::write(socket, response);
        }

        socket.shutdown(boost::asio::ip::tcp::socket::shutdown_send);
    }
    catch (const std::exception& e)
    {
        std::cerr << "Error: " << e.what() << std::endl;
    }
}

In this function, we read the incoming HTTP request from the socket, pass it to our coroutine-based handler, and write the responses back to the socket. Finally, we shut down the socket.

To complete our web server implementation, we need a main function to start accepting incoming connections:

int main()
{
    boost::asio::io_context io_context;
    boost::asio::ip::tcp::acceptor acceptor{
        io_context, {boost::asio::ip::tcp::v4(), 8080}};
    
    for (;;)
    {
        auto socket = acceptor.accept();
        std::thread{handle_connection, std::move(socket)}.detach();
    }
}

In this function, we create an io_context and an acceptor to listen on the specified port. We then enter an infinite loop, accepting incoming connections and spawning a new thread to handle each connection.

Conclusion

Using coroutines for building web servers in C++ provides a more readable and maintainable approach to handling asynchronous code. With the standardization of coroutines in C++20 and libraries like Boost.Beast, it has become easier than ever to harness the power of coroutines in your projects. Keep in mind that this is just a simple example, and you can extend it to fulfill your specific requirements. Happy coding!

#webdevelopment #cplusplus #coroutines