...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
This example assumes you have gone through the setup.
/** * This example demonstrates how to use connection_pool * to implement a server for a simple custom TCP-based protocol. * It also demonstrates how to set timeouts with asio::cancel_after. * * The protocol can be used to retrieve the full name of an * employee, given their ID. It works as follows: * - The client connects. * - The client sends the employee ID, as a big-endian 64-bit signed int. * - The server responds with a string containing the employee full name. * - The connection is closed. * * This tutorial doesn't include proper error handling. * We will build it in the next one. * * It uses Boost.Pfr for reflection, which requires C++20. * You can backport it to C++14 if you need by using Boost.Describe. * It uses C++20 coroutines. If you need, you can backport * it to C++11 by using callbacks, asio::yield_context * or sync functions instead of coroutines. * * This example uses the 'boost_mysql_examples' database, which you * can get by running db_setup.sql. */ #include <boost/mysql/connection_pool.hpp> #include <boost/mysql/error_with_diagnostics.hpp> #include <boost/mysql/pfr.hpp> #include <boost/mysql/pool_params.hpp> #include <boost/mysql/static_results.hpp> #include <boost/mysql/with_params.hpp> #include <boost/asio/awaitable.hpp> #include <boost/asio/buffer.hpp> #include <boost/asio/cancel_after.hpp> #include <boost/asio/co_spawn.hpp> #include <boost/asio/detached.hpp> #include <boost/asio/io_context.hpp> #include <boost/asio/ip/tcp.hpp> #include <boost/asio/read.hpp> #include <boost/asio/signal_set.hpp> #include <boost/asio/this_coro.hpp> #include <boost/asio/write.hpp> #include <boost/endian/conversion.hpp> #include <boost/system/error_code.hpp> #include <chrono> #include <cstdint> #include <exception> #include <iostream> #include <string> namespace mysql = boost::mysql; namespace asio = boost::asio; // Should contain a member for each field of interest present in our query struct employee { std::string first_name; std::string last_name; }; // Encapsulates the database access logic. // Given an employee_id, retrieves the employee details to be sent to the client. asio::awaitable<std::string> get_employee_details(mysql::connection_pool& pool, std::int64_t employee_id) { // Get a connection from the pool. // This will wait until a healthy connection is ready to be used. // pooled_connection grants us exclusive access to the connection until // the object is destroyed. // Fail the operation if no connection becomes available in the next 20 seconds. mysql::pooled_connection conn = co_await pool.async_get_connection( asio::cancel_after(std::chrono::seconds(1)) ); // Use the connection normally to query the database. // operator-> returns a reference to an any_connection, // so we can apply all what we learnt in previous tutorials mysql::static_results<mysql::pfr_by_name<employee>> result; co_await conn->async_execute( mysql::with_params("SELECT first_name, last_name FROM employee WHERE id = {}", employee_id), result ); // Compose the message to be sent back to the client if (result.rows().empty()) { co_return "NOT_FOUND"; } else { const auto& emp = result.rows()[0]; co_return emp.first_name + ' ' + emp.last_name; } // When the pooled_connection is destroyed, the connection is returned // to the pool, so it can be re-used. } asio::awaitable<void> handle_session(mysql::connection_pool& pool, asio::ip::tcp::socket client_socket) { // Read the request from the client. // async_read ensures that the 8-byte buffer is filled, handling partial reads. unsigned char message[8]{}; co_await asio::async_read(client_socket, asio::buffer(message)); // Parse the 64-bit big-endian int into a native int64_t std::int64_t employee_id = boost::endian::load_big_s64(message); // Invoke the database handling logic std::string response = co_await get_employee_details(pool, employee_id); // Write the response back to the client. // async_write ensures that the entire message is written, handling partial writes co_await asio::async_write(client_socket, asio::buffer(response)); // The socket's destructor will close the client connection } asio::awaitable<void> listener(mysql::connection_pool& pool, unsigned short port) { // An object that accepts incoming TCP connections. asio::ip::tcp::acceptor acc(co_await asio::this_coro::executor); // The endpoint where the server will listen. asio::ip::tcp::endpoint listening_endpoint(asio::ip::make_address("0.0.0.0"), port); // Open the acceptor acc.open(listening_endpoint.protocol()); // Allow reusing the local address, so we can restart our server // without encountering errors in bind acc.set_option(asio::socket_base::reuse_address(true)); // Bind to the local address acc.bind(listening_endpoint); // Start listening for connections acc.listen(); std::cout << "Server listening at " << acc.local_endpoint() << std::endl; // Start the accept loop while (true) { // Accept a new connection auto sock = co_await acc.async_accept(); // Function implementing our session logic. // Take ownership of the socket. // Having this as a named variable workarounds a gcc bug // (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107288) auto session_logic = [&pool, s = std::move(sock)]() mutable { return handle_session(pool, std::move(s)); }; // Launch a coroutine that runs our session logic. // We don't co_await this coroutine so we can listen // to new connections while the session is running. asio::co_spawn( // Use the same executor as the current coroutine co_await asio::this_coro::executor, // Session logic std::move(session_logic), // Propagate exceptions thrown in handle_session [](std::exception_ptr ex) { if (ex) std::rethrow_exception(ex); } ); } } void main_impl(int argc, char** argv) { if (argc != 5) { std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname> <listener-port>\n"; exit(1); } const char* username = argv[1]; const char* password = argv[2]; const char* server_hostname = argv[3]; auto listener_port = static_cast<unsigned short>(std::stoi(argv[4])); // Create an I/O context, required by all I/O objects asio::io_context ctx; // pool_params contains configuration for the pool. // You must specify enough information to establish a connection, // including the server address and credentials. // You can configure a lot of other things, like pool limits mysql::pool_params params; params.server_address.emplace_host_and_port(server_hostname); params.username = username; params.password = password; params.database = "boost_mysql_examples"; // Construct the pool. // ctx will be used to create the connections and other I/O objects mysql::connection_pool pool(ctx, std::move(params)); // You need to call async_run on the pool before doing anything useful with it. // async_run creates connections and keeps them healthy. It must be called // only once per pool. // The detached completion token means that we don't want to be notified when // the operation ends. It's similar to a no-op callback. pool.async_run(asio::detached); // signal_set is an I/O object that allows waiting for signals asio::signal_set signals(ctx, SIGINT, SIGTERM); // Wait for signals signals.async_wait([&](boost::system::error_code, int) { // Stop the execution context. This will cause io_context::run to return ctx.stop(); }); // Launch our listener asio::co_spawn( ctx, [&pool, listener_port] { return listener(pool, listener_port); }, // If any exception is thrown in the coroutine body, rethrow it. [](std::exception_ptr ptr) { if (ptr) { std::rethrow_exception(ptr); } } ); // Calling run will actually execute the coroutine until completion ctx.run(); } int main(int argc, char** argv) { try { main_impl(argc, argv); } catch (const boost::mysql::error_with_diagnostics& err) { // Some errors include additional diagnostics, like server-provided error messages. // Security note: diagnostics::server_message may contain user-supplied values (e.g. the // field value that caused the error) and is encoded using to the connection's character set // (UTF-8 by default). Treat is as untrusted input. std::cerr << "Error: " << err.what() << ", error code: " << err.code() << '\n' << "Server diagnostics: " << err.get_diagnostics().server_message() << std::endl; return 1; } catch (const std::exception& err) { std::cerr << "Error: " << err.what() << std::endl; return 1; } }