mysql/example/1_tutorial/6_connection_pool.cpp

//
// Copyright (c) 2019-2025 Ruben Perez Hidalgo (rubenperez038 at gmail dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#include <boost/mysql/pfr.hpp>

#include <boost/asio/awaitable.hpp>
#if defined(BOOST_ASIO_HAS_CO_AWAIT) && BOOST_PFR_CORE_NAME_ENABLED

//[example_tutorial_connection_pool

/**
 * 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;
};

//[tutorial_connection_pool_db
// 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)
{
    //[tutorial_connection_pool_get_connection_timeout
    // 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))
    );
    //]

    //[tutorial_connection_pool_use
    // 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.
}
//]

//[tutorial_connection_pool_session
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
}
//]

//[tutorial_connection_pool_listener
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.
        // Takes 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]));

    //[tutorial_connection_pool_main
    //[tutorial_connection_pool_create
    // 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));
    //]

    //[tutorial_connection_pool_run
    // 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);
    //]

    //[tutorial_connection_pool_signals
    // 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;
    }
}

//]

#else

#include <iostream>

int main()
{
    std::cout << "Sorry, your compiler doesn't have the required capabilities to run this example"
              << std::endl;
}

#endif