mysql/example/timeouts.cpp

//
// Copyright (c) 2019-2022 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)
//

//[example_timeouts


#include <boost/asio/steady_timer.hpp>
#include <boost/mysql.hpp>
#include <boost/asio/ssl/context.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/system/system_error.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/use_awaitable.hpp>
#include <boost/asio/awaitable.hpp>
#include <boost/asio/detached.hpp>
#include <chrono>
#include <exception>
#include <iostream>
#include <stdexcept>


#ifdef BOOST_ASIO_HAS_CO_AWAIT

#include <boost/asio/experimental/awaitable_operators.hpp>

using namespace boost::asio::experimental::awaitable_operators;
using boost::mysql::error_code;
using boost::asio::use_awaitable;

constexpr std::chrono::milliseconds TIMEOUT (2000);

void print_employee(const boost::mysql::row& employee)
{
    std::cout << "Employee '"
              << employee.values()[0] << " "                   // first_name (type boost::string_view)
              << employee.values()[1] << "' earns "            // last_name  (type boost::string_view)
              << employee.values()[2] << " dollars yearly\n";  // salary     (type double)
}

/**
 * Helper functions to check whether an async operation, launched in parallel with
 * a timer, was successful or instead timed out. The timer is always the first operation.
 * If the variant holds the first altrnative, that means that the timer fired before
 * the async operation completed, which means a timeout.
 */
template <class T>
T check_timeout(std::variant<std::monostate, T>&& op_result)
{
    if (op_result.index() == 0) {
        throw std::runtime_error("Operation timed out");
    }
    return std::get<1>(std::move(op_result));
}


/**
 * We use Boost.Asio's cancellation capabilities to implement timeouts for our
 * asynchronous operations. This is not something specific to Boost.Mysql, and
 * can be used with any other asynchronous operation that follows Asio's model.
 *
 * Each time we invoke an asynchronous operation, we also call steady_timer::async_wait.
 * We then use Asio's overload for operator || to run the timer wait and the async operation
 * in parallel. Once the first of them finishes, the other operation is cancelled
 * (the behavior is similar to JavaScripts's Promise.race).
 * If we co_await the awaitable returned by operator ||, we get a std::variant<std::monostate, T>,
 * where T is the async operation's result type. If the timer wait finishes first (we have a timeout),
 * the variant will hold the std::monostate at index 0; otherwise, it will have the async operation's
 * result at index 1. The function check_timeout throws an exception in the case of timeout and
 * extracts the operation's result otherwise.
 *
 * If any of the MySQL specific operations result in a timeout, the connection is left
 * in an unspecified state. You should close it and re-open it to get it working again.
 */
boost::asio::awaitable<void> start_query(
    boost::mysql::tcp_ssl_connection& conn,
    boost::asio::ip::tcp::resolver& resolver,
    boost::asio::steady_timer& timer,
    const boost::mysql::connection_params& params,
    const char* hostname
)
{
    try 
    {
        // Resolve hostname
        timer.expires_after(TIMEOUT);
        auto endpoints = check_timeout(co_await (
            timer.async_wait(use_awaitable) ||
            resolver.async_resolve(
                hostname,
                boost::mysql::default_port_string,
                use_awaitable
            )
        ));

        // Connect to server. Note that we need to reset the timer before using it again.
        timer.expires_after(TIMEOUT);
        check_timeout(co_await (
            timer.async_wait(use_awaitable) ||
            conn.async_connect(*endpoints.begin(), params, use_awaitable)
        ));

        // Issue the query to the server
        const char* sql = "SELECT first_name, last_name, salary FROM employee WHERE company_id = 'HGS'";
        timer.expires_after(TIMEOUT);
        auto result = check_timeout(co_await (
            timer.async_wait(use_awaitable) ||
            conn.async_query(sql, use_awaitable)
        ));

        // Read all rows
        boost::mysql::row row;
        bool more_rows = true;
        while (more_rows)
        {
            timer.expires_after(TIMEOUT);
            more_rows = check_timeout(co_await (
                timer.async_wait(use_awaitable) ||
                result.async_read_one(row, use_awaitable)
            ));
            print_employee(row);
        }

        // Notify the MySQL server we want to quit, then close the underlying connection.
        check_timeout(co_await (
            timer.async_wait(use_awaitable) ||
            conn.async_close(use_awaitable)
        ));
    } 
    catch (const boost::system::system_error& err)
    {
        std::cerr << "Error: " << err.what() << ", error code: " << err.code() << std::endl;
    }
    catch (const std::exception& err)
    {
        std::cerr << "Error: " << err.what() << std::endl;
    }
}

void main_impl(int argc, char** argv)
{
    if (argc != 4)
    {
        std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname>\n";
        exit(1);
    }

    const char* hostname = argv[3];

    // I/O context and connection. We use SSL because MySQL 8+ default settings require it.
    boost::asio::io_context ctx;
    boost::asio::ssl::context ssl_ctx (boost::asio::ssl::context::tls_client);
    boost::mysql::tcp_ssl_connection conn (ctx, ssl_ctx);
    boost::asio::steady_timer timer (ctx.get_executor());

    // Connection parameters
    boost::mysql::connection_params params (
        argv[1],               // username
        argv[2],               // password
        "boost_mysql_examples" // database to use; leave empty or omit the parameter for no database
    );

    // Resolver for hostname resolution
    boost::asio::ip::tcp::resolver resolver (ctx.get_executor());

    /**
     * The entry point. We pass in a function returning
     * a boost::asio::awaitable<void>, as required.
     */
    boost::asio::co_spawn(ctx.get_executor(), [&conn, &resolver, &timer, params, hostname] {
        return start_query(conn, resolver, timer, params, hostname);
    }, boost::asio::detached);

    // Calling run will actually start the requested operations.
    ctx.run();
}

#else

void main_impl(int, char**)
{
    std::cout << "Sorry, your compiler does not support C++20 coroutines" << std::endl;
}

#endif

int main(int argc, char** argv)
{
    try
    {
        main_impl(argc, argv);
    }
    catch (const std::exception& err)
    {
        std::cerr << "Error: " << err.what() << std::endl;
        return 1;
    }
}

//]