mysql/example/query_async_coroutines.cpp

//
// Copyright (c) 2019-2020 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/connection.hpp"
#include <boost/asio/io_service.hpp>
#include <boost/system/system_error.hpp>
#include <boost/asio/spawn.hpp>
#include <iostream>

using boost::mysql::error_code;
using boost::mysql::error_info;

/**
 * For this example, we will be using the 'mysql_asio_examples' database.
 * You can get this database by running db_setup.sql.
 * This example assumes you are connecting to a localhost MySQL server.
 *
 * This example uses asynchronous functions with coroutines.
 *
 * This example assumes you are already familiar with the basic concepts
 * of mysql-asio (tcp_connection, resultset, rows, values). If you are not,
 * please have a look to the query_sync.cpp example.
 *
 * In this library, all asynchronous operations follow Boost.Asio universal
 * asynchronous models, and thus may be used with callbacks, coroutines or futures.
 * The handler signature is always one of:
 *   - void(error_code): for operations that do not have a "return type" (e.g. handshake)
 *   - void(error_code, T): for operations that have a "return type" (e.g. query, for which
 *     T = resultset<StreamType>).
 *
 * All asynchronous operations accept a last optional error_info* parameter. error_info
 * contains additional diagnostic information returned by the server. If you
 * pass a non-nullptr value, it will be populated in case of error if any extra information
 * is available.
 *
 * Design note: handler signatures in Boost.Asio should have two parameters, at
 * most, and the first one should be an error_code - otherwise some of the asynchronous
 * features (e.g. coroutines) won't work. This is why error_info is not part of any
 * of the handler signatures.
 */

void print_employee(const boost::mysql::row& employee)
{
    using boost::mysql::operator<<; // Required for mysql::value objects to be streamable, due to ADL rules
    std::cout << "Employee '"
              << employee.values()[0] << " "                   // first_name (type std::string_view)
              << employee.values()[1] << "' earns "            // last_name  (type std::string_view)
              << employee.values()[2] << " dollars yearly\n";  // salary     (type double)
}

// Throws an exception if an operation failed
void check_error(
    const error_code& err,
    const error_info& info = {}
)
{
    if (err)
    {
        throw boost::system::system_error(err, info.message());
    }
}


void main_impl(int argc, char** argv)
{
    if (argc != 3)
    {
        std::cerr << "Usage: " << argv[0] << " <username> <password>\n";
        exit(1);
    }

    // Context and connections
    boost::asio::io_context ctx;
    boost::mysql::tcp_connection conn (ctx);

    boost::asio::ip::tcp::endpoint ep (
        boost::asio::ip::address_v4::loopback(), // host
        boost::mysql::default_port                 // port
    );
    boost::mysql::connection_params params (
        argv[1],               // username
        argv[2],               // password
        "mysql_asio_examples"  // database to use; leave empty or omit the parameter for no database
    );

    /**
     * The entry point. We spawn a stackful coroutine using boost::asio::spawn
     * (see https://www.boost.org/doc/libs/1_72_0/doc/html/boost_asio/reference/spawn.html).
     *
     * The coroutine will actually start running when we call io_context::run().
     * It will suspend every time we call one of the asyncrhonous functions, saving
     * all information it needs for resuming. When the asynchronous operation completes,
     * the coroutine will resume in the point it was left.
     *
     * The return type of a coroutine is the second argument to the handler signature
     * for the asynchronous operation. For example, connection::query has a handler
     * signature of void(error_code, resultset<Stream>), so the coroutine return
     * type is resultset<Stream>.
     *
     */
    boost::asio::spawn(ctx.get_executor(), [&conn, ep, params](boost::asio::yield_context yield) {
        // This error_code and error_info will be filled if an
        // operation fails. We will check them for every operation we perform.
        boost::mysql::error_code ec;
        boost::mysql::error_info additional_info;

        // TCP connect
        conn.next_layer().async_connect(ep, yield[ec]);
        check_error(ec);

        // MySQL handshake
        conn.async_handshake(params, yield[ec], &additional_info);
        check_error(ec, additional_info);

        // Issue the query to the server
        const char* sql = "SELECT first_name, last_name, salary FROM employee WHERE company_id = 'HGS'";
        boost::mysql::tcp_resultset result = conn.async_query(sql, yield[ec], &additional_info);
        check_error(ec, additional_info);

        /**
         * Get all rows in the resultset. We will employ resultset::async_fetch_one(),
         * which returns a single row at every call. The returned row is a pointer
         * to memory owned by the resultset, and is re-used for each row. Thus, returned
         * rows remain valid until the next call to async_fetch_one(). When no more
         * rows are available, async_fetch_one returns nullptr.
         */
        while (true)
        {
            const boost::mysql::row* row = result.async_fetch_one(yield[ec], &additional_info);
            check_error(ec, additional_info);
            if (!row) break; // No more rows available
            print_employee(*row);
        }
    });

    // Don't forget to call run()! Otherwise, your program
    // will not spawn the coroutine and will do nothing.
    ctx.run();
}

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