mysql/example/query_async_callbacks.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/coroutine.hpp>
#include <boost/asio/yield.hpp>
#include <iostream>

using boost::mysql::error_code;
using boost::mysql::error_info;
using boost::mysql::tcp_resultset;
using boost::mysql::owning_row;

/**
 * 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 callbacks.
 *
 * 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)
}

void die_on_error(
    const error_code& err,
    const boost::mysql::error_info& info = boost::mysql::error_info()
)
{
    if (err)
    {
        std::cerr << "Error: " << err << ": " << info.message() << std::endl;
        exit(1);
    }
}

class application
{
    boost::asio::ip::tcp::endpoint ep;            // Physical endpoint to connect to
    boost::mysql::connection_params conn_params;  // MySQL credentials and other connection config
    boost::asio::io_context ctx;                  // boost::asio context
    boost::mysql::tcp_connection connection;      // Represents the connection to the MySQL server
    boost::mysql::tcp_resultset resultset;        // A result from a query
    boost::mysql::error_info additional_info;     // Will be populated with additional information about any errors
public:
    application(const char* username, const char* password) :
        ep (boost::asio::ip::address_v4::loopback(), boost::mysql::default_port),
        conn_params(username, password, "mysql_asio_examples"),
        connection(ctx)
    {
    }

    void start() { connect(); }

    void connect()
    {
        connection.next_layer().async_connect(ep, [this](error_code err) {
            die_on_error(err);
            connection.async_handshake(conn_params, [this](error_code err) {
                die_on_error(err, additional_info);
                query_employees();
            }, &additional_info);
        });
    }

    void query_employees()
    {
        const char* sql = "SELECT first_name, last_name, salary FROM employee WHERE company_id = 'HGS'";
        connection.async_query(sql, [this](error_code err, tcp_resultset&& result) {
            die_on_error(err, additional_info);
            resultset = std::move(result);
            resultset.async_fetch_all([this](error_code err, const std::vector<owning_row>& rows) {
                die_on_error(err, additional_info);
                for (const auto& employee: rows)
                {
                    print_employee(employee);
                }
                update_slacker();
            }, &additional_info);
        }, &additional_info);
    }

    void update_slacker()
    {
        const char* sql = "UPDATE employee SET salary = 15000 WHERE last_name = 'Slacker'";
        connection.async_query(sql, [this](error_code err, [[maybe_unused]] tcp_resultset&& result) {
            die_on_error(err, additional_info);
            assert(result.fields().size() == 0);
            query_intern();
        }, &additional_info);
    }

    void query_intern()
    {
        const char* sql = "SELECT salary FROM employee WHERE last_name = 'Slacker'";
        connection.async_query(sql, [this](error_code err, tcp_resultset&& result) {
            die_on_error(err, additional_info);
            resultset = std::move(result);
            resultset.async_fetch_all([this](error_code err, const std::vector<owning_row>& rows) {
                die_on_error(err, additional_info);
                assert(rows.size() == 1);
                [[maybe_unused]] auto salary = std::get<double>(rows[0].values()[0]);
                assert(salary == 15000);
            }, &additional_info);
        }, &additional_info);
    }

    auto& context() { return ctx; }
};

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

    application app (argv[1], argv[2]);
    app.start(); // starts the async chain
    app.context().run(); // run the asio::io_context until the async chain finishes
}

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;
    }
}