Db.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Cron\Model\Config\Converter;

/**
 * Convert data incoming from data base storage
 */
class Db implements \Magento\Framework\Config\ConverterInterface
{
    /**
     * Convert data
     *
     * @param array $source
     * @return array
     */
    public function convert($source)
    {
        $cronTab = isset($source['crontab']) ? $source['crontab'] : [];

        if (empty($cronTab)) {
            return $cronTab;
        }
        return $this->_extractParams($cronTab);
    }

    /**
     * Extract and prepare cron job data
     *
     * @param array $jobs
     * @return array
     */
    protected function _extractParams(array $cronTab)
    {
        $result = [];
        foreach ($cronTab as $groupName => $groupConfig) {
            $jobs = $groupConfig['jobs'];
            foreach ($jobs as $jobName => $value) {
                $result[$groupName][$jobName] = $value;

                if (isset($value['schedule']) && is_array($value['schedule'])) {
                    $this->_processConfigParam($value, $jobName, $result[$groupName]);
                    $this->_processScheduleParam($value, $jobName, $result[$groupName]);
                }

                $this->_processRunModel($value, $jobName, $result[$groupName]);
            }
        }
        return $result;
    }

    /**
     * Fetch parameter 'config_path' from 'schedule' container
     *
     * @param array  $jobConfig
     * @param string $jobName
     * @param array  $result
     * @return void
     */
    protected function _processConfigParam(array $jobConfig, $jobName, array &$result)
    {
        if (array_key_exists('config_path', $jobConfig['schedule'])) {
            $result[$jobName]['config_path'] = $jobConfig['schedule']['config_path'];
        }
    }

    /**
     * Fetch parameter 'cron_expr' from 'schedule' container, reassign it
     *
     * @param array  $jobConfig
     * @param string $jobName
     * @param array  $result
     * @return void
     */
    protected function _processScheduleParam(array $jobConfig, $jobName, array &$result)
    {
        if (array_key_exists('cron_expr', $jobConfig['schedule'])) {
            $result[$jobName]['schedule'] = $jobConfig['schedule']['cron_expr'];
        }
    }

    /**
     * Fetch parameters from 'run' container and save it by reference
     *
     * @param array  $jobConfig
     * @param string $jobName
     * @param array  $result
     * @return void
     */
    protected function _processRunModel(array $jobConfig, $jobName, array &$result)
    {
        if (isset($jobConfig['run']) && is_array($jobConfig['run']) && array_key_exists('model', $jobConfig['run'])) {
            $callPath = explode('::', $jobConfig['run']['model']);

            if (empty($callPath) || empty($callPath[0]) || empty($callPath[1])) {
                unset($result[$jobName]['run']);
                return;
            }

            $result[$jobName]['instance'] = $callPath[0];
            $result[$jobName]['method'] = $callPath[1];
            unset($result[$jobName]['run']);
        }
    }
}