eventespresso / event-espresso-core

core/EE_System.core.php

Indentation +1309 added lines, -1309 removed lines

@@ -27,1313 +27,1313 @@
 final class EE_System implements ResettableInterface
 {
 
-    /**
-     * indicates this is a 'normal' request. Ie, not activation, nor upgrade, nor activation.
-     * So examples of this would be a normal GET request on the frontend or backend, or a POST, etc
-     */
-    const req_type_normal = 0;
-
-    /**
-     * Indicates this is a brand new installation of EE so we should install
-     * tables and default data etc
-     */
-    const req_type_new_activation = 1;
-
-    /**
-     * we've detected that EE has been reactivated (or EE was activated during maintenance mode,
-     * and we just exited maintenance mode). We MUST check the database is setup properly
-     * and that default data is setup too
-     */
-    const req_type_reactivation = 2;
-
-    /**
-     * indicates that EE has been upgraded since its previous request.
-     * We may have data migration scripts to call and will want to trigger maintenance mode
-     */
-    const req_type_upgrade = 3;
-
-    /**
-     * TODO  will detect that EE has been DOWNGRADED. We probably don't want to run in this case...
-     */
-    const req_type_downgrade = 4;
-
-    /**
-     * @deprecated since version 4.6.0.dev.006
-     * Now whenever a new_activation is detected the request type is still just
-     * new_activation (same for reactivation, upgrade, downgrade etc), but if we'r ein maintenance mode
-     * EE_System::initialize_db_if_no_migrations_required and EE_Addon::initialize_db_if_no_migrations_required
-     * will instead enqueue that EE plugin's db initialization for when we're taken out of maintenance mode.
-     * (Specifically, when the migration manager indicates migrations are finished
-     * EE_Data_Migration_Manager::initialize_db_for_enqueued_ee_plugins() will be called)
-     */
-    const req_type_activation_but_not_installed = 5;
-
-    /**
-     * option prefix for recording the activation history (like core's "espresso_db_update") of addons
-     */
-    const addon_activation_history_option_prefix = 'ee_addon_activation_history_';
-
-    /**
-     * @var EE_System $_instance
-     */
-    private static $_instance;
-
-    /**
-     * @var EE_Registry $registry
-     */
-    private $registry;
-
-    /**
-     * @var LoaderInterface $loader
-     */
-    private $loader;
-
-    /**
-     * @var EE_Capabilities $capabilities
-     */
-    private $capabilities;
-
-    /**
-     * @var RequestInterface $request
-     */
-    private $request;
-
-    /**
-     * @var EE_Maintenance_Mode $maintenance_mode
-     */
-    private $maintenance_mode;
-
-    /**
-     * Stores which type of request this is, options being one of the constants on EE_System starting with req_type_*.
-     * It can be a brand-new activation, a reactivation, an upgrade, a downgrade, or a normal request.
-     *
-     * @var int $_req_type
-     */
-    private $_req_type;
-
-    /**
-     * Whether or not there was a non-micro version change in EE core version during this request
-     *
-     * @var boolean $_major_version_change
-     */
-    private $_major_version_change = false;
-
-    /**
-     * A Context DTO dedicated solely to identifying the current request type.
-     *
-     * @var RequestTypeContextCheckerInterface $request_type
-     */
-    private $request_type;
-
-
-    /**
-     * @singleton method used to instantiate class object
-     * @param EE_Registry|null         $registry
-     * @param LoaderInterface|null     $loader
-     * @param RequestInterface|null    $request
-     * @param EE_Maintenance_Mode|null $maintenance_mode
-     * @return EE_System
-     */
-    public static function instance(
-        EE_Registry $registry = null,
-        LoaderInterface $loader = null,
-        RequestInterface $request = null,
-        EE_Maintenance_Mode $maintenance_mode = null
-    ) {
-        // check if class object is instantiated
-        if (! self::$_instance instanceof EE_System) {
-            self::$_instance = new self($registry, $loader, $request, $maintenance_mode);
-        }
-        return self::$_instance;
-    }
-
-
-    /**
-     * resets the instance and returns it
-     *
-     * @return EE_System
-     */
-    public static function reset()
-    {
-        self::$_instance->_req_type = null;
-        // make sure none of the old hooks are left hanging around
-        remove_all_actions('AHEE__EE_System__perform_activations_upgrades_and_migrations');
-        // we need to reset the migration manager in order for it to detect DMSs properly
-        EE_Data_Migration_Manager::reset();
-        self::instance()->detect_activations_or_upgrades();
-        self::instance()->perform_activations_upgrades_and_migrations();
-        return self::instance();
-    }
-
-
-    /**
-     * sets hooks for running rest of system
-     * provides "AHEE__EE_System__construct__complete" hook for EE Addons to use as their starting point
-     * starting EE Addons from any other point may lead to problems
-     *
-     * @param EE_Registry         $registry
-     * @param LoaderInterface     $loader
-     * @param RequestInterface    $request
-     * @param EE_Maintenance_Mode $maintenance_mode
-     */
-    private function __construct(
-        EE_Registry $registry,
-        LoaderInterface $loader,
-        RequestInterface $request,
-        EE_Maintenance_Mode $maintenance_mode
-    ) {
-        $this->registry = $registry;
-        $this->loader = $loader;
-        $this->request = $request;
-        $this->maintenance_mode = $maintenance_mode;
-        do_action('AHEE__EE_System__construct__begin', $this);
-        add_action(
-            'AHEE__EE_Bootstrap__load_espresso_addons',
-            array($this, 'loadCapabilities'),
-            5
-        );
-        add_action(
-            'AHEE__EE_Bootstrap__load_espresso_addons',
-            array($this, 'loadCommandBus'),
-            7
-        );
-        add_action(
-            'AHEE__EE_Bootstrap__load_espresso_addons',
-            array($this, 'loadPluginApi'),
-            9
-        );
-        // allow addons to load first so that they can register autoloaders, set hooks for running DMS's, etc
-        add_action(
-            'AHEE__EE_Bootstrap__load_espresso_addons',
-            array($this, 'load_espresso_addons')
-        );
-        // when an ee addon is activated, we want to call the core hook(s) again
-        // because the newly-activated addon didn't get a chance to run at all
-        add_action('activate_plugin', array($this, 'load_espresso_addons'), 1);
-        // detect whether install or upgrade
-        add_action(
-            'AHEE__EE_Bootstrap__detect_activations_or_upgrades',
-            array($this, 'detect_activations_or_upgrades'),
-            3
-        );
-        // load EE_Config, EE_Textdomain, etc
-        add_action(
-            'AHEE__EE_Bootstrap__load_core_configuration',
-            array($this, 'load_core_configuration'),
-            5
-        );
-        // load specifications for matching routes to current request
-        add_action(
-            'AHEE__EE_Bootstrap__load_core_configuration',
-            array($this, 'loadRouteMatchSpecifications')
-        );
-        // load EE_Config, EE_Textdomain, etc
-        add_action(
-            'AHEE__EE_Bootstrap__register_shortcodes_modules_and_widgets',
-            array($this, 'register_shortcodes_modules_and_widgets'),
-            7
-        );
-        // you wanna get going? I wanna get going... let's get going!
-        add_action(
-            'AHEE__EE_Bootstrap__brew_espresso',
-            array($this, 'brew_espresso'),
-            9
-        );
-        // other housekeeping
-        // exclude EE critical pages from wp_list_pages
-        add_filter(
-            'wp_list_pages_excludes',
-            array($this, 'remove_pages_from_wp_list_pages'),
-            10
-        );
-        // ALL EE Addons should use the following hook point to attach their initial setup too
-        // it's extremely important for EE Addons to register any class autoloaders so that they can be available when the EE_Config loads
-        do_action('AHEE__EE_System__construct__complete', $this);
-    }
-
-
-    /**
-     * load and setup EE_Capabilities
-     *
-     * @return void
-     * @throws EE_Error
-     */
-    public function loadCapabilities()
-    {
-        $this->capabilities = $this->loader->getShared('EE_Capabilities');
-        add_action(
-            'AHEE__EE_Capabilities__init_caps__before_initialization',
-            function () {
-                LoaderFactory::getLoader()->getShared('EE_Payment_Method_Manager');
-            }
-        );
-    }
-
-
-    /**
-     * create and cache the CommandBus, and also add middleware
-     * The CapChecker middleware requires the use of EE_Capabilities
-     * which is why we need to load the CommandBus after Caps are set up
-     *
-     * @return void
-     * @throws EE_Error
-     */
-    public function loadCommandBus()
-    {
-        $this->loader->getShared(
-            'CommandBusInterface',
-            array(
-                null,
-                apply_filters(
-                    'FHEE__EE_Load_Espresso_Core__handle_request__CommandBus_middleware',
-                    array(
-                        $this->loader->getShared('EventEspresso\core\services\commands\middleware\CapChecker'),
-                        $this->loader->getShared('EventEspresso\core\services\commands\middleware\AddActionHook'),
-                    )
-                ),
-            )
-        );
-    }
-
-
-    /**
-     * @return void
-     * @throws EE_Error
-     */
-    public function loadPluginApi()
-    {
-        // set autoloaders for all of the classes implementing EEI_Plugin_API
-        // which provide helpers for EE plugin authors to more easily register certain components with EE.
-        EEH_Autoloader::instance()->register_autoloaders_for_each_file_in_folder(EE_LIBRARIES . 'plugin_api');
-        $this->loader->getShared('EE_Request_Handler');
-    }
-
-
-    /**
-     * @param string $addon_name
-     * @param string $version_constant
-     * @param string $min_version_required
-     * @param string $load_callback
-     * @param string $plugin_file_constant
-     * @return void
-     */
-    private function deactivateIncompatibleAddon(
-        $addon_name,
-        $version_constant,
-        $min_version_required,
-        $load_callback,
-        $plugin_file_constant
-    ) {
-        if (! defined($version_constant)) {
-            return;
-        }
-        $addon_version = constant($version_constant);
-        if ($addon_version && version_compare($addon_version, $min_version_required, '<')) {
-            remove_action('AHEE__EE_System__load_espresso_addons', $load_callback);
-            if (! function_exists('deactivate_plugins')) {
-                require_once ABSPATH . 'wp-admin/includes/plugin.php';
-            }
-            deactivate_plugins(plugin_basename(constant($plugin_file_constant)));
-            unset($_GET['activate'], $_REQUEST['activate'], $_GET['activate-multi'], $_REQUEST['activate-multi']);
-            EE_Error::add_error(
-                sprintf(
-                    esc_html__(
-                        'We\'re sorry, but the Event Espresso %1$s addon was deactivated because version %2$s or higher is required with this version of Event Espresso core.',
-                        'event_espresso'
-                    ),
-                    $addon_name,
-                    $min_version_required
-                ),
-                __FILE__,
-                __FUNCTION__ . "({$addon_name})",
-                __LINE__
-            );
-            EE_Error::get_notices(false, true);
-        }
-    }
-
-
-    /**
-     * load_espresso_addons
-     * allow addons to load first so that they can set hooks for running DMS's, etc
-     * this is hooked into both:
-     *    'AHEE__EE_Bootstrap__load_core_configuration'
-     *        which runs during the WP 'plugins_loaded' action at priority 5
-     *    and the WP 'activate_plugin' hook point
-     *
-     * @access public
-     * @return void
-     */
-    public function load_espresso_addons()
-    {
-        $this->deactivateIncompatibleAddon(
-            'Wait Lists',
-            'EE_WAIT_LISTS_VERSION',
-            '1.0.0.beta.074',
-            'load_espresso_wait_lists',
-            'EE_WAIT_LISTS_PLUGIN_FILE'
-        );
-        $this->deactivateIncompatibleAddon(
-            'Automated Upcoming Event Notifications',
-            'EE_AUTOMATED_UPCOMING_EVENT_NOTIFICATION_VERSION',
-            '1.0.0.beta.091',
-            'load_espresso_automated_upcoming_event_notification',
-            'EE_AUTOMATED_UPCOMING_EVENT_NOTIFICATION_PLUGIN_FILE'
-        );
-        do_action('AHEE__EE_System__load_espresso_addons');
-        // if the WP API basic auth plugin isn't already loaded, load it now.
-        // We want it for mobile apps. Just include the entire plugin
-        // also, don't load the basic auth when a plugin is getting activated, because
-        // it could be the basic auth plugin, and it doesn't check if its methods are already defined
-        // and causes a fatal error
-        if (($this->request->isWordPressApi() || $this->request->isApi())
-            && $this->request->getRequestParam('activate') !== 'true'
-            && ! function_exists('json_basic_auth_handler')
-            && ! function_exists('json_basic_auth_error')
-            && ! in_array(
-                $this->request->getRequestParam('action'),
-                array('activate', 'activate-selected'),
-                true
-            )
-        ) {
-            include_once EE_THIRD_PARTY . 'wp-api-basic-auth' . DS . 'basic-auth.php';
-        }
-        do_action('AHEE__EE_System__load_espresso_addons__complete');
-    }
-
-
-    /**
-     * detect_activations_or_upgrades
-     * Checks for activation or upgrade of core first;
-     * then also checks if any registered addons have been activated or upgraded
-     * This is hooked into 'AHEE__EE_Bootstrap__detect_activations_or_upgrades'
-     * which runs during the WP 'plugins_loaded' action at priority 3
-     *
-     * @access public
-     * @return void
-     */
-    public function detect_activations_or_upgrades()
-    {
-        // first off: let's make sure to handle core
-        $this->detect_if_activation_or_upgrade();
-        foreach ($this->registry->addons as $addon) {
-            if ($addon instanceof EE_Addon) {
-                // detect teh request type for that addon
-                $addon->detect_activation_or_upgrade();
-            }
-        }
-    }
-
-
-    /**
-     * detect_if_activation_or_upgrade
-     * Takes care of detecting whether this is a brand new install or code upgrade,
-     * and either setting up the DB or setting up maintenance mode etc.
-     *
-     * @access public
-     * @return void
-     */
-    public function detect_if_activation_or_upgrade()
-    {
-        do_action('AHEE__EE_System___detect_if_activation_or_upgrade__begin');
-        // check if db has been updated, or if its a brand-new installation
-        $espresso_db_update = $this->fix_espresso_db_upgrade_option();
-        $request_type = $this->detect_req_type($espresso_db_update);
-        // EEH_Debug_Tools::printr( $request_type, '$request_type', __FILE__, __LINE__ );
-        switch ($request_type) {
-            case EE_System::req_type_new_activation:
-                do_action('AHEE__EE_System__detect_if_activation_or_upgrade__new_activation');
-                $this->_handle_core_version_change($espresso_db_update);
-                break;
-            case EE_System::req_type_reactivation:
-                do_action('AHEE__EE_System__detect_if_activation_or_upgrade__reactivation');
-                $this->_handle_core_version_change($espresso_db_update);
-                break;
-            case EE_System::req_type_upgrade:
-                do_action('AHEE__EE_System__detect_if_activation_or_upgrade__upgrade');
-                // migrations may be required now that we've upgraded
-                $this->maintenance_mode->set_maintenance_mode_if_db_old();
-                $this->_handle_core_version_change($espresso_db_update);
-                break;
-            case EE_System::req_type_downgrade:
-                do_action('AHEE__EE_System__detect_if_activation_or_upgrade__downgrade');
-                // its possible migrations are no longer required
-                $this->maintenance_mode->set_maintenance_mode_if_db_old();
-                $this->_handle_core_version_change($espresso_db_update);
-                break;
-            case EE_System::req_type_normal:
-            default:
-                break;
-        }
-        do_action('AHEE__EE_System__detect_if_activation_or_upgrade__complete');
-    }
-
-
-    /**
-     * Updates the list of installed versions and sets hooks for
-     * initializing the database later during the request
-     *
-     * @param array $espresso_db_update
-     */
-    private function _handle_core_version_change($espresso_db_update)
-    {
-        $this->update_list_of_installed_versions($espresso_db_update);
-        // get ready to verify the DB is ok (provided we aren't in maintenance mode, of course)
-        add_action(
-            'AHEE__EE_System__perform_activations_upgrades_and_migrations',
-            array($this, 'initialize_db_if_no_migrations_required')
-        );
-    }
-
-
-    /**
-     * standardizes the wp option 'espresso_db_upgrade' which actually stores
-     * information about what versions of EE have been installed and activated,
-     * NOT necessarily the state of the database
-     *
-     * @param mixed $espresso_db_update           the value of the WordPress option.
-     *                                            If not supplied, fetches it from the options table
-     * @return array the correct value of 'espresso_db_upgrade', after saving it, if it needed correction
-     */
-    private function fix_espresso_db_upgrade_option($espresso_db_update = null)
-    {
-        do_action('FHEE__EE_System__manage_fix_espresso_db_upgrade_option__begin', $espresso_db_update);
-        if (! $espresso_db_update) {
-            $espresso_db_update = get_option('espresso_db_update');
-        }
-        // check that option is an array
-        if (! is_array($espresso_db_update)) {
-            // if option is FALSE, then it never existed
-            if ($espresso_db_update === false) {
-                // make $espresso_db_update an array and save option with autoload OFF
-                $espresso_db_update = array();
-                add_option('espresso_db_update', $espresso_db_update, '', 'no');
-            } else {
-                // option is NOT FALSE but also is NOT an array, so make it an array and save it
-                $espresso_db_update = array($espresso_db_update => array());
-                update_option('espresso_db_update', $espresso_db_update);
-            }
-        } else {
-            $corrected_db_update = array();
-            // if IS an array, but is it an array where KEYS are version numbers, and values are arrays?
-            foreach ($espresso_db_update as $should_be_version_string => $should_be_array) {
-                if (is_int($should_be_version_string) && ! is_array($should_be_array)) {
-                    // the key is an int, and the value IS NOT an array
-                    // so it must be numerically-indexed, where values are versions installed...
-                    // fix it!
-                    $version_string = $should_be_array;
-                    $corrected_db_update[ $version_string ] = array('unknown-date');
-                } else {
-                    // ok it checks out
-                    $corrected_db_update[ $should_be_version_string ] = $should_be_array;
-                }
-            }
-            $espresso_db_update = $corrected_db_update;
-            update_option('espresso_db_update', $espresso_db_update);
-        }
-        do_action('FHEE__EE_System__manage_fix_espresso_db_upgrade_option__complete', $espresso_db_update);
-        return $espresso_db_update;
-    }
-
-
-    /**
-     * Does the traditional work of setting up the plugin's database and adding default data.
-     * If migration script/process did not exist, this is what would happen on every activation/reactivation/upgrade.
-     * NOTE: if we're in maintenance mode (which would be the case if we detect there are data
-     * migration scripts that need to be run and a version change happens), enqueues core for database initialization,
-     * so that it will be done when migrations are finished
-     *
-     * @param boolean $initialize_addons_too if true, we double-check addons' database tables etc too;
-     * @param boolean $verify_schema         if true will re-check the database tables have the correct schema.
-     *                                       This is a resource-intensive job
-     *                                       so we prefer to only do it when necessary
-     * @return void
-     * @throws EE_Error
-     */
-    public function initialize_db_if_no_migrations_required($initialize_addons_too = false, $verify_schema = true)
-    {
-        $request_type = $this->detect_req_type();
-        // only initialize system if we're not in maintenance mode.
-        if ($this->maintenance_mode->level() !== EE_Maintenance_Mode::level_2_complete_maintenance) {
-            /** @var EventEspresso\core\domain\services\custom_post_types\RewriteRules $rewrite_rules */
-            $rewrite_rules = $this->loader->getShared(
-                'EventEspresso\core\domain\services\custom_post_types\RewriteRules'
-            );
-            $rewrite_rules->flush();
-            if ($verify_schema) {
-                EEH_Activation::initialize_db_and_folders();
-            }
-            EEH_Activation::initialize_db_content();
-            EEH_Activation::system_initialization();
-            if ($initialize_addons_too) {
-                $this->initialize_addons();
-            }
-        } else {
-            EE_Data_Migration_Manager::instance()->enqueue_db_initialization_for('Core');
-        }
-        if ($request_type === EE_System::req_type_new_activation
-            || $request_type === EE_System::req_type_reactivation
-            || (
-                $request_type === EE_System::req_type_upgrade
-                && $this->is_major_version_change()
-            )
-        ) {
-            add_action('AHEE__EE_System__initialize_last', array($this, 'redirect_to_about_ee'), 9);
-        }
-    }
-
-
-    /**
-     * Initializes the db for all registered addons
-     *
-     * @throws EE_Error
-     */
-    public function initialize_addons()
-    {
-        // foreach registered addon, make sure its db is up-to-date too
-        foreach ($this->registry->addons as $addon) {
-            if ($addon instanceof EE_Addon) {
-                $addon->initialize_db_if_no_migrations_required();
-            }
-        }
-    }
-
-
-    /**
-     * Adds the current code version to the saved wp option which stores a list of all ee versions ever installed.
-     *
-     * @param    array  $version_history
-     * @param    string $current_version_to_add version to be added to the version history
-     * @return    boolean success as to whether or not this option was changed
-     */
-    public function update_list_of_installed_versions($version_history = null, $current_version_to_add = null)
-    {
-        if (! $version_history) {
-            $version_history = $this->fix_espresso_db_upgrade_option($version_history);
-        }
-        if ($current_version_to_add === null) {
-            $current_version_to_add = espresso_version();
-        }
-        $version_history[ $current_version_to_add ][] = date('Y-m-d H:i:s', time());
-        // re-save
-        return update_option('espresso_db_update', $version_history);
-    }
-
-
-    /**
-     * Detects if the current version indicated in the has existed in the list of
-     * previously-installed versions of EE (espresso_db_update). Does NOT modify it (ie, no side-effect)
-     *
-     * @param array $espresso_db_update array from the wp option stored under the name 'espresso_db_update'.
-     *                                  If not supplied, fetches it from the options table.
-     *                                  Also, caches its result so later parts of the code can also know whether
-     *                                  there's been an update or not. This way we can add the current version to
-     *                                  espresso_db_update, but still know if this is a new install or not
-     * @return int one of the constants on EE_System::req_type_
-     */
-    public function detect_req_type($espresso_db_update = null)
-    {
-        if ($this->_req_type === null) {
-            $espresso_db_update = ! empty($espresso_db_update)
-                ? $espresso_db_update
-                : $this->fix_espresso_db_upgrade_option();
-            $this->_req_type = EE_System::detect_req_type_given_activation_history(
-                $espresso_db_update,
-                'ee_espresso_activation',
-                espresso_version()
-            );
-            $this->_major_version_change = $this->_detect_major_version_change($espresso_db_update);
-            $this->request->setIsActivation($this->_req_type !== EE_System::req_type_normal);
-        }
-        return $this->_req_type;
-    }
-
-
-    /**
-     * Returns whether or not there was a non-micro version change (ie, change in either
-     * the first or second number in the version. Eg 4.9.0.rc.001 to 4.10.0.rc.000,
-     * but not 4.9.0.rc.0001 to 4.9.1.rc.0001
-     *
-     * @param $activation_history
-     * @return bool
-     */
-    private function _detect_major_version_change($activation_history)
-    {
-        $previous_version = EE_System::_get_most_recently_active_version_from_activation_history($activation_history);
-        $previous_version_parts = explode('.', $previous_version);
-        $current_version_parts = explode('.', espresso_version());
-        return isset($previous_version_parts[0], $previous_version_parts[1], $current_version_parts[0], $current_version_parts[1])
-               && ($previous_version_parts[0] !== $current_version_parts[0]
-                   || $previous_version_parts[1] !== $current_version_parts[1]
-               );
-    }
-
-
-    /**
-     * Returns true if either the major or minor version of EE changed during this request.
-     * Eg 4.9.0.rc.001 to 4.10.0.rc.000, but not 4.9.0.rc.0001 to 4.9.1.rc.0001
-     *
-     * @return bool
-     */
-    public function is_major_version_change()
-    {
-        return $this->_major_version_change;
-    }
-
-
-    /**
-     * Determines the request type for any ee addon, given three piece of info: the current array of activation
-     * histories (for core that' 'espresso_db_update' wp option); the name of the WordPress option which is temporarily
-     * set upon activation of the plugin (for core it's 'ee_espresso_activation'); and the version that this plugin was
-     * just activated to (for core that will always be espresso_version())
-     *
-     * @param array  $activation_history_for_addon     the option's value which stores the activation history for this
-     *                                                 ee plugin. for core that's 'espresso_db_update'
-     * @param string $activation_indicator_option_name the name of the WordPress option that is temporarily set to
-     *                                                 indicate that this plugin was just activated
-     * @param string $version_to_upgrade_to            the version that was just upgraded to (for core that will be
-     *                                                 espresso_version())
-     * @return int one of the constants on EE_System::req_type_*
-     */
-    public static function detect_req_type_given_activation_history(
-        $activation_history_for_addon,
-        $activation_indicator_option_name,
-        $version_to_upgrade_to
-    ) {
-        $version_is_higher = self::_new_version_is_higher($activation_history_for_addon, $version_to_upgrade_to);
-        if ($activation_history_for_addon) {
-            // it exists, so this isn't a completely new install
-            // check if this version already in that list of previously installed versions
-            if (! isset($activation_history_for_addon[ $version_to_upgrade_to ])) {
-                // it a version we haven't seen before
-                if ($version_is_higher === 1) {
-                    $req_type = EE_System::req_type_upgrade;
-                } else {
-                    $req_type = EE_System::req_type_downgrade;
-                }
-                delete_option($activation_indicator_option_name);
-            } else {
-                // its not an update. maybe a reactivation?
-                if (get_option($activation_indicator_option_name, false)) {
-                    if ($version_is_higher === -1) {
-                        $req_type = EE_System::req_type_downgrade;
-                    } elseif ($version_is_higher === 0) {
-                        // we've seen this version before, but it's an activation. must be a reactivation
-                        $req_type = EE_System::req_type_reactivation;
-                    } else {// $version_is_higher === 1
-                        $req_type = EE_System::req_type_upgrade;
-                    }
-                    delete_option($activation_indicator_option_name);
-                } else {
-                    // we've seen this version before and the activation indicate doesn't show it was just activated
-                    if ($version_is_higher === -1) {
-                        $req_type = EE_System::req_type_downgrade;
-                    } elseif ($version_is_higher === 0) {
-                        // we've seen this version before and it's not an activation. its normal request
-                        $req_type = EE_System::req_type_normal;
-                    } else {// $version_is_higher === 1
-                        $req_type = EE_System::req_type_upgrade;
-                    }
-                }
-            }
-        } else {
-            // brand new install
-            $req_type = EE_System::req_type_new_activation;
-            delete_option($activation_indicator_option_name);
-        }
-        return $req_type;
-    }
-
-
-    /**
-     * Detects if the $version_to_upgrade_to is higher than the most recent version in
-     * the $activation_history_for_addon
-     *
-     * @param array  $activation_history_for_addon (keys are versions, values are arrays of times activated,
-     *                                             sometimes containing 'unknown-date'
-     * @param string $version_to_upgrade_to        (current version)
-     * @return int results of version_compare( $version_to_upgrade_to, $most_recently_active_version ).
-     *                                             ie, -1 if $version_to_upgrade_to is LOWER (downgrade);
-     *                                             0 if $version_to_upgrade_to MATCHES (reactivation or normal request);
-     *                                             1 if $version_to_upgrade_to is HIGHER (upgrade) ;
-     */
-    private static function _new_version_is_higher($activation_history_for_addon, $version_to_upgrade_to)
-    {
-        // find the most recently-activated version
-        $most_recently_active_version =
-            EE_System::_get_most_recently_active_version_from_activation_history($activation_history_for_addon);
-        return version_compare($version_to_upgrade_to, $most_recently_active_version);
-    }
-
-
-    /**
-     * Gets the most recently active version listed in the activation history,
-     * and if none are found (ie, it's a brand new install) returns '0.0.0.dev.000'.
-     *
-     * @param array $activation_history  (keys are versions, values are arrays of times activated,
-     *                                   sometimes containing 'unknown-date'
-     * @return string
-     */
-    private static function _get_most_recently_active_version_from_activation_history($activation_history)
-    {
-        $most_recently_active_version_activation = '1970-01-01 00:00:00';
-        $most_recently_active_version = '0.0.0.dev.000';
-        if (is_array($activation_history)) {
-            foreach ($activation_history as $version => $times_activated) {
-                // check there is a record of when this version was activated. Otherwise,
-                // mark it as unknown
-                if (! $times_activated) {
-                    $times_activated = array('unknown-date');
-                }
-                if (is_string($times_activated)) {
-                    $times_activated = array($times_activated);
-                }
-                foreach ($times_activated as $an_activation) {
-                    if ($an_activation !== 'unknown-date'
-                        && $an_activation
-                           > $most_recently_active_version_activation) {
-                        $most_recently_active_version = $version;
-                        $most_recently_active_version_activation = $an_activation === 'unknown-date'
-                            ? '1970-01-01 00:00:00'
-                            : $an_activation;
-                    }
-                }
-            }
-        }
-        return $most_recently_active_version;
-    }
-
-
-    /**
-     * This redirects to the about EE page after activation
-     *
-     * @return void
-     */
-    public function redirect_to_about_ee()
-    {
-        $notices = EE_Error::get_notices(false);
-        // if current user is an admin and it's not an ajax or rest request
-        if (! isset($notices['errors'])
-            && $this->request->isAdmin()
-            && apply_filters(
-                'FHEE__EE_System__redirect_to_about_ee__do_redirect',
-                $this->capabilities->current_user_can('manage_options', 'espresso_about_default')
-            )
-        ) {
-            $query_params = array('page' => 'espresso_about');
-            if (EE_System::instance()->detect_req_type() === EE_System::req_type_new_activation) {
-                $query_params['new_activation'] = true;
-            }
-            if (EE_System::instance()->detect_req_type() === EE_System::req_type_reactivation) {
-                $query_params['reactivation'] = true;
-            }
-            $url = add_query_arg($query_params, admin_url('admin.php'));
-            wp_safe_redirect($url);
-            exit();
-        }
-    }
-
-
-    /**
-     * load_core_configuration
-     * this is hooked into 'AHEE__EE_Bootstrap__load_core_configuration'
-     * which runs during the WP 'plugins_loaded' action at priority 5
-     *
-     * @return void
-     * @throws ReflectionException
-     * @throws Exception
-     */
-    public function load_core_configuration()
-    {
-        do_action('AHEE__EE_System__load_core_configuration__begin', $this);
-        $this->loader->getShared('EE_Load_Textdomain');
-        // load textdomain
-        EE_Load_Textdomain::load_textdomain();
-        // load and setup EE_Config and EE_Network_Config
-        $config = $this->loader->getShared('EE_Config');
-        $this->loader->getShared('EE_Network_Config');
-        // setup autoloaders
-        // enable logging?
-        if ($config->admin->use_full_logging) {
-            $this->loader->getShared('EE_Log');
-        }
-        // check for activation errors
-        $activation_errors = get_option('ee_plugin_activation_errors', false);
-        if ($activation_errors) {
-            EE_Error::add_error($activation_errors, __FILE__, __FUNCTION__, __LINE__);
-            update_option('ee_plugin_activation_errors', false);
-        }
-        // get model names
-        $this->_parse_model_names();
-        // load caf stuff a chance to play during the activation process too.
-        $this->_maybe_brew_regular();
-        // configure custom post type definitions
-        $this->loader->getShared('EventEspresso\core\domain\entities\custom_post_types\CustomTaxonomyDefinitions');
-        $this->loader->getShared('EventEspresso\core\domain\entities\custom_post_types\CustomPostTypeDefinitions');
-        do_action('AHEE__EE_System__load_core_configuration__complete', $this);
-    }
-
-
-    /**
-     * cycles through all of the models/*.model.php files, and assembles an array of model names
-     *
-     * @return void
-     * @throws ReflectionException
-     */
-    private function _parse_model_names()
-    {
-        // get all the files in the EE_MODELS folder that end in .model.php
-        $models = glob(EE_MODELS . '*.model.php');
-        $model_names = array();
-        $non_abstract_db_models = array();
-        foreach ($models as $model) {
-            // get model classname
-            $classname = EEH_File::get_classname_from_filepath_with_standard_filename($model);
-            $short_name = str_replace('EEM_', '', $classname);
-            $reflectionClass = new ReflectionClass($classname);
-            if ($reflectionClass->isSubclassOf('EEM_Base') && ! $reflectionClass->isAbstract()) {
-                $non_abstract_db_models[ $short_name ] = $classname;
-            }
-            $model_names[ $short_name ] = $classname;
-        }
-        $this->registry->models = apply_filters('FHEE__EE_System__parse_model_names', $model_names);
-        $this->registry->non_abstract_db_models = apply_filters(
-            'FHEE__EE_System__parse_implemented_model_names',
-            $non_abstract_db_models
-        );
-    }
-
-
-    /**
-     * The purpose of this method is to simply check for a file named "caffeinated/brewing_regular.php" for any hooks
-     * that need to be setup before our EE_System launches.
-     *
-     * @return void
-     * @throws DomainException
-     * @throws InvalidArgumentException
-     * @throws InvalidDataTypeException
-     * @throws InvalidInterfaceException
-     * @throws InvalidClassException
-     * @throws InvalidFilePathException
-     */
-    private function _maybe_brew_regular()
-    {
-        /** @var Domain $domain */
-        $domain = DomainFactory::getShared(
-            new FullyQualifiedName(
-                'EventEspresso\core\domain\Domain'
-            ),
-            array(
-                new FilePath(EVENT_ESPRESSO_MAIN_FILE),
-                Version::fromString(espresso_version()),
-            )
-        );
-        if ($domain->isCaffeinated()) {
-            require_once EE_CAFF_PATH . 'brewing_regular.php';
-        }
-    }
-
-
-    /**
-     * @since 4.9.71.p
-     * @throws Exception
-     */
-    public function loadRouteMatchSpecifications()
-    {
-        try {
-            $this->loader->getShared(
-                'EventEspresso\core\services\route_match\RouteMatchSpecificationManager'
-            );
-        } catch (Exception $exception) {
-            new ExceptionStackTraceDisplay($exception);
-        }
-        do_action('AHEE__EE_System__loadRouteMatchSpecifications');
-    }
-
-
-    /**
-     * register_shortcodes_modules_and_widgets
-     * generate lists of shortcodes and modules, then verify paths and classes
-     * This is hooked into 'AHEE__EE_Bootstrap__register_shortcodes_modules_and_widgets'
-     * which runs during the WP 'plugins_loaded' action at priority 7
-     *
-     * @access public
-     * @return void
-     * @throws Exception
-     */
-    public function register_shortcodes_modules_and_widgets()
-    {
-        if ($this->request->isFrontend() || $this->request->isIframe() || $this->request->isAjax()) {
-            try {
-                // load, register, and add shortcodes the new way
-                $this->loader->getShared(
-                    'EventEspresso\core\services\shortcodes\ShortcodesManager',
-                    array(
-                        // and the old way, but we'll put it under control of the new system
-                        EE_Config::getLegacyShortcodesManager(),
-                    )
-                );
-            } catch (Exception $exception) {
-                new ExceptionStackTraceDisplay($exception);
-            }
-        }
-        do_action('AHEE__EE_System__register_shortcodes_modules_and_widgets');
-        // check for addons using old hook point
-        if (has_action('AHEE__EE_System__register_shortcodes_modules_and_addons')) {
-            $this->_incompatible_addon_error();
-        }
-    }
-
-
-    /**
-     * _incompatible_addon_error
-     *
-     * @access public
-     * @return void
-     */
-    private function _incompatible_addon_error()
-    {
-        // get array of classes hooking into here
-        $class_names = EEH_Class_Tools::get_class_names_for_all_callbacks_on_hook(
-            'AHEE__EE_System__register_shortcodes_modules_and_addons'
-        );
-        if (! empty($class_names)) {
-            $msg = __(
-                'The following plugins, addons, or modules appear to be incompatible with this version of Event Espresso and were automatically deactivated to avoid fatal errors:',
-                'event_espresso'
-            );
-            $msg .= '<ul>';
-            foreach ($class_names as $class_name) {
-                $msg .= '<li><b>Event Espresso - '
-                        . str_replace(
-                            array('EE_', 'EEM_', 'EED_', 'EES_', 'EEW_'),
-                            '',
-                            $class_name
-                        ) . '</b></li>';
-            }
-            $msg .= '</ul>';
-            $msg .= __(
-                'Compatibility issues can be avoided and/or resolved by keeping addons and plugins updated to the latest version.',
-                'event_espresso'
-            );
-            // save list of incompatible addons to wp-options for later use
-            add_option('ee_incompatible_addons', $class_names, '', 'no');
-            if (is_admin()) {
-                EE_Error::add_error($msg, __FILE__, __FUNCTION__, __LINE__);
-            }
-        }
-    }
-
-
-    /**
-     * brew_espresso
-     * begins the process of setting hooks for initializing EE in the correct order
-     * This is happening on the 'AHEE__EE_Bootstrap__brew_espresso' hook point
-     * which runs during the WP 'plugins_loaded' action at priority 9
-     *
-     * @return void
-     */
-    public function brew_espresso()
-    {
-        do_action('AHEE__EE_System__brew_espresso__begin', $this);
-        // load some final core systems
-        add_action('init', array($this, 'set_hooks_for_core'), 1);
-        add_action('init', array($this, 'perform_activations_upgrades_and_migrations'), 3);
-        add_action('init', array($this, 'load_CPTs_and_session'), 5);
-        add_action('init', array($this, 'load_controllers'), 7);
-        add_action('init', array($this, 'core_loaded_and_ready'), 9);
-        add_action('init', array($this, 'initialize'), 10);
-        add_action('init', array($this, 'initialize_last'), 100);
-        if (is_admin() && apply_filters('FHEE__EE_System__brew_espresso__load_pue', true)) {
-            // pew pew pew
-            $this->loader->getShared('EventEspresso\core\services\licensing\LicenseService');
-            do_action('AHEE__EE_System__brew_espresso__after_pue_init');
-        }
-        do_action('AHEE__EE_System__brew_espresso__complete', $this);
-    }
-
-
-    /**
-     *    set_hooks_for_core
-     *
-     * @access public
-     * @return    void
-     * @throws EE_Error
-     */
-    public function set_hooks_for_core()
-    {
-        $this->_deactivate_incompatible_addons();
-        do_action('AHEE__EE_System__set_hooks_for_core');
-        $this->loader->getShared('EventEspresso\core\domain\values\session\SessionLifespan');
-        // caps need to be initialized on every request so that capability maps are set.
-        // @see https://events.codebasehq.com/projects/event-espresso/tickets/8674
-        $this->registry->CAP->init_caps();
-    }
-
-
-    /**
-     * Using the information gathered in EE_System::_incompatible_addon_error,
-     * deactivates any addons considered incompatible with the current version of EE
-     */
-    private function _deactivate_incompatible_addons()
-    {
-        $incompatible_addons = get_option('ee_incompatible_addons', array());
-        if (! empty($incompatible_addons)) {
-            $active_plugins = get_option('active_plugins', array());
-            foreach ($active_plugins as $active_plugin) {
-                foreach ($incompatible_addons as $incompatible_addon) {
-                    if (strpos($active_plugin, $incompatible_addon) !== false) {
-                        unset($_GET['activate']);
-                        espresso_deactivate_plugin($active_plugin);
-                    }
-                }
-            }
-        }
-    }
-
-
-    /**
-     *    perform_activations_upgrades_and_migrations
-     *
-     * @access public
-     * @return    void
-     */
-    public function perform_activations_upgrades_and_migrations()
-    {
-        do_action('AHEE__EE_System__perform_activations_upgrades_and_migrations');
-    }
-
-
-    /**
-     * @return void
-     * @throws DomainException
-     */
-    public function load_CPTs_and_session()
-    {
-        do_action('AHEE__EE_System__load_CPTs_and_session__start');
-        /** @var EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomies $register_custom_taxonomies */
-        $register_custom_taxonomies = $this->loader->getShared(
-            'EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomies'
-        );
-        $register_custom_taxonomies->registerCustomTaxonomies();
-        /** @var EventEspresso\core\domain\services\custom_post_types\RegisterCustomPostTypes $register_custom_post_types */
-        $register_custom_post_types = $this->loader->getShared(
-            'EventEspresso\core\domain\services\custom_post_types\RegisterCustomPostTypes'
-        );
-        $register_custom_post_types->registerCustomPostTypes();
-        /** @var EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomyTerms $register_custom_taxonomy_terms */
-        $register_custom_taxonomy_terms = $this->loader->getShared(
-            'EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomyTerms'
-        );
-        $register_custom_taxonomy_terms->registerCustomTaxonomyTerms();
-        // load legacy Custom Post Types and Taxonomies
-        $this->loader->getShared('EE_Register_CPTs');
-        do_action('AHEE__EE_System__load_CPTs_and_session__complete');
-    }
-
-
-    /**
-     * load_controllers
-     * this is the best place to load any additional controllers that needs access to EE core.
-     * it is expected that all basic core EE systems, that are not dependant on the current request are loaded at this
-     * time
-     *
-     * @access public
-     * @return void
-     */
-    public function load_controllers()
-    {
-        do_action('AHEE__EE_System__load_controllers__start');
-        // let's get it started
-        if (! $this->maintenance_mode->level()
-            && ($this->request->isFrontend() || $this->request->isFrontAjax())
-        ) {
-            do_action('AHEE__EE_System__load_controllers__load_front_controllers');
-            $this->loader->getShared('EE_Front_Controller');
-        } elseif ($this->request->isAdmin() || $this->request->isAdminAjax()) {
-            do_action('AHEE__EE_System__load_controllers__load_admin_controllers');
-            $this->loader->getShared('EE_Admin');
-        } elseif ($this->request->isWordPressHeartbeat()) {
-            $this->loader->getShared('EventEspresso\core\domain\services\admin\ajax\WordpressHeartbeat');
-        }
-        do_action('AHEE__EE_System__load_controllers__complete');
-    }
-
-
-    /**
-     * core_loaded_and_ready
-     * all of the basic EE core should be loaded at this point and available regardless of M-Mode
-     *
-     * @access public
-     * @return void
-     * @throws Exception
-     */
-    public function core_loaded_and_ready()
-    {
-        if ($this->request->isAdmin()
-            || $this->request->isFrontend()
-            || $this->request->isIframe()
-            || $this->request->isWordPressApi()
-        ) {
-            try {
-                $this->loader->getShared('EventEspresso\core\services\assets\Registry');
-                $this->loader->getShared('EventEspresso\core\domain\services\assets\CoreAssetManager');
-                if ($this->canLoadBlocks()) {
-                    $this->loader->getShared(
-                        'EventEspresso\core\services\editor\BlockRegistrationManager'
-                    );
-                }
-            } catch (Exception $exception) {
-                new ExceptionStackTraceDisplay($exception);
-            }
-        }
-        if ($this->request->isAdmin()
-            || $this->request->isEeAjax()
-            || $this->request->isFrontend()
-        ) {
-            $this->loader->getShared('EE_Session');
-        }
-        // integrate WP_Query with the EE models
-        $this->loader->getShared('EE_CPT_Strategy');
-        do_action('AHEE__EE_System__core_loaded_and_ready');
-        // always load template tags, because it's faster than checking if it's a front-end request, and many page
-        // builders require these even on the front-end
-        require_once EE_PUBLIC . 'template_tags.php';
-        do_action('AHEE__EE_System__set_hooks_for_shortcodes_modules_and_addons');
-    }
-
-
-    /**
-     * initialize
-     * this is the best place to begin initializing client code
-     *
-     * @access public
-     * @return void
-     */
-    public function initialize()
-    {
-        do_action('AHEE__EE_System__initialize');
-    }
-
-
-    /**
-     * initialize_last
-     * this is run really late during the WP init hook point, and ensures that mostly everything else that needs to
-     * initialize has done so
-     *
-     * @access public
-     * @return void
-     */
-    public function initialize_last()
-    {
-        do_action('AHEE__EE_System__initialize_last');
-        /** @var EventEspresso\core\domain\services\custom_post_types\RewriteRules $rewrite_rules */
-        $rewrite_rules = $this->loader->getShared(
-            'EventEspresso\core\domain\services\custom_post_types\RewriteRules'
-        );
-        $rewrite_rules->flushRewriteRules();
-        add_action('admin_bar_init', array($this, 'addEspressoToolbar'));
-        if (($this->request->isAjax() || $this->request->isAdmin())
-            && $this->maintenance_mode->models_can_query()) {
-            $this->loader->getShared('EventEspresso\core\services\privacy\export\PersonalDataExporterManager');
-            $this->loader->getShared('EventEspresso\core\services\privacy\erasure\PersonalDataEraserManager');
-        }
-    }
-
-
-    /**
-     * @return void
-     * @throws EE_Error
-     */
-    public function addEspressoToolbar()
-    {
-        $this->loader->getShared(
-            'EventEspresso\core\domain\services\admin\AdminToolBar',
-            array($this->registry->CAP)
-        );
-    }
-
-
-    /**
-     * do_not_cache
-     * sets no cache headers and defines no cache constants for WP plugins
-     *
-     * @access public
-     * @return void
-     */
-    public static function do_not_cache()
-    {
-        // set no cache constants
-        if (! defined('DONOTCACHEPAGE')) {
-            define('DONOTCACHEPAGE', true);
-        }
-        if (! defined('DONOTCACHCEOBJECT')) {
-            define('DONOTCACHCEOBJECT', true);
-        }
-        if (! defined('DONOTCACHEDB')) {
-            define('DONOTCACHEDB', true);
-        }
-        // add no cache headers
-        add_action('send_headers', array('EE_System', 'nocache_headers'), 10);
-        // plus a little extra for nginx and Google Chrome
-        add_filter('nocache_headers', array('EE_System', 'extra_nocache_headers'), 10, 1);
-        // prevent browsers from prefetching of the rel='next' link, because it may contain content that interferes with the registration process
-        remove_action('wp_head', 'adjacent_posts_rel_link_wp_head');
-    }
-
-
-    /**
-     *    extra_nocache_headers
-     *
-     * @access    public
-     * @param $headers
-     * @return    array
-     */
-    public static function extra_nocache_headers($headers)
-    {
-        // for NGINX
-        $headers['X-Accel-Expires'] = 0;
-        // plus extra for Google Chrome since it doesn't seem to respect "no-cache", but WILL respect "no-store"
-        $headers['Cache-Control'] = 'no-store, no-cache, must-revalidate, max-age=0';
-        return $headers;
-    }
-
-
-    /**
-     *    nocache_headers
-     *
-     * @access    public
-     * @return    void
-     */
-    public static function nocache_headers()
-    {
-        nocache_headers();
-    }
-
-
-    /**
-     * simply hooks into "wp_list_pages_exclude" filter (for wp_list_pages method) and makes sure EE critical pages are
-     * never returned with the function.
-     *
-     * @param  array $exclude_array any existing pages being excluded are in this array.
-     * @return array
-     */
-    public function remove_pages_from_wp_list_pages($exclude_array)
-    {
-        return array_merge($exclude_array, $this->registry->CFG->core->get_critical_pages_array());
-    }
-
-
-    /**
-     * Return whether blocks can be registered/loaded or not.
-     * @return bool
-     */
-    private function canLoadBlocks()
-    {
-        return apply_filters('FHEE__EE_System__canLoadBlocks', true)
-               && function_exists('register_block_type')
-               // don't load blocks if in the Divi page builder editor context
-               // @see https://github.com/eventespresso/event-espresso-core/issues/814
-               && ! $this->request->getRequestParam('et_fb', false);
-    }
+	/**
+	 * indicates this is a 'normal' request. Ie, not activation, nor upgrade, nor activation.
+	 * So examples of this would be a normal GET request on the frontend or backend, or a POST, etc
+	 */
+	const req_type_normal = 0;
+
+	/**
+	 * Indicates this is a brand new installation of EE so we should install
+	 * tables and default data etc
+	 */
+	const req_type_new_activation = 1;
+
+	/**
+	 * we've detected that EE has been reactivated (or EE was activated during maintenance mode,
+	 * and we just exited maintenance mode). We MUST check the database is setup properly
+	 * and that default data is setup too
+	 */
+	const req_type_reactivation = 2;
+
+	/**
+	 * indicates that EE has been upgraded since its previous request.
+	 * We may have data migration scripts to call and will want to trigger maintenance mode
+	 */
+	const req_type_upgrade = 3;
+
+	/**
+	 * TODO  will detect that EE has been DOWNGRADED. We probably don't want to run in this case...
+	 */
+	const req_type_downgrade = 4;
+
+	/**
+	 * @deprecated since version 4.6.0.dev.006
+	 * Now whenever a new_activation is detected the request type is still just
+	 * new_activation (same for reactivation, upgrade, downgrade etc), but if we'r ein maintenance mode
+	 * EE_System::initialize_db_if_no_migrations_required and EE_Addon::initialize_db_if_no_migrations_required
+	 * will instead enqueue that EE plugin's db initialization for when we're taken out of maintenance mode.
+	 * (Specifically, when the migration manager indicates migrations are finished
+	 * EE_Data_Migration_Manager::initialize_db_for_enqueued_ee_plugins() will be called)
+	 */
+	const req_type_activation_but_not_installed = 5;
+
+	/**
+	 * option prefix for recording the activation history (like core's "espresso_db_update") of addons
+	 */
+	const addon_activation_history_option_prefix = 'ee_addon_activation_history_';
+
+	/**
+	 * @var EE_System $_instance
+	 */
+	private static $_instance;
+
+	/**
+	 * @var EE_Registry $registry
+	 */
+	private $registry;
+
+	/**
+	 * @var LoaderInterface $loader
+	 */
+	private $loader;
+
+	/**
+	 * @var EE_Capabilities $capabilities
+	 */
+	private $capabilities;
+
+	/**
+	 * @var RequestInterface $request
+	 */
+	private $request;
+
+	/**
+	 * @var EE_Maintenance_Mode $maintenance_mode
+	 */
+	private $maintenance_mode;
+
+	/**
+	 * Stores which type of request this is, options being one of the constants on EE_System starting with req_type_*.
+	 * It can be a brand-new activation, a reactivation, an upgrade, a downgrade, or a normal request.
+	 *
+	 * @var int $_req_type
+	 */
+	private $_req_type;
+
+	/**
+	 * Whether or not there was a non-micro version change in EE core version during this request
+	 *
+	 * @var boolean $_major_version_change
+	 */
+	private $_major_version_change = false;
+
+	/**
+	 * A Context DTO dedicated solely to identifying the current request type.
+	 *
+	 * @var RequestTypeContextCheckerInterface $request_type
+	 */
+	private $request_type;
+
+
+	/**
+	 * @singleton method used to instantiate class object
+	 * @param EE_Registry|null         $registry
+	 * @param LoaderInterface|null     $loader
+	 * @param RequestInterface|null    $request
+	 * @param EE_Maintenance_Mode|null $maintenance_mode
+	 * @return EE_System
+	 */
+	public static function instance(
+		EE_Registry $registry = null,
+		LoaderInterface $loader = null,
+		RequestInterface $request = null,
+		EE_Maintenance_Mode $maintenance_mode = null
+	) {
+		// check if class object is instantiated
+		if (! self::$_instance instanceof EE_System) {
+			self::$_instance = new self($registry, $loader, $request, $maintenance_mode);
+		}
+		return self::$_instance;
+	}
+
+
+	/**
+	 * resets the instance and returns it
+	 *
+	 * @return EE_System
+	 */
+	public static function reset()
+	{
+		self::$_instance->_req_type = null;
+		// make sure none of the old hooks are left hanging around
+		remove_all_actions('AHEE__EE_System__perform_activations_upgrades_and_migrations');
+		// we need to reset the migration manager in order for it to detect DMSs properly
+		EE_Data_Migration_Manager::reset();
+		self::instance()->detect_activations_or_upgrades();
+		self::instance()->perform_activations_upgrades_and_migrations();
+		return self::instance();
+	}
+
+
+	/**
+	 * sets hooks for running rest of system
+	 * provides "AHEE__EE_System__construct__complete" hook for EE Addons to use as their starting point
+	 * starting EE Addons from any other point may lead to problems
+	 *
+	 * @param EE_Registry         $registry
+	 * @param LoaderInterface     $loader
+	 * @param RequestInterface    $request
+	 * @param EE_Maintenance_Mode $maintenance_mode
+	 */
+	private function __construct(
+		EE_Registry $registry,
+		LoaderInterface $loader,
+		RequestInterface $request,
+		EE_Maintenance_Mode $maintenance_mode
+	) {
+		$this->registry = $registry;
+		$this->loader = $loader;
+		$this->request = $request;
+		$this->maintenance_mode = $maintenance_mode;
+		do_action('AHEE__EE_System__construct__begin', $this);
+		add_action(
+			'AHEE__EE_Bootstrap__load_espresso_addons',
+			array($this, 'loadCapabilities'),
+			5
+		);
+		add_action(
+			'AHEE__EE_Bootstrap__load_espresso_addons',
+			array($this, 'loadCommandBus'),
+			7
+		);
+		add_action(
+			'AHEE__EE_Bootstrap__load_espresso_addons',
+			array($this, 'loadPluginApi'),
+			9
+		);
+		// allow addons to load first so that they can register autoloaders, set hooks for running DMS's, etc
+		add_action(
+			'AHEE__EE_Bootstrap__load_espresso_addons',
+			array($this, 'load_espresso_addons')
+		);
+		// when an ee addon is activated, we want to call the core hook(s) again
+		// because the newly-activated addon didn't get a chance to run at all
+		add_action('activate_plugin', array($this, 'load_espresso_addons'), 1);
+		// detect whether install or upgrade
+		add_action(
+			'AHEE__EE_Bootstrap__detect_activations_or_upgrades',
+			array($this, 'detect_activations_or_upgrades'),
+			3
+		);
+		// load EE_Config, EE_Textdomain, etc
+		add_action(
+			'AHEE__EE_Bootstrap__load_core_configuration',
+			array($this, 'load_core_configuration'),
+			5
+		);
+		// load specifications for matching routes to current request
+		add_action(
+			'AHEE__EE_Bootstrap__load_core_configuration',
+			array($this, 'loadRouteMatchSpecifications')
+		);
+		// load EE_Config, EE_Textdomain, etc
+		add_action(
+			'AHEE__EE_Bootstrap__register_shortcodes_modules_and_widgets',
+			array($this, 'register_shortcodes_modules_and_widgets'),
+			7
+		);
+		// you wanna get going? I wanna get going... let's get going!
+		add_action(
+			'AHEE__EE_Bootstrap__brew_espresso',
+			array($this, 'brew_espresso'),
+			9
+		);
+		// other housekeeping
+		// exclude EE critical pages from wp_list_pages
+		add_filter(
+			'wp_list_pages_excludes',
+			array($this, 'remove_pages_from_wp_list_pages'),
+			10
+		);
+		// ALL EE Addons should use the following hook point to attach their initial setup too
+		// it's extremely important for EE Addons to register any class autoloaders so that they can be available when the EE_Config loads
+		do_action('AHEE__EE_System__construct__complete', $this);
+	}
+
+
+	/**
+	 * load and setup EE_Capabilities
+	 *
+	 * @return void
+	 * @throws EE_Error
+	 */
+	public function loadCapabilities()
+	{
+		$this->capabilities = $this->loader->getShared('EE_Capabilities');
+		add_action(
+			'AHEE__EE_Capabilities__init_caps__before_initialization',
+			function () {
+				LoaderFactory::getLoader()->getShared('EE_Payment_Method_Manager');
+			}
+		);
+	}
+
+
+	/**
+	 * create and cache the CommandBus, and also add middleware
+	 * The CapChecker middleware requires the use of EE_Capabilities
+	 * which is why we need to load the CommandBus after Caps are set up
+	 *
+	 * @return void
+	 * @throws EE_Error
+	 */
+	public function loadCommandBus()
+	{
+		$this->loader->getShared(
+			'CommandBusInterface',
+			array(
+				null,
+				apply_filters(
+					'FHEE__EE_Load_Espresso_Core__handle_request__CommandBus_middleware',
+					array(
+						$this->loader->getShared('EventEspresso\core\services\commands\middleware\CapChecker'),
+						$this->loader->getShared('EventEspresso\core\services\commands\middleware\AddActionHook'),
+					)
+				),
+			)
+		);
+	}
+
+
+	/**
+	 * @return void
+	 * @throws EE_Error
+	 */
+	public function loadPluginApi()
+	{
+		// set autoloaders for all of the classes implementing EEI_Plugin_API
+		// which provide helpers for EE plugin authors to more easily register certain components with EE.
+		EEH_Autoloader::instance()->register_autoloaders_for_each_file_in_folder(EE_LIBRARIES . 'plugin_api');
+		$this->loader->getShared('EE_Request_Handler');
+	}
+
+
+	/**
+	 * @param string $addon_name
+	 * @param string $version_constant
+	 * @param string $min_version_required
+	 * @param string $load_callback
+	 * @param string $plugin_file_constant
+	 * @return void
+	 */
+	private function deactivateIncompatibleAddon(
+		$addon_name,
+		$version_constant,
+		$min_version_required,
+		$load_callback,
+		$plugin_file_constant
+	) {
+		if (! defined($version_constant)) {
+			return;
+		}
+		$addon_version = constant($version_constant);
+		if ($addon_version && version_compare($addon_version, $min_version_required, '<')) {
+			remove_action('AHEE__EE_System__load_espresso_addons', $load_callback);
+			if (! function_exists('deactivate_plugins')) {
+				require_once ABSPATH . 'wp-admin/includes/plugin.php';
+			}
+			deactivate_plugins(plugin_basename(constant($plugin_file_constant)));
+			unset($_GET['activate'], $_REQUEST['activate'], $_GET['activate-multi'], $_REQUEST['activate-multi']);
+			EE_Error::add_error(
+				sprintf(
+					esc_html__(
+						'We\'re sorry, but the Event Espresso %1$s addon was deactivated because version %2$s or higher is required with this version of Event Espresso core.',
+						'event_espresso'
+					),
+					$addon_name,
+					$min_version_required
+				),
+				__FILE__,
+				__FUNCTION__ . "({$addon_name})",
+				__LINE__
+			);
+			EE_Error::get_notices(false, true);
+		}
+	}
+
+
+	/**
+	 * load_espresso_addons
+	 * allow addons to load first so that they can set hooks for running DMS's, etc
+	 * this is hooked into both:
+	 *    'AHEE__EE_Bootstrap__load_core_configuration'
+	 *        which runs during the WP 'plugins_loaded' action at priority 5
+	 *    and the WP 'activate_plugin' hook point
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public function load_espresso_addons()
+	{
+		$this->deactivateIncompatibleAddon(
+			'Wait Lists',
+			'EE_WAIT_LISTS_VERSION',
+			'1.0.0.beta.074',
+			'load_espresso_wait_lists',
+			'EE_WAIT_LISTS_PLUGIN_FILE'
+		);
+		$this->deactivateIncompatibleAddon(
+			'Automated Upcoming Event Notifications',
+			'EE_AUTOMATED_UPCOMING_EVENT_NOTIFICATION_VERSION',
+			'1.0.0.beta.091',
+			'load_espresso_automated_upcoming_event_notification',
+			'EE_AUTOMATED_UPCOMING_EVENT_NOTIFICATION_PLUGIN_FILE'
+		);
+		do_action('AHEE__EE_System__load_espresso_addons');
+		// if the WP API basic auth plugin isn't already loaded, load it now.
+		// We want it for mobile apps. Just include the entire plugin
+		// also, don't load the basic auth when a plugin is getting activated, because
+		// it could be the basic auth plugin, and it doesn't check if its methods are already defined
+		// and causes a fatal error
+		if (($this->request->isWordPressApi() || $this->request->isApi())
+			&& $this->request->getRequestParam('activate') !== 'true'
+			&& ! function_exists('json_basic_auth_handler')
+			&& ! function_exists('json_basic_auth_error')
+			&& ! in_array(
+				$this->request->getRequestParam('action'),
+				array('activate', 'activate-selected'),
+				true
+			)
+		) {
+			include_once EE_THIRD_PARTY . 'wp-api-basic-auth' . DS . 'basic-auth.php';
+		}
+		do_action('AHEE__EE_System__load_espresso_addons__complete');
+	}
+
+
+	/**
+	 * detect_activations_or_upgrades
+	 * Checks for activation or upgrade of core first;
+	 * then also checks if any registered addons have been activated or upgraded
+	 * This is hooked into 'AHEE__EE_Bootstrap__detect_activations_or_upgrades'
+	 * which runs during the WP 'plugins_loaded' action at priority 3
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public function detect_activations_or_upgrades()
+	{
+		// first off: let's make sure to handle core
+		$this->detect_if_activation_or_upgrade();
+		foreach ($this->registry->addons as $addon) {
+			if ($addon instanceof EE_Addon) {
+				// detect teh request type for that addon
+				$addon->detect_activation_or_upgrade();
+			}
+		}
+	}
+
+
+	/**
+	 * detect_if_activation_or_upgrade
+	 * Takes care of detecting whether this is a brand new install or code upgrade,
+	 * and either setting up the DB or setting up maintenance mode etc.
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public function detect_if_activation_or_upgrade()
+	{
+		do_action('AHEE__EE_System___detect_if_activation_or_upgrade__begin');
+		// check if db has been updated, or if its a brand-new installation
+		$espresso_db_update = $this->fix_espresso_db_upgrade_option();
+		$request_type = $this->detect_req_type($espresso_db_update);
+		// EEH_Debug_Tools::printr( $request_type, '$request_type', __FILE__, __LINE__ );
+		switch ($request_type) {
+			case EE_System::req_type_new_activation:
+				do_action('AHEE__EE_System__detect_if_activation_or_upgrade__new_activation');
+				$this->_handle_core_version_change($espresso_db_update);
+				break;
+			case EE_System::req_type_reactivation:
+				do_action('AHEE__EE_System__detect_if_activation_or_upgrade__reactivation');
+				$this->_handle_core_version_change($espresso_db_update);
+				break;
+			case EE_System::req_type_upgrade:
+				do_action('AHEE__EE_System__detect_if_activation_or_upgrade__upgrade');
+				// migrations may be required now that we've upgraded
+				$this->maintenance_mode->set_maintenance_mode_if_db_old();
+				$this->_handle_core_version_change($espresso_db_update);
+				break;
+			case EE_System::req_type_downgrade:
+				do_action('AHEE__EE_System__detect_if_activation_or_upgrade__downgrade');
+				// its possible migrations are no longer required
+				$this->maintenance_mode->set_maintenance_mode_if_db_old();
+				$this->_handle_core_version_change($espresso_db_update);
+				break;
+			case EE_System::req_type_normal:
+			default:
+				break;
+		}
+		do_action('AHEE__EE_System__detect_if_activation_or_upgrade__complete');
+	}
+
+
+	/**
+	 * Updates the list of installed versions and sets hooks for
+	 * initializing the database later during the request
+	 *
+	 * @param array $espresso_db_update
+	 */
+	private function _handle_core_version_change($espresso_db_update)
+	{
+		$this->update_list_of_installed_versions($espresso_db_update);
+		// get ready to verify the DB is ok (provided we aren't in maintenance mode, of course)
+		add_action(
+			'AHEE__EE_System__perform_activations_upgrades_and_migrations',
+			array($this, 'initialize_db_if_no_migrations_required')
+		);
+	}
+
+
+	/**
+	 * standardizes the wp option 'espresso_db_upgrade' which actually stores
+	 * information about what versions of EE have been installed and activated,
+	 * NOT necessarily the state of the database
+	 *
+	 * @param mixed $espresso_db_update           the value of the WordPress option.
+	 *                                            If not supplied, fetches it from the options table
+	 * @return array the correct value of 'espresso_db_upgrade', after saving it, if it needed correction
+	 */
+	private function fix_espresso_db_upgrade_option($espresso_db_update = null)
+	{
+		do_action('FHEE__EE_System__manage_fix_espresso_db_upgrade_option__begin', $espresso_db_update);
+		if (! $espresso_db_update) {
+			$espresso_db_update = get_option('espresso_db_update');
+		}
+		// check that option is an array
+		if (! is_array($espresso_db_update)) {
+			// if option is FALSE, then it never existed
+			if ($espresso_db_update === false) {
+				// make $espresso_db_update an array and save option with autoload OFF
+				$espresso_db_update = array();
+				add_option('espresso_db_update', $espresso_db_update, '', 'no');
+			} else {
+				// option is NOT FALSE but also is NOT an array, so make it an array and save it
+				$espresso_db_update = array($espresso_db_update => array());
+				update_option('espresso_db_update', $espresso_db_update);
+			}
+		} else {
+			$corrected_db_update = array();
+			// if IS an array, but is it an array where KEYS are version numbers, and values are arrays?
+			foreach ($espresso_db_update as $should_be_version_string => $should_be_array) {
+				if (is_int($should_be_version_string) && ! is_array($should_be_array)) {
+					// the key is an int, and the value IS NOT an array
+					// so it must be numerically-indexed, where values are versions installed...
+					// fix it!
+					$version_string = $should_be_array;
+					$corrected_db_update[ $version_string ] = array('unknown-date');
+				} else {
+					// ok it checks out
+					$corrected_db_update[ $should_be_version_string ] = $should_be_array;
+				}
+			}
+			$espresso_db_update = $corrected_db_update;
+			update_option('espresso_db_update', $espresso_db_update);
+		}
+		do_action('FHEE__EE_System__manage_fix_espresso_db_upgrade_option__complete', $espresso_db_update);
+		return $espresso_db_update;
+	}
+
+
+	/**
+	 * Does the traditional work of setting up the plugin's database and adding default data.
+	 * If migration script/process did not exist, this is what would happen on every activation/reactivation/upgrade.
+	 * NOTE: if we're in maintenance mode (which would be the case if we detect there are data
+	 * migration scripts that need to be run and a version change happens), enqueues core for database initialization,
+	 * so that it will be done when migrations are finished
+	 *
+	 * @param boolean $initialize_addons_too if true, we double-check addons' database tables etc too;
+	 * @param boolean $verify_schema         if true will re-check the database tables have the correct schema.
+	 *                                       This is a resource-intensive job
+	 *                                       so we prefer to only do it when necessary
+	 * @return void
+	 * @throws EE_Error
+	 */
+	public function initialize_db_if_no_migrations_required($initialize_addons_too = false, $verify_schema = true)
+	{
+		$request_type = $this->detect_req_type();
+		// only initialize system if we're not in maintenance mode.
+		if ($this->maintenance_mode->level() !== EE_Maintenance_Mode::level_2_complete_maintenance) {
+			/** @var EventEspresso\core\domain\services\custom_post_types\RewriteRules $rewrite_rules */
+			$rewrite_rules = $this->loader->getShared(
+				'EventEspresso\core\domain\services\custom_post_types\RewriteRules'
+			);
+			$rewrite_rules->flush();
+			if ($verify_schema) {
+				EEH_Activation::initialize_db_and_folders();
+			}
+			EEH_Activation::initialize_db_content();
+			EEH_Activation::system_initialization();
+			if ($initialize_addons_too) {
+				$this->initialize_addons();
+			}
+		} else {
+			EE_Data_Migration_Manager::instance()->enqueue_db_initialization_for('Core');
+		}
+		if ($request_type === EE_System::req_type_new_activation
+			|| $request_type === EE_System::req_type_reactivation
+			|| (
+				$request_type === EE_System::req_type_upgrade
+				&& $this->is_major_version_change()
+			)
+		) {
+			add_action('AHEE__EE_System__initialize_last', array($this, 'redirect_to_about_ee'), 9);
+		}
+	}
+
+
+	/**
+	 * Initializes the db for all registered addons
+	 *
+	 * @throws EE_Error
+	 */
+	public function initialize_addons()
+	{
+		// foreach registered addon, make sure its db is up-to-date too
+		foreach ($this->registry->addons as $addon) {
+			if ($addon instanceof EE_Addon) {
+				$addon->initialize_db_if_no_migrations_required();
+			}
+		}
+	}
+
+
+	/**
+	 * Adds the current code version to the saved wp option which stores a list of all ee versions ever installed.
+	 *
+	 * @param    array  $version_history
+	 * @param    string $current_version_to_add version to be added to the version history
+	 * @return    boolean success as to whether or not this option was changed
+	 */
+	public function update_list_of_installed_versions($version_history = null, $current_version_to_add = null)
+	{
+		if (! $version_history) {
+			$version_history = $this->fix_espresso_db_upgrade_option($version_history);
+		}
+		if ($current_version_to_add === null) {
+			$current_version_to_add = espresso_version();
+		}
+		$version_history[ $current_version_to_add ][] = date('Y-m-d H:i:s', time());
+		// re-save
+		return update_option('espresso_db_update', $version_history);
+	}
+
+
+	/**
+	 * Detects if the current version indicated in the has existed in the list of
+	 * previously-installed versions of EE (espresso_db_update). Does NOT modify it (ie, no side-effect)
+	 *
+	 * @param array $espresso_db_update array from the wp option stored under the name 'espresso_db_update'.
+	 *                                  If not supplied, fetches it from the options table.
+	 *                                  Also, caches its result so later parts of the code can also know whether
+	 *                                  there's been an update or not. This way we can add the current version to
+	 *                                  espresso_db_update, but still know if this is a new install or not
+	 * @return int one of the constants on EE_System::req_type_
+	 */
+	public function detect_req_type($espresso_db_update = null)
+	{
+		if ($this->_req_type === null) {
+			$espresso_db_update = ! empty($espresso_db_update)
+				? $espresso_db_update
+				: $this->fix_espresso_db_upgrade_option();
+			$this->_req_type = EE_System::detect_req_type_given_activation_history(
+				$espresso_db_update,
+				'ee_espresso_activation',
+				espresso_version()
+			);
+			$this->_major_version_change = $this->_detect_major_version_change($espresso_db_update);
+			$this->request->setIsActivation($this->_req_type !== EE_System::req_type_normal);
+		}
+		return $this->_req_type;
+	}
+
+
+	/**
+	 * Returns whether or not there was a non-micro version change (ie, change in either
+	 * the first or second number in the version. Eg 4.9.0.rc.001 to 4.10.0.rc.000,
+	 * but not 4.9.0.rc.0001 to 4.9.1.rc.0001
+	 *
+	 * @param $activation_history
+	 * @return bool
+	 */
+	private function _detect_major_version_change($activation_history)
+	{
+		$previous_version = EE_System::_get_most_recently_active_version_from_activation_history($activation_history);
+		$previous_version_parts = explode('.', $previous_version);
+		$current_version_parts = explode('.', espresso_version());
+		return isset($previous_version_parts[0], $previous_version_parts[1], $current_version_parts[0], $current_version_parts[1])
+			   && ($previous_version_parts[0] !== $current_version_parts[0]
+				   || $previous_version_parts[1] !== $current_version_parts[1]
+			   );
+	}
+
+
+	/**
+	 * Returns true if either the major or minor version of EE changed during this request.
+	 * Eg 4.9.0.rc.001 to 4.10.0.rc.000, but not 4.9.0.rc.0001 to 4.9.1.rc.0001
+	 *
+	 * @return bool
+	 */
+	public function is_major_version_change()
+	{
+		return $this->_major_version_change;
+	}
+
+
+	/**
+	 * Determines the request type for any ee addon, given three piece of info: the current array of activation
+	 * histories (for core that' 'espresso_db_update' wp option); the name of the WordPress option which is temporarily
+	 * set upon activation of the plugin (for core it's 'ee_espresso_activation'); and the version that this plugin was
+	 * just activated to (for core that will always be espresso_version())
+	 *
+	 * @param array  $activation_history_for_addon     the option's value which stores the activation history for this
+	 *                                                 ee plugin. for core that's 'espresso_db_update'
+	 * @param string $activation_indicator_option_name the name of the WordPress option that is temporarily set to
+	 *                                                 indicate that this plugin was just activated
+	 * @param string $version_to_upgrade_to            the version that was just upgraded to (for core that will be
+	 *                                                 espresso_version())
+	 * @return int one of the constants on EE_System::req_type_*
+	 */
+	public static function detect_req_type_given_activation_history(
+		$activation_history_for_addon,
+		$activation_indicator_option_name,
+		$version_to_upgrade_to
+	) {
+		$version_is_higher = self::_new_version_is_higher($activation_history_for_addon, $version_to_upgrade_to);
+		if ($activation_history_for_addon) {
+			// it exists, so this isn't a completely new install
+			// check if this version already in that list of previously installed versions
+			if (! isset($activation_history_for_addon[ $version_to_upgrade_to ])) {
+				// it a version we haven't seen before
+				if ($version_is_higher === 1) {
+					$req_type = EE_System::req_type_upgrade;
+				} else {
+					$req_type = EE_System::req_type_downgrade;
+				}
+				delete_option($activation_indicator_option_name);
+			} else {
+				// its not an update. maybe a reactivation?
+				if (get_option($activation_indicator_option_name, false)) {
+					if ($version_is_higher === -1) {
+						$req_type = EE_System::req_type_downgrade;
+					} elseif ($version_is_higher === 0) {
+						// we've seen this version before, but it's an activation. must be a reactivation
+						$req_type = EE_System::req_type_reactivation;
+					} else {// $version_is_higher === 1
+						$req_type = EE_System::req_type_upgrade;
+					}
+					delete_option($activation_indicator_option_name);
+				} else {
+					// we've seen this version before and the activation indicate doesn't show it was just activated
+					if ($version_is_higher === -1) {
+						$req_type = EE_System::req_type_downgrade;
+					} elseif ($version_is_higher === 0) {
+						// we've seen this version before and it's not an activation. its normal request
+						$req_type = EE_System::req_type_normal;
+					} else {// $version_is_higher === 1
+						$req_type = EE_System::req_type_upgrade;
+					}
+				}
+			}
+		} else {
+			// brand new install
+			$req_type = EE_System::req_type_new_activation;
+			delete_option($activation_indicator_option_name);
+		}
+		return $req_type;
+	}
+
+
+	/**
+	 * Detects if the $version_to_upgrade_to is higher than the most recent version in
+	 * the $activation_history_for_addon
+	 *
+	 * @param array  $activation_history_for_addon (keys are versions, values are arrays of times activated,
+	 *                                             sometimes containing 'unknown-date'
+	 * @param string $version_to_upgrade_to        (current version)
+	 * @return int results of version_compare( $version_to_upgrade_to, $most_recently_active_version ).
+	 *                                             ie, -1 if $version_to_upgrade_to is LOWER (downgrade);
+	 *                                             0 if $version_to_upgrade_to MATCHES (reactivation or normal request);
+	 *                                             1 if $version_to_upgrade_to is HIGHER (upgrade) ;
+	 */
+	private static function _new_version_is_higher($activation_history_for_addon, $version_to_upgrade_to)
+	{
+		// find the most recently-activated version
+		$most_recently_active_version =
+			EE_System::_get_most_recently_active_version_from_activation_history($activation_history_for_addon);
+		return version_compare($version_to_upgrade_to, $most_recently_active_version);
+	}
+
+
+	/**
+	 * Gets the most recently active version listed in the activation history,
+	 * and if none are found (ie, it's a brand new install) returns '0.0.0.dev.000'.
+	 *
+	 * @param array $activation_history  (keys are versions, values are arrays of times activated,
+	 *                                   sometimes containing 'unknown-date'
+	 * @return string
+	 */
+	private static function _get_most_recently_active_version_from_activation_history($activation_history)
+	{
+		$most_recently_active_version_activation = '1970-01-01 00:00:00';
+		$most_recently_active_version = '0.0.0.dev.000';
+		if (is_array($activation_history)) {
+			foreach ($activation_history as $version => $times_activated) {
+				// check there is a record of when this version was activated. Otherwise,
+				// mark it as unknown
+				if (! $times_activated) {
+					$times_activated = array('unknown-date');
+				}
+				if (is_string($times_activated)) {
+					$times_activated = array($times_activated);
+				}
+				foreach ($times_activated as $an_activation) {
+					if ($an_activation !== 'unknown-date'
+						&& $an_activation
+						   > $most_recently_active_version_activation) {
+						$most_recently_active_version = $version;
+						$most_recently_active_version_activation = $an_activation === 'unknown-date'
+							? '1970-01-01 00:00:00'
+							: $an_activation;
+					}
+				}
+			}
+		}
+		return $most_recently_active_version;
+	}
+
+
+	/**
+	 * This redirects to the about EE page after activation
+	 *
+	 * @return void
+	 */
+	public function redirect_to_about_ee()
+	{
+		$notices = EE_Error::get_notices(false);
+		// if current user is an admin and it's not an ajax or rest request
+		if (! isset($notices['errors'])
+			&& $this->request->isAdmin()
+			&& apply_filters(
+				'FHEE__EE_System__redirect_to_about_ee__do_redirect',
+				$this->capabilities->current_user_can('manage_options', 'espresso_about_default')
+			)
+		) {
+			$query_params = array('page' => 'espresso_about');
+			if (EE_System::instance()->detect_req_type() === EE_System::req_type_new_activation) {
+				$query_params['new_activation'] = true;
+			}
+			if (EE_System::instance()->detect_req_type() === EE_System::req_type_reactivation) {
+				$query_params['reactivation'] = true;
+			}
+			$url = add_query_arg($query_params, admin_url('admin.php'));
+			wp_safe_redirect($url);
+			exit();
+		}
+	}
+
+
+	/**
+	 * load_core_configuration
+	 * this is hooked into 'AHEE__EE_Bootstrap__load_core_configuration'
+	 * which runs during the WP 'plugins_loaded' action at priority 5
+	 *
+	 * @return void
+	 * @throws ReflectionException
+	 * @throws Exception
+	 */
+	public function load_core_configuration()
+	{
+		do_action('AHEE__EE_System__load_core_configuration__begin', $this);
+		$this->loader->getShared('EE_Load_Textdomain');
+		// load textdomain
+		EE_Load_Textdomain::load_textdomain();
+		// load and setup EE_Config and EE_Network_Config
+		$config = $this->loader->getShared('EE_Config');
+		$this->loader->getShared('EE_Network_Config');
+		// setup autoloaders
+		// enable logging?
+		if ($config->admin->use_full_logging) {
+			$this->loader->getShared('EE_Log');
+		}
+		// check for activation errors
+		$activation_errors = get_option('ee_plugin_activation_errors', false);
+		if ($activation_errors) {
+			EE_Error::add_error($activation_errors, __FILE__, __FUNCTION__, __LINE__);
+			update_option('ee_plugin_activation_errors', false);
+		}
+		// get model names
+		$this->_parse_model_names();
+		// load caf stuff a chance to play during the activation process too.
+		$this->_maybe_brew_regular();
+		// configure custom post type definitions
+		$this->loader->getShared('EventEspresso\core\domain\entities\custom_post_types\CustomTaxonomyDefinitions');
+		$this->loader->getShared('EventEspresso\core\domain\entities\custom_post_types\CustomPostTypeDefinitions');
+		do_action('AHEE__EE_System__load_core_configuration__complete', $this);
+	}
+
+
+	/**
+	 * cycles through all of the models/*.model.php files, and assembles an array of model names
+	 *
+	 * @return void
+	 * @throws ReflectionException
+	 */
+	private function _parse_model_names()
+	{
+		// get all the files in the EE_MODELS folder that end in .model.php
+		$models = glob(EE_MODELS . '*.model.php');
+		$model_names = array();
+		$non_abstract_db_models = array();
+		foreach ($models as $model) {
+			// get model classname
+			$classname = EEH_File::get_classname_from_filepath_with_standard_filename($model);
+			$short_name = str_replace('EEM_', '', $classname);
+			$reflectionClass = new ReflectionClass($classname);
+			if ($reflectionClass->isSubclassOf('EEM_Base') && ! $reflectionClass->isAbstract()) {
+				$non_abstract_db_models[ $short_name ] = $classname;
+			}
+			$model_names[ $short_name ] = $classname;
+		}
+		$this->registry->models = apply_filters('FHEE__EE_System__parse_model_names', $model_names);
+		$this->registry->non_abstract_db_models = apply_filters(
+			'FHEE__EE_System__parse_implemented_model_names',
+			$non_abstract_db_models
+		);
+	}
+
+
+	/**
+	 * The purpose of this method is to simply check for a file named "caffeinated/brewing_regular.php" for any hooks
+	 * that need to be setup before our EE_System launches.
+	 *
+	 * @return void
+	 * @throws DomainException
+	 * @throws InvalidArgumentException
+	 * @throws InvalidDataTypeException
+	 * @throws InvalidInterfaceException
+	 * @throws InvalidClassException
+	 * @throws InvalidFilePathException
+	 */
+	private function _maybe_brew_regular()
+	{
+		/** @var Domain $domain */
+		$domain = DomainFactory::getShared(
+			new FullyQualifiedName(
+				'EventEspresso\core\domain\Domain'
+			),
+			array(
+				new FilePath(EVENT_ESPRESSO_MAIN_FILE),
+				Version::fromString(espresso_version()),
+			)
+		);
+		if ($domain->isCaffeinated()) {
+			require_once EE_CAFF_PATH . 'brewing_regular.php';
+		}
+	}
+
+
+	/**
+	 * @since 4.9.71.p
+	 * @throws Exception
+	 */
+	public function loadRouteMatchSpecifications()
+	{
+		try {
+			$this->loader->getShared(
+				'EventEspresso\core\services\route_match\RouteMatchSpecificationManager'
+			);
+		} catch (Exception $exception) {
+			new ExceptionStackTraceDisplay($exception);
+		}
+		do_action('AHEE__EE_System__loadRouteMatchSpecifications');
+	}
+
+
+	/**
+	 * register_shortcodes_modules_and_widgets
+	 * generate lists of shortcodes and modules, then verify paths and classes
+	 * This is hooked into 'AHEE__EE_Bootstrap__register_shortcodes_modules_and_widgets'
+	 * which runs during the WP 'plugins_loaded' action at priority 7
+	 *
+	 * @access public
+	 * @return void
+	 * @throws Exception
+	 */
+	public function register_shortcodes_modules_and_widgets()
+	{
+		if ($this->request->isFrontend() || $this->request->isIframe() || $this->request->isAjax()) {
+			try {
+				// load, register, and add shortcodes the new way
+				$this->loader->getShared(
+					'EventEspresso\core\services\shortcodes\ShortcodesManager',
+					array(
+						// and the old way, but we'll put it under control of the new system
+						EE_Config::getLegacyShortcodesManager(),
+					)
+				);
+			} catch (Exception $exception) {
+				new ExceptionStackTraceDisplay($exception);
+			}
+		}
+		do_action('AHEE__EE_System__register_shortcodes_modules_and_widgets');
+		// check for addons using old hook point
+		if (has_action('AHEE__EE_System__register_shortcodes_modules_and_addons')) {
+			$this->_incompatible_addon_error();
+		}
+	}
+
+
+	/**
+	 * _incompatible_addon_error
+	 *
+	 * @access public
+	 * @return void
+	 */
+	private function _incompatible_addon_error()
+	{
+		// get array of classes hooking into here
+		$class_names = EEH_Class_Tools::get_class_names_for_all_callbacks_on_hook(
+			'AHEE__EE_System__register_shortcodes_modules_and_addons'
+		);
+		if (! empty($class_names)) {
+			$msg = __(
+				'The following plugins, addons, or modules appear to be incompatible with this version of Event Espresso and were automatically deactivated to avoid fatal errors:',
+				'event_espresso'
+			);
+			$msg .= '<ul>';
+			foreach ($class_names as $class_name) {
+				$msg .= '<li><b>Event Espresso - '
+						. str_replace(
+							array('EE_', 'EEM_', 'EED_', 'EES_', 'EEW_'),
+							'',
+							$class_name
+						) . '</b></li>';
+			}
+			$msg .= '</ul>';
+			$msg .= __(
+				'Compatibility issues can be avoided and/or resolved by keeping addons and plugins updated to the latest version.',
+				'event_espresso'
+			);
+			// save list of incompatible addons to wp-options for later use
+			add_option('ee_incompatible_addons', $class_names, '', 'no');
+			if (is_admin()) {
+				EE_Error::add_error($msg, __FILE__, __FUNCTION__, __LINE__);
+			}
+		}
+	}
+
+
+	/**
+	 * brew_espresso
+	 * begins the process of setting hooks for initializing EE in the correct order
+	 * This is happening on the 'AHEE__EE_Bootstrap__brew_espresso' hook point
+	 * which runs during the WP 'plugins_loaded' action at priority 9
+	 *
+	 * @return void
+	 */
+	public function brew_espresso()
+	{
+		do_action('AHEE__EE_System__brew_espresso__begin', $this);
+		// load some final core systems
+		add_action('init', array($this, 'set_hooks_for_core'), 1);
+		add_action('init', array($this, 'perform_activations_upgrades_and_migrations'), 3);
+		add_action('init', array($this, 'load_CPTs_and_session'), 5);
+		add_action('init', array($this, 'load_controllers'), 7);
+		add_action('init', array($this, 'core_loaded_and_ready'), 9);
+		add_action('init', array($this, 'initialize'), 10);
+		add_action('init', array($this, 'initialize_last'), 100);
+		if (is_admin() && apply_filters('FHEE__EE_System__brew_espresso__load_pue', true)) {
+			// pew pew pew
+			$this->loader->getShared('EventEspresso\core\services\licensing\LicenseService');
+			do_action('AHEE__EE_System__brew_espresso__after_pue_init');
+		}
+		do_action('AHEE__EE_System__brew_espresso__complete', $this);
+	}
+
+
+	/**
+	 *    set_hooks_for_core
+	 *
+	 * @access public
+	 * @return    void
+	 * @throws EE_Error
+	 */
+	public function set_hooks_for_core()
+	{
+		$this->_deactivate_incompatible_addons();
+		do_action('AHEE__EE_System__set_hooks_for_core');
+		$this->loader->getShared('EventEspresso\core\domain\values\session\SessionLifespan');
+		// caps need to be initialized on every request so that capability maps are set.
+		// @see https://events.codebasehq.com/projects/event-espresso/tickets/8674
+		$this->registry->CAP->init_caps();
+	}
+
+
+	/**
+	 * Using the information gathered in EE_System::_incompatible_addon_error,
+	 * deactivates any addons considered incompatible with the current version of EE
+	 */
+	private function _deactivate_incompatible_addons()
+	{
+		$incompatible_addons = get_option('ee_incompatible_addons', array());
+		if (! empty($incompatible_addons)) {
+			$active_plugins = get_option('active_plugins', array());
+			foreach ($active_plugins as $active_plugin) {
+				foreach ($incompatible_addons as $incompatible_addon) {
+					if (strpos($active_plugin, $incompatible_addon) !== false) {
+						unset($_GET['activate']);
+						espresso_deactivate_plugin($active_plugin);
+					}
+				}
+			}
+		}
+	}
+
+
+	/**
+	 *    perform_activations_upgrades_and_migrations
+	 *
+	 * @access public
+	 * @return    void
+	 */
+	public function perform_activations_upgrades_and_migrations()
+	{
+		do_action('AHEE__EE_System__perform_activations_upgrades_and_migrations');
+	}
+
+
+	/**
+	 * @return void
+	 * @throws DomainException
+	 */
+	public function load_CPTs_and_session()
+	{
+		do_action('AHEE__EE_System__load_CPTs_and_session__start');
+		/** @var EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomies $register_custom_taxonomies */
+		$register_custom_taxonomies = $this->loader->getShared(
+			'EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomies'
+		);
+		$register_custom_taxonomies->registerCustomTaxonomies();
+		/** @var EventEspresso\core\domain\services\custom_post_types\RegisterCustomPostTypes $register_custom_post_types */
+		$register_custom_post_types = $this->loader->getShared(
+			'EventEspresso\core\domain\services\custom_post_types\RegisterCustomPostTypes'
+		);
+		$register_custom_post_types->registerCustomPostTypes();
+		/** @var EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomyTerms $register_custom_taxonomy_terms */
+		$register_custom_taxonomy_terms = $this->loader->getShared(
+			'EventEspresso\core\domain\services\custom_post_types\RegisterCustomTaxonomyTerms'
+		);
+		$register_custom_taxonomy_terms->registerCustomTaxonomyTerms();
+		// load legacy Custom Post Types and Taxonomies
+		$this->loader->getShared('EE_Register_CPTs');
+		do_action('AHEE__EE_System__load_CPTs_and_session__complete');
+	}
+
+
+	/**
+	 * load_controllers
+	 * this is the best place to load any additional controllers that needs access to EE core.
+	 * it is expected that all basic core EE systems, that are not dependant on the current request are loaded at this
+	 * time
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public function load_controllers()
+	{
+		do_action('AHEE__EE_System__load_controllers__start');
+		// let's get it started
+		if (! $this->maintenance_mode->level()
+			&& ($this->request->isFrontend() || $this->request->isFrontAjax())
+		) {
+			do_action('AHEE__EE_System__load_controllers__load_front_controllers');
+			$this->loader->getShared('EE_Front_Controller');
+		} elseif ($this->request->isAdmin() || $this->request->isAdminAjax()) {
+			do_action('AHEE__EE_System__load_controllers__load_admin_controllers');
+			$this->loader->getShared('EE_Admin');
+		} elseif ($this->request->isWordPressHeartbeat()) {
+			$this->loader->getShared('EventEspresso\core\domain\services\admin\ajax\WordpressHeartbeat');
+		}
+		do_action('AHEE__EE_System__load_controllers__complete');
+	}
+
+
+	/**
+	 * core_loaded_and_ready
+	 * all of the basic EE core should be loaded at this point and available regardless of M-Mode
+	 *
+	 * @access public
+	 * @return void
+	 * @throws Exception
+	 */
+	public function core_loaded_and_ready()
+	{
+		if ($this->request->isAdmin()
+			|| $this->request->isFrontend()
+			|| $this->request->isIframe()
+			|| $this->request->isWordPressApi()
+		) {
+			try {
+				$this->loader->getShared('EventEspresso\core\services\assets\Registry');
+				$this->loader->getShared('EventEspresso\core\domain\services\assets\CoreAssetManager');
+				if ($this->canLoadBlocks()) {
+					$this->loader->getShared(
+						'EventEspresso\core\services\editor\BlockRegistrationManager'
+					);
+				}
+			} catch (Exception $exception) {
+				new ExceptionStackTraceDisplay($exception);
+			}
+		}
+		if ($this->request->isAdmin()
+			|| $this->request->isEeAjax()
+			|| $this->request->isFrontend()
+		) {
+			$this->loader->getShared('EE_Session');
+		}
+		// integrate WP_Query with the EE models
+		$this->loader->getShared('EE_CPT_Strategy');
+		do_action('AHEE__EE_System__core_loaded_and_ready');
+		// always load template tags, because it's faster than checking if it's a front-end request, and many page
+		// builders require these even on the front-end
+		require_once EE_PUBLIC . 'template_tags.php';
+		do_action('AHEE__EE_System__set_hooks_for_shortcodes_modules_and_addons');
+	}
+
+
+	/**
+	 * initialize
+	 * this is the best place to begin initializing client code
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public function initialize()
+	{
+		do_action('AHEE__EE_System__initialize');
+	}
+
+
+	/**
+	 * initialize_last
+	 * this is run really late during the WP init hook point, and ensures that mostly everything else that needs to
+	 * initialize has done so
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public function initialize_last()
+	{
+		do_action('AHEE__EE_System__initialize_last');
+		/** @var EventEspresso\core\domain\services\custom_post_types\RewriteRules $rewrite_rules */
+		$rewrite_rules = $this->loader->getShared(
+			'EventEspresso\core\domain\services\custom_post_types\RewriteRules'
+		);
+		$rewrite_rules->flushRewriteRules();
+		add_action('admin_bar_init', array($this, 'addEspressoToolbar'));
+		if (($this->request->isAjax() || $this->request->isAdmin())
+			&& $this->maintenance_mode->models_can_query()) {
+			$this->loader->getShared('EventEspresso\core\services\privacy\export\PersonalDataExporterManager');
+			$this->loader->getShared('EventEspresso\core\services\privacy\erasure\PersonalDataEraserManager');
+		}
+	}
+
+
+	/**
+	 * @return void
+	 * @throws EE_Error
+	 */
+	public function addEspressoToolbar()
+	{
+		$this->loader->getShared(
+			'EventEspresso\core\domain\services\admin\AdminToolBar',
+			array($this->registry->CAP)
+		);
+	}
+
+
+	/**
+	 * do_not_cache
+	 * sets no cache headers and defines no cache constants for WP plugins
+	 *
+	 * @access public
+	 * @return void
+	 */
+	public static function do_not_cache()
+	{
+		// set no cache constants
+		if (! defined('DONOTCACHEPAGE')) {
+			define('DONOTCACHEPAGE', true);
+		}
+		if (! defined('DONOTCACHCEOBJECT')) {
+			define('DONOTCACHCEOBJECT', true);
+		}
+		if (! defined('DONOTCACHEDB')) {
+			define('DONOTCACHEDB', true);
+		}
+		// add no cache headers
+		add_action('send_headers', array('EE_System', 'nocache_headers'), 10);
+		// plus a little extra for nginx and Google Chrome
+		add_filter('nocache_headers', array('EE_System', 'extra_nocache_headers'), 10, 1);
+		// prevent browsers from prefetching of the rel='next' link, because it may contain content that interferes with the registration process
+		remove_action('wp_head', 'adjacent_posts_rel_link_wp_head');
+	}
+
+
+	/**
+	 *    extra_nocache_headers
+	 *
+	 * @access    public
+	 * @param $headers
+	 * @return    array
+	 */
+	public static function extra_nocache_headers($headers)
+	{
+		// for NGINX
+		$headers['X-Accel-Expires'] = 0;
+		// plus extra for Google Chrome since it doesn't seem to respect "no-cache", but WILL respect "no-store"
+		$headers['Cache-Control'] = 'no-store, no-cache, must-revalidate, max-age=0';
+		return $headers;
+	}
+
+
+	/**
+	 *    nocache_headers
+	 *
+	 * @access    public
+	 * @return    void
+	 */
+	public static function nocache_headers()
+	{
+		nocache_headers();
+	}
+
+
+	/**
+	 * simply hooks into "wp_list_pages_exclude" filter (for wp_list_pages method) and makes sure EE critical pages are
+	 * never returned with the function.
+	 *
+	 * @param  array $exclude_array any existing pages being excluded are in this array.
+	 * @return array
+	 */
+	public function remove_pages_from_wp_list_pages($exclude_array)
+	{
+		return array_merge($exclude_array, $this->registry->CFG->core->get_critical_pages_array());
+	}
+
+
+	/**
+	 * Return whether blocks can be registered/loaded or not.
+	 * @return bool
+	 */
+	private function canLoadBlocks()
+	{
+		return apply_filters('FHEE__EE_System__canLoadBlocks', true)
+			   && function_exists('register_block_type')
+			   // don't load blocks if in the Divi page builder editor context
+			   // @see https://github.com/eventespresso/event-espresso-core/issues/814
+			   && ! $this->request->getRequestParam('et_fb', false);
+	}
 }