yii2/docs/guide/runtime-routing.md

Routing

When the yii\web\Application::run() method is called by the entry script, the first thing it does is to resolve the incoming request and instantiate an appropriate controller action to handle the request. This process is called routing.

Resolving Route

The first step of routing is to parse the incoming request into a route which, as described in the Controllers section, is used to address a controller action. This is done by yii\web\Request::resolve() method of the request application component. The method invokes the URL manager to do the actual request parsing work.

By default, if the incoming request contains a GET parameter named r, its value will be considered as the route. However, if the yii\web\UrlManager::enablePrettyUrl is enabled, more work will be done to determine the requested route. For more details, please refer to the URL Parsing and Generation section.

In case a route cannot be determined, the request component will throw a yii\web\NotFoundHttpException.

Default Route

If an incoming request does not specify a route, which often happens to the request for homepages, the route specified by yii\web\Application::defaultRoute will be used. The default value of this property is site/index, which refers to the index action of the site controller. You may customize this property in the application configuration like the following:

return [
    // ...
    'defaultRoute' => 'main/index',
];

catchAll Route

Sometimes, you may want to put your Web application in maintenance mode temporarily and display the same informational page for all requests. There are many ways to accomplish this goal. But one of the simplest ways is to configure the yii\web\Application::catchAll property like the following in the application configuration:

return [
    // ...
    'catchAll' => ['site/offline'],
];

The catchAll property should take an array whose first element specifies a route, and the rest of the elements (name-value pairs) specify the parameters to be bound to the action.

When the catchAll property is set, it will replace any route resolved from the incoming requests. With the above configuration, the same site/offline action will be used to handle all incoming requests.

Creating Action

Once the requested route is determined, the next step is to create the action object corresponding to the route.

The route is broken down into multiple parts by the slashes in it. For example, site/index will be broken into site and index. Each part is an ID which may refer to a module, a controller or an action.

Starting from the first part in the route, the application conducts the following steps to create modules (if any), the controller and the action:

1. Set the application as the current module.
2. Check if the yii\base\Module::controllerMap of the current module contains the current ID. If so, a controller object will be created according to the controller configuration found in the map, and do Step 5 with the rest parts of the route.
3. Check if the ID refers to a module listed in the yii\base\Module::modules property of the current module. If so, a module is created according to the configuration found in the module list, and do Step 2 with the next part in the route under the context of the newly created module.
4. Treat the ID as a controller ID and create a controller object. Do the next step with the rest part of the route.
5. The controller looks for the current ID in its yii\base\Controller::actions(). If found, it creates an action according to the configuration found in the map. Otherwise, the controller will attempt to create an inline action which is defined by an action method corresponding to the current ID.

Among the above steps, if any error occurs, a yii\web\NotFoundHttpException will be thrown, indicating failure of the routing.