added bootstapping tutorial

docs/guide/README.md

@@ -47,7 +47,8 @@ Application Structure
 Handling Requests
 -----------------
 
-* **TBD** [Bootstrapping](runtime-bootstrapping.md)
+* [Overview](runtime-overview.md)
+* [Bootstrapping](runtime-bootstrapping.md)
 * **TBD** [Routing](runtime-routing.md)
 * **TBD** [Requests](runtime-requests.md)
 * **TBD** [Responses](runtime-responses.md)

docs/guide/runtime-bootstrapping.md

+Bootstrapping
+=============
+
+Bootstrapping refers to the process of preparing the environment before an application starts
+to resolve and process an incoming request. Bootstrapping is done in two places:
+the [entry script](structure-entry-scripts.md) and the [application](structure-applications.md).
+
+In the [entry script](structure-entry-scripts.md), class autoloaders for different libraries are
+registered. This includes the Composer autoloader through its `autoload.php` file and the Yii
+autoloader through its `Yii` class file. The entry script then loads the application
+[configuration](concept-configurations.md) and creates an [application](structure-applications.md) instance.
+
+In the constructor of the application, the following bootstrapping work are done:
+
+1. [[yii\base\Application::preInit()|preInit()]] is called, which configures some high priority
+   application properties, such as [[yii\base\Application::basePath|basePath]].
+2. Register the [[yii\base\Application::errorHandler|error handler]].
+3. Initialize application properties using the given application configuration.
+4. [[yii\base\Application::init()|init()]] is called which in turn calls
+   [[yii\base\Application::bootstrap()|bootstrap()]] to run bootstrapping components.
+   - Include the extension manifest file `vendor/yiisoft/extensions.php`.
+   - Create and run [bootstrap components](structure-extensions.md#bootstrapping-classes)
+     declared by extensions.
+   - Create and run [application components](structure-application-components.md) and/or
+     [modules](structure-modules.md) that are declared in the application's
+     [bootstrap property](structure-applications.md#bootstrap).
+
+Because the bootstrapping work has to be done before handling *every* request, it is very important
+to keep this process light and optimize it as much as possible.
+
+Try not to register too many bootstrapping components. A bootstrapping component is needed only
+if it wants to participate the whole life cycle of requesting handling. For example, if a module
+needs to register additional URL parsing rules, it should be listed in the
+[bootstrap property](structure-applications.md#bootstrap) so that the new URL rules can take effect
+before they are used to resolve requests.
+
+In production mode, enable bytecode cache, such as APC, to minimize the time needed for including
+and parsing PHP files.
+
+Some large applications have very complex application [configurations](concept-configurations.md)
+which are divided into many smaller configuration files. If this is the case, consider caching
+the whole configuration array and loading it directly from cache before creating the application instance
+in the entry script.

docs/guide/runtime-overview.md

+Overview
+========
+
+Each time when a Yii application handles a request, it undergoes a similar workflow.
+
+1. A user makes a request to the [entry script](structure-entry-scripts.md) `web/index.php`.
+2. The entry script loads the application [configuration](concept-configurations.md) and creates
+   an [application](structure-applications.md) instance to handle the request.
+3. The application resolves the requested [route](runtime-routing.md) with the help of
+   the [request](runtime-requests.md) application component.
+4. The application creates a [controller](structure-controllers.md) instance to handle the request.
+5. The controller creates an [action](structure-controllers.md) instance and performs the filters for the action.
+6. If any filter fails, the action is cancelled.
+7. If all filters pass, the action is executed.
+8. The action loads a data model, possibly from a database.
+9. The action renders a view, providing it with the data model.
+10. The rendered result is returned to the [response](runtime-responses.md) application component.
+11. The response component sends the rendered result to the user's browser.
+
+The following diagram shows how an application handles a request.
+
+![Request Lifecycle](images/application-lifecycle.png)
+
+In this section, we will describe in detail how some of these steps work.