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