Merge pull request #2 from /pull/8762

Imported magento devdocs 8762

Author: dobooth

src/guides/v2.4/config-guide/bootstrap/magento-bootstrap.md

@@ -0,0 +1,118 @@
+---
+group: configuration-guide
+title: Magento application initialization and bootstrap
+functional_areas:
+  - Configuration
+  - System
+  - Setup
+---
+
+## Overview of bootstrapping {#config-boot-overview}
+
+To run the Magento application, the following actions are implemented in [pub/index.php]({{ site.mage2bloburl }}/{{ page.guide_version }}/pub/index.php){:target="_blank"}:
+
+*  Include [app/bootstrap.php]({{ site.mage2bloburl }}/{{ page.guide_version }}/app/bootstrap.php){:target="_blank"} which performs essential initialization routines, such as error handling, initializing the autoloader, setting profiling options, setting the default timezone, and so on.
+*  Create an instance of [\Magento\Framework\App\Bootstrap]({{ site.mage2bloburl }}/{{ page.guide_version }}/lib/internal/Magento/Framework/App/Bootstrap.php){:target="_blank"}. <!-- It requires initialization parameters to be specified in constructor.  Normally, the $_SERVER super-global variable is supposed to be passed there. -->
+*  Create a Magento application instance ([\Magento\Framework\AppInterface]({{ site.mage2bloburl }}/{{ page.guide_version }}/lib/internal/Magento/Framework/AppInterface.php){:target="_blank"}).
+*  Run Magento
+
+## Bootstrap run logic {#config-boot-logic}
+
+[The bootstrap object]({{ site.mage2bloburl }}/{{ page.guide_version }}/app/bootstrap.php){:target="_blank"} uses the following algorithm to run the Magento application:
+
+1. Initializes the error handler.
+1. Creates the [object manager]({{ site.mage2bloburl }}/{{ page.guide_version }}/lib/internal/Magento/Framework/ObjectManager){:target="_blank"} and basic shared services that are used everywhere and are affected by the environment. The environment parameters are injected properly into these objects.
+1. Asserts that maintenance mode is *not* enabled; otherwise, terminates.
+1. Asserts that the Magento application is installed; otherwise, terminates.
+1. Starts the Magento application.
+
+   Any uncaught exception during application launch is automatically passed back to Magento in the `catchException()` method which you can use to handle the exception. The latter must return either `true` or `false`:
+
+   *  If `true`: Magento handled exception successfully. No need to do anything else.
+   *  If `false` (or any other empty result): Magento did not handle the exception. The bootstrap object performs the default exception handling subroutine.
+
+1. Sends the response provided by the application object.
+
+ {:.bs-callout-info}
+The assertions that the Magento application is installed and not in maintenance mode is the default behavior of the `\Magento\Framework\App\Bootstrap` class. You can modify it using an entry point script when creating the bootstrap object.
+
+Sample entry point script that modifies the bootstrap object:
+
+```php
+<?php
+use Magento\Framework\App\Bootstrap;
+require __DIR__ . '/app/bootstrap.php';
+
+$params = $_SERVER;
+$params[Bootstrap::PARAM_REQUIRE_MAINTENANCE] = true; // default false
+$params[Bootstrap::PARAM_REQUIRE_IS_INSTALLED] = false; // default true
+$bootstrap = Bootstrap::create(BP, $params);
+
+/** @var \Magento\Framework\App\Http $app */
+$app = $bootstrap->createApplication('Magento\Framework\App\Http');
+$bootstrap->run($app);
+```
+
+## Default exception handling {#config-boot-exception}
+
+The bootstrap object specifies how the Magento application handles uncaught exceptions as follows:
+
+*  In [developer mode]({{ page.baseurl }}/config-guide/bootstrap/magento-modes.html#developer-mode), displays the [exception](https://glossary.magento.com/exception) as-is.
+*  In any other mode, attempts to log exception and display a generic error message.
+*  Terminates Magento with error code `1`
+
+## Entry point applications {#config-boot-entry}
+
+We have the following entry point applications (that is, applications defined by Magento that are used by the web server as a directory index):
+
+*  [HTTP entry point](#config-boot-entry-http)
+*  [Static resource entry point](#config-boot-entry-static)
+*  [Media resource entry point](#config-boot-entry-media)
+
+### HTTP entry point {#config-boot-entry-http}
+
+[\Magento\Framework\App\Http]({{ site.mage2bloburl }}/{{ page.guide_version }}/lib/internal/Magento/Framework/App/Http){:target="_blank"} operates as follows:
+
+1. Determines the [application area]({{ page.baseurl }}/architecture/archi_perspectives/components/modules/mod_and_areas.html).
+1. Starts the front controller and routing systems in order to find and execute a controller action.
+1. Uses an HTTP response object to return result obtained from the controller action.
+1. Error handling (in the following priority order):
+
+   1. If you're using [developer mode]({{ page.baseurl }}/config-guide/bootstrap/magento-modes.html#developer-mode):
+     *  If the Magento application is not installed, redirect to Setup Wizard.
+     *  If the Magento application is installed, display an error and HTTP status code 500 (Internal Server Error).
+   1. If the Magento application is in maintenance mode, display a user-friendly "Service Unavailable" landing page with HTTP status code 503 (Service Temporary Unavailable).
+   1. If the Magento application is *not* installed, redirect to Setup Wizard.
+   1. If the session is invalid, redirect to the home page.
+   1. If there is any other application initialization error, display a user-friendly "Page Not Found" page with HTTP status code 404 (Not Found).
+   1. On any other error, display a user-friendly "Service Unavailable" page with HTTP response 503 and generate an error report and display its ID on the page.
+
+### Static resource entry point {#config-boot-entry-static}
+
+[\Magento\Framework\App\StaticResource]({{ site.mage2bloburl }}/{{ page.guide_version }}/lib/internal/Magento/Framework/App/StaticResource.php){:target="_blank"} is an application for retrieving static resources (for example, CSS, JavaScript, and images). It postpones any actions with a static resource until the resource is requested.
+
+ {:.bs-callout-info}
+The entry point for static view files is not used in [production mode]({{ page.baseurl }}/config-guide/bootstrap/magento-modes.html#production-mode) to avoid potential exploits on the server. In production mode, the Magento application expects that all necessary resources already exist in the `<your Magento install dir>/pub/static` directory.
+
+In default or developer mode, a request for a non-existent static resource is redirected to the static entry point according to the rewrite rules specified by the appropriate `.htaccess`.
+When the request is redirected to the entry point, the Magento application parses the requested [URL](https://glossary.magento.com/url) based on retrieved parameters and finds the requested resource.
+
+*  In developer mode, the content of the file is returned so that every time the resource is requested, the returned content is up to date.
+*  In [default]({{ page.baseurl }}/config-guide/bootstrap/magento-modes.html#default-mode) mode, the retrieved resource is published so it is accessible by the previously requested URL.
+
+   All future requests for the static resource are processed by the server the same as static files; that is, without involving the entry point. If it's necessary to synchronize published files with original ones, the `pub/static` directory should be removed; as a result, files are automatically republished with the next request.
+
+### Media resource entry point {#config-boot-entry-media}
+
+[Magento\MediaStorage\App\Media]({{ site.mage2bloburl }}/{{ page.guide_version }}/app/code/Magento/MediaStorage/App/Media.php){:target="_blank"} retrieves media resources (that is, any files uploaded to media storage) from the database. It is used whenever the database is configured as a [media storage](https://glossary.magento.com/media-storage).
+
+`\Magento\Core\App\Media` attempts to find the media file in the configured database storage and write it into the `pub/static` directory, then return its contents. On error, it returns an HTTP 404 (Not Found) status code in the header with no contents.
+
+{:.ref-header}
+Related topics
+
+This topic discussed the basics of Magento application initialization and bootstrapping. To find out how to set bootstrap environment variables, see one of the following topics:
+
+*  [Customize base directory paths (MAGE_DIRS)]({{ page.baseurl }}/config-guide/bootstrap/mage-dirs.html)
+*  [Set the mode (MAGE_MODE)]({{ page.baseurl }}/config-guide/bootstrap/magento-modes.html)
+*  [Enable an HTML profiler (MAGE_PROFILER)]({{ page.baseurl }}/config-guide/bootstrap/mage-profiler.html)