# Application Configuration

You can customize Craft’s entire [Yii application configuration](https://www.yiiframework.com/doc/guide/2.0/en/structure-applications#application-configurations) from `config/app.php`. Any items returned by that array will get merged into the main application configuration array.

You can also customize Craft’s application configuration for only web requests or console requests from `config/app.web.php` and `config/app.console.php`.

::: tip
Craft’s default configuration is defined by [src/config/app.php](https://github.com/craftcms/cms/blob/master/src/config/app.php), [app.web.php](https://github.com/craftcms/cms/blob/master/src/config/app.web.php), and [app.console.php](https://github.com/craftcms/cms/blob/master/src/config/app.console.php). Refer to these files when you need to override existing application components.
:::

[[toc]]

## Cache Component

By default, Craft will store data caches in the `storage/runtime/cache/` folder. You can configure Craft to use alternative [cache storage](https://www.yiiframework.com/doc/guide/2.0/en/caching-data#supported-cache-storage) by overriding the `cache` application component from `config/app.php`.

```php
<?php
return [
    'components' => [
        'cache' => [
            'class' => yii\caching\ApcCache::class,
            'useApcu' => true,
        ],
    ],
];
```

#### Memcached Example

```php
<?php
return [
    'components' => [
        'cache' => [
            'class' => yii\caching\MemCache::class,
            'useMemcached' => true,
            'username' => getenv('MEMCACHED_USERNAME'),
            'password' => getenv('MEMCACHED_PASSWORD'),
            'defaultDuration' => 86400,
            'servers' => [
                [
                    'host' => 'localhost',
                    'persistent' => true,
                    'port' => 11211,
                    'retryInterval' => 15,
                    'status' => true,
                    'timeout' => 15,
                    'weight' => 1,
                ],
            ],
        ],
    ],
];
```

#### Redis Example

To use Redis cache storage, you will first need to install the [yii2-redis](https://github.com/yiisoft/yii2-redis) library. Then configure Craft’s `cache` component to use it:

```php
<?php
return [
    'components' => [
        'redis' => [
            'class' => yii\redis\Connection::class,
            'hostname' => 'localhost',
            'port' => 6379,
            'password' => getenv('REDIS_PASSWORD'),
        ],
        'cache' => [
            'class' => yii\redis\Cache::class,
            'defaultDuration' => 86400,
        ],
    ],
];
```

## Session Component

In a load-balanced environment, you may want to override the default `session` component to store PHP session data in a centralized location (e.g. Redis):

```php
<?php
return [
    'components' => [
        'redis' => [
            'class' => yii\redis\Connection::class,
            'hostname' => 'localhost',
            'port' => 6379,
            'password' => getenv('REDIS_PASSWORD'),
        ],
        'session' => [
            'class' => yii\redis\Session::class,
            'as session' => craft\behaviors\SessionBehavior::class,
        ],
    ],
];
```

::: tip
The `session` component **must** be configured with the <api:craft\behaviors\SessionBehavior> behavior, which adds methods to the component that the system relies on.
:::

## Mailer Component

To override the `mailer` component config (which is responsible for sending emails), do this in `config/app.php`:

```php
<?php
return [
    'components' => [
        'mailer' => function() {
            // Get the stored email settings
            $settings = Craft::$app->systemSettings->getEmailSettings();

            // Override the transport adapter class
            $settings->transportType = craft\mailgun\MailgunAdapter::class;

            // Override the transport adapter settings
            $settings->transportSettings = [
                'domain' => 'foo.com',
                'apiKey' => 'key-xxxxxxxxxx',
            ];

            // Create a Mailer component config with these settings
            $config = craft\helpers\App::mailerConfig($settings);

            // Instantiate and return it
            return Craft::createObject($config);
        },
    ],
];
```

::: tip
Any changes you make to the Mailer component from `config/app.php` will not be reflected when testing email settings from Settings → Email.
:::

## Queue Component

Craft’s job queue is powered by the [Yii2 Queue Extension](https://github.com/yiisoft/yii2-queue). By default Craft will use a [custom queue driver](craft\queue\Queue) based on the extension’s [DB driver](https://github.com/yiisoft/yii2-queue/blob/master/docs/guide/driver-db.md), but you can switch to a different driver by overriding Craft’s `queue` component from `config/app.php`:

```php
<?php
return [
    'components' => [
        'queue' => [
            'class' => yii\queue\redis\Queue::class,
            'redis' => 'redis', // Redis connection component or its config
            'channel' => 'queue', // Queue channel key
        ],
    ],
];
```

Available drivers are listed in the [Yii2 Queue Extension documentation](https://github.com/yiisoft/yii2-queue/tree/master/docs/guide).

::: warning
Only drivers that implement <api:craft\queue\QueueInterface> will be visible within the Control Panel.
:::

::: tip
If your queue driver supplies its own worker, set the <config:runQueueAutomatically> config setting to `false` in `config/general.php`.
:::

## Modules

You can register and bootstrap custom Yii modules into the application from `config/app.php` as well. See [How to Build a Module](../extend/module-guide.md) for more info.