# Cache guide

## Cache providers

To use cache in your application, you must first define a cache provider. Each cache provider connects your app with a different cache mechanism or server, that means your app can make use of different caches at the same time.

Cache providers are configured in the [Cache configuration](https://cherrycake.tin.cat/reference/core-modules/cache) file `/config/Cache.config.php`. For example, if you want to use a simple but fast APCu cache provider, your Cache configuration file would look like this:

```php
<?php

namespace Cherrycake;

$CacheConfig = [
    "providers" => [
        "fast" => [
            "providerClassName" => "CacheProviderApcu"
        ]
    ]
];
```

> Note we called our cache provider `fast`&#x20;

You can configure more than one cache providers. For example, let's add a Redis cache provider too, called `huge`:

```php
<?php

namespace Cherrycake;

$CacheConfig = [
    "providers" => [
        "fast" => [
            "providerClassName" => "CacheProviderApcu"
        ],
        "huge" => [
            "providerClassName" => "CacheProviderRedis",
            "config" => [
                "scheme" => "tcp",
                "host" => "localhost"
                "port" => 6379,
                "database" => 0,
                "prefix" => "CherrycakeApp:"
            ]
        ]
    ]
];
```

> Since some functionalities of Cherrycake make use of caching mechanisms, there is a default cache provider called `engine` that uses an APCu cache provider. This provider is always defined, no matter what you setup in your `Cache.config.php` file.

## Modules that depend on Cache

Some Core modules make use of Cache by their own, like [Database](https://cherrycake.tin.cat/reference/core-modules/database) and [Patterns](https://cherrycake.tin.cat/reference/core-modules/patterns). Those modules always accept a configuration key to tell them the name of the cache provider to use, as defined in your `Cache.config.php`

## How long cached objects stay in cache?

The data you store in cache, as well as the objects cached by modules like [Database](https://cherrycake.tin.cat/reference/core-modules/database) and [Patterns](https://cherrycake.tin.cat/architecture/patterns), are normally persistent between requests, and it's the job of the cache mechanism to keep them there for as long as possible, or until their TTL expiration time arrives.

However, you can not rely on a cache or shared memory system as a persistent way of storing information. Generally, cached objects are deleted when the server restarts, when it runs out of memory, or if it implements certain cache eviction policies, like removing all cached objects that are too old, or that haven't been accessed too much.

> To store information in a persistent way, use [Database](https://cherrycake.tin.cat/reference/core-modules/database).

## Other ways of using shared memory

Some specific cache providers like Redis implement other useful ways of working with shared memory, like [Lists](https://cherrycake.tin.cat/guide/cache-guide/lists), [Queues](https://cherrycake.tin.cat/guide/cache-guide/queues) and [Pools](https://cherrycake.tin.cat/guide/cache-guide/pools).

This methods are specially suited for high performance operations like the storage of events in a high traffic scenario, or the intermediate storage of data that needs to be accessed extremely fast, lots of times per second.