# Cache Component

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Async, [Promise](https://github.com/reactphp/promise)-based cache interface
for [ReactPHP](https://reactphp.org/).

The cache component provides a
[Promise](https://github.com/reactphp/promise)-based
[`CacheInterface`](#cacheinterface) and an in-memory [`ArrayCache`](#arraycache)
implementation of that.
This allows consumers to type hint against the interface and third parties to
provide alternate implementations.

**Table of Contents**

* [Usage](#usage)
  * [CacheInterface](#cacheinterface)
    * [get()](#get)
    * [set()](#set)
    * [delete()](#delete)
  * [ArrayCache](#arraycache)
* [Common usage](#common-usage)
  * [Fallback get](#fallback-get)
  * [Fallback-get-and-set](#fallback-get-and-set)
* [Install](#install)
* [Tests](#tests)
* [License](#license)

## Usage

### CacheInterface

The `CacheInterface` describes the main interface of this component.
This allows consumers to type hint against the interface and third parties to
provide alternate implementations.

#### get()

The `get(string $key, mixed $default = null): PromiseInterface` method can be used to
retrieve an item from the cache.

This method will resolve with the cached value on success or with the
given `$default` value when no item can be found or when an error occurs.
Similarly, an expired cache item (once the time-to-live is expired) is
considered a cache miss.

```php
$cache
    ->get('foo')
    ->then('var_dump');
```

This example fetches the value of the key `foo` and passes it to the
`var_dump` function. You can use any of the composition provided by
[promises](https://github.com/reactphp/promise).

#### set()

The `set(string $key, mixed $value, ?float $ttl = null): PromiseInterface` method can be used to
store an item in the cache.

This method will resolve with `true` on success or `false` when an error
occurs. If the cache implementation has to go over the network to store
it, it may take a while.

The optional `$ttl` parameter sets the maximum time-to-live in seconds
for this cache item. If this parameter is omitted (or `null`), the item
will stay in the cache for as long as the underlying implementation
supports. Trying to access an expired cache item results in a cache miss,
see also [`get()`](#get).

```php
$cache->set('foo', 'bar', 60);
```

This example eventually sets the value of the key `foo` to `bar`. If it
already exists, it is overridden.

#### delete()

Deletes an item from the cache.

This method will resolve with `true` on success or `false` when an error
occurs. When no item for `$key` is found in the cache, it also resolves
to `true`. If the cache implementation has to go over the network to
delete it, it may take a while.

```php
$cache->delete('foo');
```

This example eventually deletes the key `foo` from the cache. As with
`set()`, this may not happen instantly and a promise is returned to
provide guarantees whether or not the item has been removed from cache.

### ArrayCache

The `ArrayCache` provides an in-memory implementation of the [`CacheInterface`](#cacheinterface).

```php
$cache = new ArrayCache();

$cache->set('foo', 'bar');
```

Its constructor accepts an optional `?int $limit` parameter to limit the
maximum number of entries to store in the LRU cache. If you add more
entries to this instance, it will automatically take care of removing
the one that was least recently used (LRU).

For example, this snippet will overwrite the first value and only store
the last two entries:

```php
$cache = new ArrayCache(2);

$cache->set('foo', '1');
$cache->set('bar', '2');
$cache->set('baz', '3');
```

## Common usage

### Fallback get

A common use case of caches is to attempt fetching a cached value and as a
fallback retrieve it from the original data source if not found. Here is an
example of that:

```php
$cache
    ->get('foo')
    ->then(function ($result) {
        if ($result === null) {
            return getFooFromDb();
        }
        
        return $result;
    })
    ->then('var_dump');
```

First an attempt is made to retrieve the value of `foo`. A callback function is 
registered that will call `getFooFromDb` when the resulting value is null. 
`getFooFromDb` is a function (can be any PHP callable) that will be called if the 
key does not exist in the cache.

`getFooFromDb` can handle the missing key by returning a promise for the
actual value from the database (or any other data source). As a result, this
chain will correctly fall back, and provide the value in both cases.

### Fallback get and set

To expand on the fallback get example, often you want to set the value on the
cache after fetching it from the data source.

```php
$cache
    ->get('foo')
    ->then(function ($result) {
        if ($result === null) {
            return $this->getAndCacheFooFromDb();
        }
        
        return $result;
    })
    ->then('var_dump');

public function getAndCacheFooFromDb()
{
    return $this->db
        ->get('foo')
        ->then(array($this, 'cacheFooFromDb'));
}

public function cacheFooFromDb($foo)
{
    $this->cache->set('foo', $foo);

    return $foo;
}
```

By using chaining you can easily conditionally cache the value if it is
fetched from the database.

## Install

The recommended way to install this library is [through Composer](https://getcomposer.org).
[New to Composer?](https://getcomposer.org/doc/00-intro.md)

This will install the latest supported version:

```bash
$ composer require react/cache:^0.5.0
```

See also the [CHANGELOG](CHANGELOG.md) for details about version upgrades.

This project aims to run on any platform and thus does not require any PHP
extensions and supports running on legacy PHP 5.3 through current PHP 7+ and
HHVM.
It's *highly recommended to use PHP 7+* for this project.

## Tests

To run the test suite, you first need to clone this repo and then install all
dependencies [through Composer](https://getcomposer.org):

```bash
$ composer install
```

To run the test suite, go to the project root and run:

```bash
$ php vendor/bin/phpunit
```

## License

MIT, see [LICENSE file](LICENSE).
