# Caching

ABP framework extends ASP.NET Core's distributed caching system.

## Volo.Abp.Caching Package

> This package is already installed by default with the startup template. So, most of the time, you don't need to install it manually.

Volo.Abp.Caching is the core package of the caching system. Install it to your project using the package manager console (PMC):

```
Install-Package Volo.Abp.Caching
```

Then you can add **AbpCachingModule** dependency to your module:

```c#
using Volo.Abp.Modularity;
using Volo.Abp.Caching;

namespace MyCompany.MyProject
{
    [DependsOn(typeof(AbpCachingModule))]
    public class MyModule : AbpModule
    {
        //...
    }
}
```

## `IDistributedCache` Interface

ASP.NET Core defines the `IDistributedCache` interface to get/set cache values. But it has some difficulties:

* It works with **byte arrays** rather than .NET objects. So, you need to **serialize/deserialize** the objects you need to cache.
* It provides a **single key pool** for all cache items, so;
  * You need to care about the keys to distinguish **different type of objects**.
  * You need to care about the cache items of **different tenants** (see [multi-tenancy](Multi-Tenancy.md)).

> `IDistributedCache` is defined in the `Microsoft.Extensions.Caching.Abstractions` package. That means it is not only usable for ASP.NET Core applications, but also available to **any type of applications**.

> Default implementation of the `IDistributedCache` interface is the `MemoryDistributedCache` which works **in-memory**. See [ASP.NET Core's documentation](https://docs.microsoft.com/en-us/aspnet/core/performance/caching/distributed) to see how to switch to **Redis** or another cache provider.

See [ASP.NET Core's distributed caching document](https://docs.microsoft.com/en-us/aspnet/core/performance/caching/distributed) for more information.

## `IDistributedCache<TCacheItem>` Interface

ABP framework defines the generic `IDistributedCache<TCacheItem>` interface in the [Volo.Abp.Caching](https://www.nuget.org/packages/Volo.Abp.Caching/) package. `TCacheItem` is the type of the object stored in the cache. 

`IDistributedCache<TCacheItem>` solves the difficulties explained above;

* It internally **serializes/deserializes** the cached objects. Uses **JSON** serialization by default, but can be overridden by replacing the `IDistributedCacheSerializer` service in the [dependency injection](Dependency-Injection.md) system.
* It automatically adds a **cache name** prefix to the cache keys based on the object type stored in the cache. Default cache name is the full name of the cache item class (`CacheItem` postfix is removed if your cache item class ends with it). You can use the `CacheName` attribute on the cache item class to set the cache name.
* It automatically adds the **current tenant id** to the cache key to distinguish cache items for different tenants (only works if your application is [multi-tenant](Multi-Tenancy.md)). Define `IgnoreMultiTenancy` attribute on the cache item class to disable this if you want to share the cached objects among all tenants in a multi-tenant application.
* Allows to define a **global cache key prefix** per application, so different applications can use their isolated key pools in a shared distributed cache source.

### Usage

An example class to store an item in the cache:

````csharp
public class BookCacheItem
{
    public string Name { get; set; }

    public float Price { get; set; }
}
````

You can inject and use the `IDistributedCache<BookCacheItem>` service to get/set `BookCacheItem` objects.

Example usage:

````csharp
public class BookService : ITransientDependency
{
    private readonly IDistributedCache<BookCacheItem> _cache;

    public BookService(IDistributedCache<BookCacheItem> cache)
    {
        _cache = cache;
    }

    public async Task<BookCacheItem> GetAsync(Guid bookId)
    {
        return await _cache.GetOrAddAsync(
            bookId.ToString(), //Cache key
            async () => await GetBookFromDatabaseAsync(bookId),
            () => new DistributedCacheEntryOptions
            {
                AbsoluteExpiration = DateTimeOffset.Now.AddHours(1)
            }
        );
    }

    private Task<BookCacheItem> GetBookFromDatabaseAsync(Guid bookId)
    {
        //TODO: get from database
    }
}
````

* This sample service uses the `GetOrAddAsync()` method to get a book item from the cache.
* If the book was not found in the cache, it calls the factory method (`GetBookFromDatabaseAsync` in this case) to retrieve the book item from the original source.
* `GetOrAddAsync` optionally gets a `DistributedCacheEntryOptions` which can be used to set the lifetime of the cached item.

Other methods of the `IDistributedCache<BookCacheItem>` are same as ASP.NET Core's `IDistributedCache` interface, so you can refer [it's documentation](https://docs.microsoft.com/en-us/aspnet/core/performance/caching/distributed).

## `IDistributedCache<TCacheItem, TCacheKey>` Interface

`IDistributedCache<TCacheItem>` interface assumes that the type of your cache key is `string` (so, you need to manually convert your key to string if you need to use a different kind of cache key). `IDistributedCache<TCacheItem, TCacheKey>` can be used when your cache key type is not `string`.

### Usage

An example class to store an item in the cache:

````csharp
public class BookCacheItem
{
    public string Name { get; set; }

    public float Price { get; set; }
}
````

Example usage (assumes that your cache key type is `Guid`):

````csharp
public class BookService : ITransientDependency
{
    private readonly IDistributedCache<BookCacheItem, Guid> _cache;

    public BookService(IDistributedCache<BookCacheItem, Guid> cache)
    {
        _cache = cache;
    }

    public async Task<BookCacheItem> GetAsync(Guid bookId)
    {
        return await _cache.GetOrAddAsync(
            bookId, //Guid type used as the cache key
            async () => await GetBookFromDatabaseAsync(bookId),
            () => new DistributedCacheEntryOptions
            {
                AbsoluteExpiration = DateTimeOffset.Now.AddHours(1)
            }
        );
    }
    private Task<BookCacheItem> GetBookFromDatabaseAsync(Guid bookId)
    {
        //TODO: get from database
    }
}
````

* This sample service uses the `GetOrAddAsync()` method to get a book item from the cache.
* Since cache explicitly implemented as using  `Guid` as cache key, `Guid` value passed to  `_cache_GetOrAddAsync()` method.

`IDistributedCache<TCacheItem, TCacheKey>`  internally uses `ToString()` method of the key object to convert it to a string. If you need to use a complex object as the cache key, you need to override `ToString` method of your class.

An example class that is used as a cache key:

````csharp
public class UserInOrganizationCacheKey
{
    public Guid UserId { get; set; }
    
    public Guid OrganizationId { get; set; }
    
    //Builds the cache key
    public override string ToString()
    {
        return $"{UserId}_{OrganizationId}";
    }
}
````

Example usage:

````csharp
public class BookService : ITransientDependency
{
    private readonly IDistributedCache<UserCacheItem, UserInOrganizationCacheKey> _cache;

    public BookService(
        IDistributedCache<UserCacheItem, UserInOrganizationCacheKey> cache)
    {
        _cache = cache;
    }
    
    ...
}
````

### DistributedCacheOptions

TODO