# Caching

ABP Framework extends the [ASP.NET Core distributed cache](https://docs.microsoft.com/en-us/aspnet/core/performance/caching/distributed).

## Installation

> This package is already installed by default with the [application startup template](Startup-Templates/Application.md). So, most of the time, you don't need to install it manually.

[Volo.Abp.Caching](https://www.nuget.org/packages/Volo.Abp.Caching) is the main package of the caching system. You can install it a project using the add-package command of the [ABP CLI](CLI.md):

```
abp add-package Volo.Abp.Caching
```

You need to run this command on a command line terminal in a folder containing a `csproj` file (see [other options](https://abp.io/package-detail/Volo.Abp.Caching) to install).

## Usage

### `IDistributedCache` Interface

ASP.NET Core defines the `IDistributedCache` interface to get/set the 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** in a [multi-tenant](Multi-Tenancy.md) system.

> `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. Also, see the [Redis Cache](Redis-Cache.md) document if you want to use the Redis as the distributed cache server.

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 (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 server.
* It **can tolerate errors** wherever possible and bypasses the cache. This is useful when you have temporary problems on the cache server.
* It has methods like `GetManyAsync` and `SetManyAsync` which significantly improve the performance on **batch operations**.

**Example: Store Book names and prices in the cache**

````csharp
namespace MyProject
{
    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:

````csharp
using System;
using System.Threading.Tasks;
using Microsoft.Extensions.Caching.Distributed;
using Volo.Abp.Caching;
using Volo.Abp.DependencyInjection;

namespace MyProject
{
    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. `GetOrAddAsync` is an additional method that was added by the ABP Framework to the standard ASP.NET Core distributed cache methods.
* 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.

`IDistributedCache<BookCacheItem>` supports the same methods of the ASP.NET Core's standard `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). While this is not a big deal, `IDistributedCache<TCacheItem, TCacheKey>` can be used when your cache key type is not `string`.

**Example: Store Book names and prices in the cache**

````csharp
using Volo.Abp.Caching;

namespace MyProject
{
    [CacheName("Books")]
    public class BookCacheItem
    {
        public string Name { get; set; }

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

* This example uses the `CacheName` attribute for the `BookCacheItem` class to set the cache name.

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

````csharp
using System;
using System.Threading.Tasks;
using Microsoft.Extensions.Caching.Distributed;
using Volo.Abp.Caching;
using Volo.Abp.DependencyInjection;

namespace MyProject
{
    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.

#### Complex Types as the Cache Key

`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;
    }
    
    ...
}
````

## Configuration

### AbpDistributedCacheOptions

`AbpDistributedCacheOptions` is the main [options class](Options.md) to configure the caching.

**Example: Set the cache key prefix for the application**

```csharp
Configure<AbpDistributedCacheOptions>(options =>
{
    options.KeyPrefix = "MyApp1";
});
```

> Write that code inside the `ConfigureServices` method of your [module class](Module-Development-Basics.md).

#### Available Options

* `HideErrors` (`bool`, default: `true`): Enables/disables hiding the errors on writing/reading values from the cache server.
* `KeyPrefix` (`string`, default: `null`): If your cache server is shared by multiple applications, you can set a prefix for the cache keys for your application. In this case, different applications can not overwrite each other's cache items.
* `GlobalCacheEntryOptions` (`DistributedCacheEntryOptions`): Used to set default distributed cache options (like `AbsoluteExpiration` and `SlidingExpiration`) used when you don't specify the options while saving cache items. Default value uses the `SlidingExpiration` as 20 minutes.

## Error Handling

When you design a cache for your objects, you typically try to get the value from cache first. If not found in the cache, you query the object from the **original source**. It may be located in a **database** or may require to perform an HTTP call to a remote server.

In most cases, you want to **tolerate the cache errors**; If you get error from the cache server you don't want to cancel the operation. Instead, you silently hide (and log) the error and **query from the original source**. This is what the ABP Framework does by default.

ABP's Distributed Cache [handle](Exception-Handling.md), log and hide errors by default. There is an option to change this globally (see the options below).

In addition, all of the `IDistributedCache<TCacheItem>` (and `IDistributedCache<TCacheItem, TCacheKey>`) methods have an optional `hideErrors` parameter, which is `null` by default. The global value is used if this parameter left as `null`, otherwise you can decide to hide or throw the exceptions for individual method calls.

## Batch Operations

ABP's distributed cache interfaces provide methods to perform batch get/set methods those improves the performance when you want to get or set multiple cache items in a single method call.

* `SetManyAsync` and `SetMany` methods can be used to set multiple values to the cache.
* `GetManyAsync` and `GetMany` methods can be used to retrieve multiple values from the cache.

> These are not standard methods of the ASP.NET Core caching. So, some providers may not support them. They are supported by the [ABP Redis Cache integration package](Redis-Cache.md). If the provider doesn't support, it fallbacks to `SetAsync` and `GetAsync` methods (called once for each item).

## Advanced Topics

### Unit Of Work Level Cache

Distributed cache service provides an interesting feature. Assume that you've updated the price of a book in the database, then set the new price to the cache, so you can use the cached value later. What if you have an exception after setting the cache and you **rollback the transaction** that updates the price of the book? In this case, cache value will be incorrect.

`IDistributedCache<..>` methods gets an optional parameter, named `considerOuw`, which is `false` by default. If you set it to `true`, then the changes you made for the cache are not actually applied to the real cache store, but associated with the current [unit of work](Unit-Of-Work.md). You get the value you set in the same unit of work, but the changes are applied **only if the current unit of work succeed**.

### IDistributedCacheSerializer

`IDistributedCacheSerializer` service is used to serialize and deserialize the cache items. Default implementation is the `Utf8JsonDistributedCacheSerializer` class that uses `IJsonSerializer` service to convert objects to [JSON](Json.md) and vice verse. Then it uses UTC8 encoding to convert the JSON string to a byte array which is accepted by the distributed cache.

You can [replace](Dependency-Injection.md) this service by your own implementation if you want to implement your own serialization logic.

### IDistributedCacheKeyNormalizer

`IDistributedCacheKeyNormalizer` is implemented by the `DistributedCacheKeyNormalizer` class by default. It adds cache name, application cache prefix and current tenant id to the cache key. If you need a more advanced key normalization, you can [replace](Dependency-Injection.md) this service by your own implementation.

## See Also

* [Redis Cache](Redis-Cache.md)