Distributed caching in ASP.NET Core

By Mohsin Nasir and smandia

A distributed cache is a cache shared by multiple app servers, typically maintained as an external service to the app servers that access it. A distributed cache can improve the performance and scalability of an ASP.NET Core app, especially when the app is hosted by a cloud service or a server farm.

A distributed cache has several advantages over other caching scenarios where cached data is stored on individual app servers.

When cached data is distributed, the data:

• Is coherent (consistent) across requests to multiple servers.
• Survives server restarts and app deployments.
• Doesn't use local memory.

Distributed cache configuration is implementation specific. This article describes how to configure SQL Server and Redis distributed caches. Third party implementations are also available, such as NCache (NCache on GitHub). Regardless of which implementation is selected, the app interacts with the cache using the IDistributedCache interface.

View or download sample code (how to download)

Warning

This article uses a local database that doesn't require the user to be authenticated. Production apps should use the most secure authentication flow available. For more information on authentication for deployed test and production apps, see Secure authentication flows.

Prerequisites

Add a package reference for the distributed cache provider used:

• For a Redis distributed cache, Microsoft.Extensions.Caching.StackExchangeRedis.
• For SQL Server, Microsoft.Extensions.Caching.SqlServer.
• For the NCache distributed cache, NCache.Microsoft.Extensions.Caching.OpenSource.

IDistributedCache interface

The IDistributedCache interface provides the following methods to manipulate items in the distributed cache implementation:

• Get, GetAsync: Accepts a string key and retrieves a cached item as a byte[] array if found in the cache.
• Set, SetAsync: Adds an item (as byte[] array) to the cache using a string key.
• Refresh, RefreshAsync: Refreshes an item in the cache based on its key, resetting its sliding expiration timeout (if any).
• Remove, RemoveAsync: Removes a cache item based on its string key.

Establish distributed caching services

Register an implementation of IDistributedCache in Program.cs. Framework-provided implementations described in this topic include:

• Distributed Redis cache
• Distributed Memory Cache
• Distributed SQL Server cache
• Distributed NCache cache
• Distributed Azure CosmosDB cache

Distributed Redis Cache

We recommend production apps use the Distributed Redis Cache because it's the most performant. For more information see Recommendations.

Redis is an open source in-memory data store, which is often used as a distributed cache. You can configure an Azure Cache for Redis for an Azure-hosted ASP.NET Core app, and use an Azure Cache for Redis for local development.

An app configures the cache implementation using a RedisCache instance, by calling AddStackExchangeRedisCache. For output caching, use AddStackExchangeRedisOutputCache.

1. Create an Azure Cache for Redis.
2. Copy the Primary connection string (StackExchange.Redis) to Configuration.
   • Local development: Save the connection string with Secret Manager.
   • Azure: Save the connection string in a secure store such as Azure Key Vault

The following code enables the Azure Cache for Redis:

builder.Services.AddStackExchangeRedisCache(options =>
 {
     options.Configuration = builder.Configuration.GetConnectionString("MyRedisConStr");
     options.InstanceName = "SampleInstance";
 });

The preceding code assumes the Primary connection string (StackExchange.Redis) was saved in configuration with the key name MyRedisConStr.

For more information, see Azure Cache for Redis.

See this GitHub issue for a discussion on alternative approaches to a local Redis cache.

Distributed Memory Cache

The Distributed Memory Cache (AddDistributedMemoryCache) is a framework-provided implementation of IDistributedCache that stores items in memory. The Distributed Memory Cache isn't an actual distributed cache. Cached items are stored by the app instance on the server where the app is running.

The Distributed Memory Cache is a useful implementation:

• In development and testing scenarios.
• When a single server is used in production and memory consumption isn't an issue. Implementing the Distributed Memory Cache abstracts cached data storage. It allows for implementing a true distributed caching solution in the future if multiple nodes or fault tolerance become necessary.

The sample app makes use of the Distributed Memory Cache when the app is run in the Development environment in Program.cs:

builder.Services.AddDistributedMemoryCache();

Distributed SQL Server Cache

The Distributed SQL Server Cache implementation (AddDistributedSqlServerCache) allows the distributed cache to use a SQL Server database as its backing store. To create a SQL Server cached item table in a SQL Server instance, you can use the sql-cache tool. The tool creates a table with the name and schema that you specify.

Create a table in SQL Server by running the sql-cache create command. Provide the SQL Server instance (Data Source), database (Initial Catalog), schema (for example, dbo), and table name (for example, TestCache):

dotnet sql-cache create "Data Source=(localdb)/MSSQLLocalDB;Initial Catalog=DistCache;Integrated Security=True;" dbo TestCache

A message is logged to indicate that the tool was successful:

Table and index were created successfully.

The table created by the sql-cache tool has the following schema:

Note

An app should manipulate cache values using an instance of IDistributedCache, not a SqlServerCache.

The sample app implements SqlServerCache in a non-Development environment in Program.cs:

builder.Services.AddDistributedSqlServerCache(options =>
{
    options.ConnectionString = builder.Configuration.GetConnectionString(
        "DistCache_ConnectionString");
    options.SchemaName = "dbo";
    options.TableName = "TestCache";
});

Note

A ConnectionString (and optionally, SchemaName and TableName) are typically stored outside of source control (for example, stored by the Secret Manager or in appsettings.json/appsettings.{Environment}.json files). The connection string may contain credentials that should be kept out of source control systems.

Distributed NCache Cache

NCache is an open source in-memory distributed cache developed natively in .NET and .NET Core. NCache works both locally and configured as a distributed cache cluster for an ASP.NET Core app running in Azure or on other hosting platforms.

To install and configure NCache on your local machine, see Getting Started Guide for Windows (.NET and .NET Core).

To configure NCache:

1. Install NCache open source NuGet.
2. Configure the cache cluster in client.ncconf.
3. Add the following code to Program.cs:

builder.Services.AddNCacheDistributedCache(configuration =>
{
    configuration.CacheName = "democache";
    configuration.EnableLogs = true;
    configuration.ExceptionsEnabled = true;
});

Distributed Azure CosmosDB Cache

Azure Cosmos DB can be used in ASP.NET Core as a session state provider by using the IDistributedCache interface. Azure Cosmos DB is a fully managed NoSQL and relational database for modern app development that offers high availability, scalability, and low-latency access to data for mission-critical applications.

After installing the Microsoft.Extensions.Caching.Cosmos NuGet package, configure an Azure Cosmos DB distributed cache as follows:

Reuse an existing client

The easiest way to configure distributed cache is by reusing an existing Azure Cosmos DB client. In this case, the CosmosClient instance won't be disposed when the provider is disposed.

services.AddCosmosCache((CosmosCacheOptions cacheOptions) =>
{
    cacheOptions.ContainerName = Configuration["CosmosCacheContainer"];
    cacheOptions.DatabaseName = Configuration["CosmosCacheDatabase"];
    cacheOptions.CosmosClient = existingCosmosClient;
    cacheOptions.CreateIfNotExists = true;
});

Create a new client

Alternatively, instantiate a new client. In this case, the CosmosClient instance will get disposed when the provider is disposed.

services.AddCosmosCache((CosmosCacheOptions cacheOptions) =>
{
    cacheOptions.ContainerName = Configuration["CosmosCacheContainer"];
    cacheOptions.DatabaseName = Configuration["CosmosCacheDatabase"];
    cacheOptions.ClientBuilder = new CosmosClientBuilder(Configuration["CosmosConnectionString"]);
    cacheOptions.CreateIfNotExists = true;
});

Use the distributed cache

To use the IDistributedCache interface, request an instance of IDistributedCache in the app. The instance is provided by dependency injection (DI).

When the sample app starts, IDistributedCache is injected into Program.cs. The current time is cached using IHostApplicationLifetime (for more information, see Generic Host: IHostApplicationLifetime):

app.Lifetime.ApplicationStarted.Register(() =>
{
    var currentTimeUTC = DateTime.UtcNow.ToString();
    byte[] encodedCurrentTimeUTC = System.Text.Encoding.UTF8.GetBytes(currentTimeUTC);
    var options = new DistributedCacheEntryOptions()
        .SetSlidingExpiration(TimeSpan.FromSeconds(20));
    app.Services.GetService<IDistributedCache>()
                              .Set("cachedTimeUTC", encodedCurrentTimeUTC, options);
});

The sample app injects IDistributedCache into the IndexModel for use by the Index page.

Each time the Index page is loaded, the cache is checked for the cached time in OnGetAsync. If the cached time hasn't expired, the time is displayed. If 20 seconds have elapsed since the last time the cached time was accessed (the last time this page was loaded), the page displays Cached Time Expired.

Immediately update the cached time to the current time by selecting the Reset Cached Time button. The button triggers the OnPostResetCachedTime handler method.

public class IndexModel : PageModel
{
    private readonly IDistributedCache _cache;

    public IndexModel(IDistributedCache cache)
    {
        _cache = cache;
    }

    public string? CachedTimeUTC { get; set; }
    public string? ASP_Environment { get; set; }

    public async Task OnGetAsync()
    {
        CachedTimeUTC = "Cached Time Expired";
        var encodedCachedTimeUTC = await _cache.GetAsync("cachedTimeUTC");

        if (encodedCachedTimeUTC != null)
        {
            CachedTimeUTC = Encoding.UTF8.GetString(encodedCachedTimeUTC);
        }

        ASP_Environment = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");
        if (String.IsNullOrEmpty(ASP_Environment))
        {
            ASP_Environment = "Null, so Production";
        }
    }

    public async Task<IActionResult> OnPostResetCachedTime()
    {
        var currentTimeUTC = DateTime.UtcNow.ToString();
        byte[] encodedCurrentTimeUTC = Encoding.UTF8.GetBytes(currentTimeUTC);
        var options = new DistributedCacheEntryOptions()
            .SetSlidingExpiration(TimeSpan.FromSeconds(20));
        await _cache.SetAsync("cachedTimeUTC", encodedCurrentTimeUTC, options);

        return RedirectToPage();
    }
}

There's no need to use a Singleton or Scoped lifetime for IDistributedCache instances with the built-in implementations.

You can also create an IDistributedCache instance wherever you might need one instead of using DI, but creating an instance in code can make your code harder to test and violates the Explicit Dependencies Principle.

Recommendations

When deciding which implementation of IDistributedCache is best for your app, consider the following:

• Existing infrastructure
• Performance requirements
• Cost
• Team experience

Caching solutions usually rely on in-memory storage to provide fast retrieval of cached data, but memory is a limited resource and costly to expand. Only store commonly used data in a cache.

For most apps, a Redis cache provides higher throughput and lower latency than a SQL Server cache. However, benchmarking is recommended to determine the performance characteristics of caching strategies.

When SQL Server is used as a distributed cache backing store, use of the same database for the cache and the app's ordinary data storage and retrieval can negatively impact the performance of both. We recommend using a dedicated SQL Server instance for the distributed cache backing store.

Additional resources

• Redis Cache on Azure
• SQL Database on Azure
• ASP.NET Core IDistributedCache Provider for NCache in Web Farms (NCache on GitHub)
• Repository README file for Microsoft.Extensions.Caching.Cosmos
• Cache in-memory in ASP.NET Core
• Detect changes with change tokens in ASP.NET Core
• Response caching in ASP.NET Core
• Response Caching Middleware in ASP.NET Core
• Cache Tag Helper in ASP.NET Core MVC
• Distributed Cache Tag Helper in ASP.NET Core
• Host ASP.NET Core in a web farm