Update cache in background (#18295)

* Update cache in background

* Update cache in background

Author: Rick-Anderson

aspnetcore/performance/caching/memory.md

@@ -175,6 +175,10 @@ Using a <xref:System.Threading.CancellationTokenSource> allows multiple cache en
 * Use <xref:Microsoft.Extensions.Caching.Memory.ICacheEntry.PostEvictionCallbacks> to set the callbacks that will be fired after the cache entry is evicted from the cache.
 * For most apps, `IMemoryCache` is enabled. For example, calling `AddMvc`, `AddControllersWithViews`, `AddRazorPages`, `AddMvcCore().AddRazorViewEngine`, and many other `Add{Service}` methods in `ConfigureServices`, enables `IMemoryCache`. For apps that are not calling one of the preceding `Add{Service}` methods, it may be necessary to call <xref:Microsoft.Extensions.DependencyInjection.MemoryCacheServiceCollectionExtensions.AddMemoryCache*> in `ConfigureServices`.
 
+## Background cache update
+
+Use a [background service](xref:fundamentals/host/hosted-services) such as <xref:Microsoft.Extensions.Hosting.IHostedService> to update the cache. The background service can recompute the entries and then assign them to the cache only when they’re ready.
+
 ## Additional resources
 
 * <xref:performance/caching/distributed>
@@ -342,6 +346,10 @@ Using a `CancellationTokenSource` allows multiple cache entries to be evicted as
 
 * Use [PostEvictionCallbacks](/dotnet/api/microsoft.extensions.caching.memory.icacheentry.postevictioncallbacks#Microsoft_Extensions_Caching_Memory_ICacheEntry_PostEvictionCallbacks) to set the callbacks that will be fired after the cache entry is evicted from the cache.
 
+## Background cache update
+
+Use a [background service](xref:fundamentals/host/hosted-services) such as <xref:Microsoft.Extensions.Hosting.IHostedService> to update the cache. The background service can recompute the entries and then assign them to the cache only when they’re ready.
+
 ## Additional resources
 
 * <xref:performance/caching/distributed>