Fix update cache example. Related to #33589.

docs/core/extensions/caching.md

@@ -217,17 +217,20 @@ In the preceding C# code:
 
 Consumers of this service are free to call `GetPhotosAsync` method, and handle photos accordingly. No `HttpClient` is required as the cache contains the photos.
 
+The asynchronous signal is based on an encapsulated <xref:System.Threading.SemaphoreSlim> instance, within a generic-type constrained singleton. The `CacheSignal<T>` relies on an instance of `SemaphoreSlim`:
+
+:::code source="snippets/caching/memory-worker/CacheSignal.cs":::
+
+In the preceding C# code, the decorator pattern is used to wrap an instance of the `SemaphoreSlim`. Since the `CacheSignal<T>` is registered as a singleton, it can be used across all service lifetimes with any generic type &mdash; in this case, the `Photo`. It is responsible for signaling the seeding of the cache.
+
 The `CacheWorker` is a subclass of <xref:Microsoft.Extensions.Hosting.BackgroundService>:
 
 :::code source="snippets/caching/memory-worker/CacheWorker.cs":::
 
-> [!IMPORTANT]
-> You need to `override` <xref:Microsoft.Extensions.Hosting.BackgroundService.StartAsync%2A?displayProperty=nameWithType> and call `await _cacheSignal.WaitAsync()` in order to prevent a race condition between the starting of the `CacheWorker` and a call to `PhotoService.GetPhotosAsync`.
-
 In the preceding C# code:
 
-- The constructor requires an `ILogger`, `HttpClient`, `CacheSignal<Photo>`, and `IMemoryCache`.
-- The defines an `_updateInterval` of three hours.
+- The constructor requires an `ILogger`, `HttpClient`, and `IMemoryCache`.
+- The `_updateInterval` is defined for three hours.
 - The `ExecuteAsync` method:
   - Loops while the app is running.
   - Makes an HTTP request to `"https://jsonplaceholder.typicode.com/photos"`, and maps the response as an array of `Photo` objects.
@@ -236,11 +239,7 @@ In the preceding C# code:
   - The call to <xref:System.Threading.Tasks.Task.Delay%2A?displayProperty=nameWithType> is awaited, given the update interval.
   - After delaying for three hours, the cache is again updated.
 
-The asynchronous signal is based on an encapsulated <xref:System.Threading.SemaphoreSlim> instance, within a generic-type constrained singleton. The `CacheSignal<T>` relies on an instance of `SemaphoreSlim`:
-
-:::code source="snippets/caching/memory-worker/CacheSignal.cs":::
-
-In the preceding C# code, the decorator pattern is used to wrap an instance of the `SemaphoreSlim`. Since the `CacheSignal<T>` is registered as a singleton, it can be used across all service lifetimes with any generic type &mdash; in this case, the `Photo`. It is responsible for signaling the seeding of the cache.
+Consumers in the same process could ask the `IMemoryCache` for the photos, but the `CacheWorker` is responsible for updating the cache.
 
 ## Distributed caching
 

docs/core/extensions/snippets/caching/memory-worker/CacheWorker.cs

@@ -11,6 +11,8 @@ public sealed class CacheWorker : BackgroundService
     private readonly IMemoryCache _cache;
     private readonly TimeSpan _updateInterval = TimeSpan.FromHours(3);
 
+    private bool _isCacheInitialized = false;
+
     private const string Url = "https://jsonplaceholder.typicode.com/photos";
 
     public CacheWorker(
@@ -52,7 +54,11 @@ await _httpClient.GetFromJsonAsync<Photo[]>(
             }
             finally
             {
-                _cacheSignal.Release();
+                if (!_isCacheInitialized)
+                {
+                    _cacheSignal.Release();
+                    _isCacheInitialized = true;
+                }
             }
 
             try

docs/core/extensions/snippets/caching/memory-worker/Program.cs

@@ -9,6 +9,7 @@
         services.AddScoped<PhotoService>();
         services.AddSingleton(typeof(CacheSignal<>));
     })
+    .UseConsoleLifetime()
     .Build();
 
 await host.StartAsync();
@@ -26,11 +27,10 @@
 
         PhotoService service =
             scope.ServiceProvider.GetRequiredService<PhotoService>();
-
-        IAsyncEnumerable<Photo> photos = service.GetPhotosAsync(p => p.AlbumId == id);
-        await foreach (Photo photo in photos)
+
+        await foreach (Photo photo in service.GetPhotosAsync(p => p.AlbumId == id))
         {
-            logger.LogInformation(photo.ToString());
+            logger.LogInformation("{PhotoDetails}", photo.ToString());
         }
 
         logger.LogInformation("");