fleshout backend

backend/Data/PersistenceServiceCollectionExtensions.cs

@@ -0,0 +1,37 @@
+using Backend.Data.InMemory;
+
+namespace Backend.Data;
+
+/// <summary>
+/// Single place where a storage backend is chosen. To add a real database, implement
+/// <see cref="IUserRepository"/> and register it under a new provider name here;
+/// controllers stay untouched.
+/// </summary>
+public static class PersistenceServiceCollectionExtensions
+{
+    public static IServiceCollection AddPersistence(
+        this IServiceCollection services,
+        IConfiguration configuration)
+    {
+        services.Configure<PersistenceOptions>(configuration.GetSection(PersistenceOptions.SectionName));
+
+        var provider = configuration.GetSection(PersistenceOptions.SectionName).Get<PersistenceOptions>()?.Provider
+            ?? PersistenceProviders.InMemory;
+
+        switch (provider)
+        {
+            case PersistenceProviders.InMemory:
+                // Singleton because the store itself holds the data.
+                services.AddSingleton<IUserRepository, InMemoryUserRepository>();
+                break;
+
+            default:
+                throw new InvalidOperationException(
+                    $"Unknown persistence provider '{provider}'. Supported providers: " +
+                    $"'{PersistenceProviders.InMemory}'. Register additional providers in " +
+                    $"{nameof(PersistenceServiceCollectionExtensions)}.");
+        }
+
+        return services;
+    }
+}