Commented som things

src/DatabaseFactory.cs

@@ -15,6 +15,14 @@ internal class DatabaseFactory : IDatabaseFactory
         private static Dictionary<string, KormBuilder> AddBuildersDictionary(IServiceCollection services)
             => _builders.GetOrAdd(services, _ => new Dictionary<string, KormBuilder>());
 
+        /// <summary>
+        /// Adds <paramref name="builder"/> with name <paramref name="name"/> into the list of builders for
+        /// <paramref name="services"/>.
+        /// </summary>
+        /// <param name="services">Service collection for which the builder is added.</param>
+        /// <param name="name">The name of the database builder.</param>
+        /// <param name="builder">Database builder.</param>
+        /// <returns><see langword="true"/>, if this was the first builder added, otherwise <see langword="false"/>.</returns>
         internal static bool AddBuilder(IServiceCollection services, string name, KormBuilder builder)
         {
             Dictionary<string, KormBuilder> builders = AddBuildersDictionary(services);
@@ -23,6 +31,9 @@ internal static bool AddBuilder(IServiceCollection services, string name, KormBu
                 throw new ArgumentException(string.Format(Resources.DuplicateDatabaseName, name), nameof(name));
             }
             builders.Add(name, builder);
+
+            // We need to know if it was the first builder added.
+            // The first builder is added into the service container also as IDatabase dependency.
             return builders.Count == 1;
         }
 

src/ServiceCollectionExtensions.cs

@@ -25,7 +25,9 @@ public static class ServiceCollectionExtensions
         /// </summary>
         public const string KormAutoMigrateKey = "KormAutoMigrate";
 
-        private static ConcurrentDictionary<IServiceCollection, bool> _databaseFactoryAdded =
+        // Only one IDatabaseFactory for service collection must be registered.
+        // This dictionary holds flags for already used service collections.
+        private static readonly ConcurrentDictionary<IServiceCollection, bool> _databaseFactoryAdded =
             new ConcurrentDictionary<IServiceCollection, bool>();
 
         /// <summary>