Factory Pattern in ASP.Net Core

Question

I'm working on Worker Service in .Net Core and Added DDD to it. In the Application Layer, I have created a Factory Pattern to Pick a specific Message Handler to handle different Kafka Topics. This approach is working fine. But, wanted to know is it the right way of implementing it? I am a bit confused about injecting constructor parameters in MessageFactory

public interface IMessageFactory
{
    IMessageHandler CreateMessage(string topicName);
}

Implementation of Factory Method

public class MessageFactory: IMessageFactory
    {
        private readonly ILogger<MessageFactory> _logger;
        private readonly IOneRepository _oneRepository;
        private readonly ITwoRepository _twoRepositoty;

        public MessageFactory(ILogger<MessageFactory> logger,
            IOneRepository oneRepository,
            ITwoRepository twoRepositoty)
        {
            _logger = logger;
            _oneRepository = oneRepository;
            _twoRepositoty = twoRepositoty;
        }

        public IMessageHandler CreateMessage(string topicName)
        {
            switch (topicName.ToUpper())
            {
                case KafkaConstants.OneMaster:
                    return new OneMessageHandler(_logger, _oneRepository );
                case KafkaConstants.TwoMaster:
                    return new TwoMessageHandler(_logger, _twoRepositoty );
                default:
                    return null;
            }
        }
    }
}

Individual Handlers

public class OneMessageHandler : IMessageHandler
{
    private readonly ILogger<MessageFactory> _logger;
    private readonly IOneRepository _oneRepository;

    public OneMessageHandler (ILogger<MessageFactory> logger,
        IOneRepository _oneRepository)
    {
        _logger = logger;
       _oneRepository = oneRepository;
    }

    public async Task<bool> ProcessMessage(string message)
    {
        ..........
    }
}

public class TwoMessageHandler : IMessageHandler
{
    private readonly ILogger<MessageFactory> _logger;
    private readonly ITwoRepository _twoRepository;

    public TwoMessageHandler(ILogger<MessageFactory> logger,
        ITwoRepository twoRepository)
    {
        _logger = logger;
        _twoRepository = twoRepository;
    }

    public async Task<bool> ProcessMessage(string message)
    {
        ..........
    }
}

Worker Service

public class Worker: BackgroundService
{
    private readonly IMessageFactory _messageFactory;
   
    public Worker(IMessageFactory messageFactory)
    {
        _messageFactory = messageFactory;
    }
    IMessageHandler messageHandler = _messageFactory.CreateMessage("OneMaster");
    messageHandler.ProcessMessage(message_from_kafka_response));
}

Dependency Injection

services.AddTransient<IMessageFactory, MessageFactory>();

Answer 1

Well, I would make everything injectable, so I can register it like this (all classes self explanatory) without using factory and just using container:

container.RegisterSingleton<IHostedService>(x =>
    new RepeatBackgroundWorker(
        repeatInterval,
        new MessageHandler<TKey, TPayload>( //this can also be just something like void IWorker.RunAsync(cancellationToken)
            new KafkaMessageProducer<TKey, TPayload>(/*settings like name, consumer group and connection settings*/),
            x.GetByKey<IMessageProcessor<TKey, TPayload>>(/*processor name*/))));


//RepeatBackgroundWorker - it will generally repeat some background worker
//MessageHandler - it will consume message, process it, set new offset and until message producer enumerable reaches the end (end of queue)
//KafkaMessageProducer - it will just return something like IAsyncEnumerable<Message<TKey, TPayload>>
//IMessageProcessor - it will be your processor

as you see you have mutiple benefits:

• can additionally wrap any part into general logging or metrics system, like MessageProcessorLogger, MessageProducerLogger, etc
• can apply general filtering to producer (like grouping by Key or some field in payload, for example: grouping messages by UserId and taking last action performed, or wrap it into some batch wrapper, etc)
• can manipulate handler behavior (how and when it restarts, what to do on errors, etc)
• can manipulate various sources (like, changing kafka to rabbitmq etc)
• can put entire factory into container or runtime
• easy to use in tests, changing producer to some custom implementation, or mocking processor/consumer with something
• this way you basically need to just implement a processor/consumer and specify name of queue (+consumer group for kafka) and it will automatically provide you with fine-named metrics and logs

PS I didnt implement all classes, because it depends on business requirements

PS2 This is basically what I use in production to squeeze kafka performance, get all logs into kibana and all metrics (message timestamp is particullary useful when you have spikes and want to rebalance up/down) to prometheus+grafana from all sources out of the box.