What is the prescribed methodology for integrating Wolverine framework within a HostedService?

Question

So far I have been using

MediatR.Courier

for subscribing to messages in my background services / asp.net hosted services.

Why, because Hosted services are singletons, and NotificationHandler are not, and I needed a way to handle notification messages from MediatR in a hosted service.

Now I am in the process of migrating to

Wolverine

framework, and I want to know if there is a way of using Wolverine in a singleton hosted service like there is MediatR.Courier or is the Wolverine Handle method just supposed to work?

Answer 1

Wolverine is that it is built to take advantage of the ASP.NET Core dependency injection (DI) container: see "Wolverine / Message Handlers / Method Injection", and it naturally fits into the lifecycle of ASP.NET Core applications, including hosted services. Integrating Wolverine into a singleton HostedService should not require a separate courier-like mechanism, as you would with MediatR.

In Wolverine, message handlers are just classes that you have indicated should handle specific types of messages. You will define these handlers similar to how you would in MediatR, but make sure they are recognizable to Wolverine's scanning mechanisms.

Within your singleton HostedService, you can inject Wolverine's IMessageContext to publish or send messages. Wolverine handles the resolution of handlers from the DI container, making sure that they are executed in response to messages.
As an example, similar to PlayingWithWolverineMarten Worker.Ping/Workers/PingWorker.cs:

public class MyHostedService : BackgroundService
{
    private readonly IMessageContext _messageContext;

    public MyHostedService(IMessageContext messageContext)
    {
        _messageContext = messageContext;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            // Example of sending a message
            await _messageContext.Send(new MyMessage());
            await Task.Delay(1000, stoppingToken);
        }
    }
}

And a simple message handler could look like:

public class MyMessageHandler
{
    public async Task Handle(MyMessage message)
    {
        // Handle the message here
    }
}

Make sure your message handlers are registered with the DI container, either by manually registering them or by using Wolverine's assembly scanning capabilities to automatically discover them.

---

I'm OK with publishing, that is easy.
But how can I handle subscribing to a message in a background/hosted service that is singleton?

I want my background/hosted service to handle some message.
So if I implement a Handle method that is not static in my background/hosted service will it work?
I'm asking because my tough is that each Wolverine handler is a new instance, so having it as a singleton...

True: in Wolverine, message handlers are typically instantiated per message handling operation, which means they are created for the scope of a single message handling and do not persist across multiple messages. That approach makes sure handlers are stateless, thread-safe, and do not retain any data between handling different messages, aligning with best practices for handling concurrent operations in a scalable way.

However, when integrating message handling directly into a singleton HostedService, you would need to consider:

• the singleton lifecycle: A singleton service, including a HostedService, is created once per application lifetime and shares its state across the entire application: that differs from Wolverine's design of handling messages through transient or scoped handlers.

• handler registration: Wolverine expects handlers to be registered and recognized through its configuration and discovery mechanisms. Simply adding a handler method within a HostedService does not automatically register it with Wolverine's messaging infrastructure.

So, instead of having a singleton service listen to and handle messages, you might consider:

• separating Handlers from the hosted service: that allows Wolverine to manage message handler lifecycles according to its conventions and makes sure your application adheres to the separation of concerns principle.

• communicating within the application: If your HostedService needs to be aware of or react to messages, you can use application-wide events, a shared state (with thread-safety in mind), or other messaging mechanisms (like MediatR within the same application) to notify the HostedService of the necessary actions.

• injecting shared service or state management mechanism into your handlers, if the handlers need to trigger actions within the HostedService or affect its state. That service can then be used to communicate with the HostedService, making sure that any state changes are thread-safe.

Example:

public class MyMessageHandler
{
    private readonly IMyHostedServiceAction _serviceAction;

    public MyMessageHandler(IMyHostedServiceAction serviceAction)
    {
        _serviceAction = serviceAction;
    }

    public async Task Handle(MyMessage message)
    {
        // Trigger action or update state in HostedService
        await _serviceAction.PerformActionAsync(message.Data);
    }
}

public interface IMyHostedServiceAction
{
    Task PerformActionAsync(string data);
}

// Implement this interface in your HostedService and register it as a singleton

That would make sure your singleton HostedService can participate in message handling indirectly while adhering to Wolverine's architecture and the recommended practices for .NET applications.

• The MyMessageHandler class is defined separately from any HostedService. This separation allows Wolverine to manage the lifecycle of MyMessageHandler instances according to its internal logic, which typically involves creating a new instance for each message to be handled.

• The MyMessageHandler class communicates with the HostedService through an abstraction (IMyHostedServiceAction). This interface defines a contract (PerformActionAsync) that the HostedService can implement to react to messages. This allows for loose coupling between the message handler and the service, facilitating communication within the application without directly linking the message handler to the implementation details of the HostedService.

• The MyMessageHandler constructor takes an instance of IMyHostedServiceAction as a dependency, which is injected by the ASP.NET Core dependency injection container at runtime. This demonstrates injecting services into handlers, allowing the handlers to interact with other parts of the application, such as a HostedService. The HostedService would implement IMyHostedServiceAction and register itself (or a delegate) with the DI container to fulfill this contract.
  The handler can trigger actions or change the state within the HostedService in a thread-safe manner, as all the operations that modify the state or behavior of the service go through a well-defined interface.

The handlers remain focused on responding to messages, while the HostedService can handle background tasks and react to messages without being directly responsible for message handling logic.