Steeltoe Messaging with RabbitMQ - Listener and Dependency Injection issue

Question

I'm using Steeltoe to implement Event Messaging, with RabbitMQ, between microservices, but I'm having an issue when I register my Listener service and it not recognizing other DI services.

In my Startup.cs file I have my services being registered as follows:

public void ConfigureServices(IServiceCollection servcies)
{
    ...

    // Add my custom service as a scoped service
    services.AddScoped<IMyService>(provider => new MyService());

    var rabbitSection = configuration.GetSection(RabbitOptions.PREFIX);
    services.Configure<RabbitOptions>(rabbitSection);

    services.AddRabbitServices();
    services.AddRabbitAdmin();
    services.AddRabbitTemplate();

    // Add Rabbit Listener Service
    services.AddSingleton<MyRabbitListenerService>();
    services.AddRabbitListeners<MyRabbitListenerService>();

    ...
}

... then in my MyRabbitListenerService.cs class:

public class MyRabbitListenerService
{
    private readonly IMyService _myService;

    public MyRabbitListenerService(IMyService myService)
    {
        _myService = myService;
    }

    [RabbitListener("MyQueue")]
    public async Task MessageListener(byte[] message)
    {
        // Do stuff ...
    }
}

When I run this, I'm getting an error indicating the IMyService couldn't be injected into the Listener service as it wasn't registered. I can't work out why this isn't working. Is it because I'm trying to inject a scoped service into a singleton service?

UPDATE

I did some testing and changing IMyService from a scoped service to a singleton made it work. Now I need to figure out how to get around this because in my situation it doesn't make sense to register IMyService as a singleton.

score 2 · Answer 1 · answered Jan 08 '21 at 20:44

2

The reason for this error is that you cannot consume a scoped service from a singleton. Scoped has per request semantics, which for messaging doesn't make sense. Perhaps you mean AddTransient? That works. If this doesn't work for you, Can you give more detail on why MyService cannot be just transient?

answered Jan 08 '21 at 20:44

Hananiel

421
2
9

I could make MyService transient, but in my case, this service is a wrapper around methods to interact with an external Web Api. I would imagine this would be more suitable to Scoped (or maybe even Singleton). – Dazfl Jan 11 '21 at 06:12
Scoped is per HttpWebRequest, which is for incoming requests through ANC pipeline, where your application is acting as a server. The RabbitListener is a long running process which listens to incoming messages but these are not HttpWebRequests. If you want a new object per each rabbitMQ message, the correct type would be transient. In your case, if you are using httpClient, singleton would be the correct type. – Hananiel Jan 13 '21 at 17:08

Steeltoe Messaging with RabbitMQ - Listener and Dependency Injection issue

1 Answers1