Module-based web app: Custom startup pipeline not supported

[maelscuttle]

In order to better separate application concerns I tried to introduce a module-based approach for configuring the web-host and the application pipeline, however I am failing to understand what "magic conventions" or restrictions are at work within mostly Microsoft.AspNetCore.Hosting since nothing I try seems to work.

I created a new interface like this:

public interface IWebModule {
    void ConfigureServices(IServiceCollection services);
    void Configure(IApplicationBuilder app);
}

At first I included the following into a custom convention-based startup:

class MyStartup {
    public MyStartup(IEnumerable<IWebModule> modules) { 
        //.ctor stuff
    }
}

This obviously didn't work since there is no real IServiceProvider yet available. It seems there is only a pseudo-service locator within GenericWebHostBuilder.
Instead of relying on WebHostBuilder.UseStartup<T>(), I then tried to utilize WebHostBuilder.Configure(Action<IApplicationBuilder>) and call the following:

builder.Configure(app => {
    var modules = app.ApplicationServices.GetServices<IWebModule>();
    foreach(var module) {
        // service reg left out for readability
        module.Configure(app);
    }));

While just calling builder.Configure() works, this one then throws exception such as: InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Mvc.ApplicationParts.ApplicationPartManager' while attempting to activate 'Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation.RuntimeViewCompilerProvider'.

Next I tried again with a custom startup implementing IStartup, but resolving services beforehand.

builder.ConfigureServices(services => {
    services.AddSingleton<IStartup>(provider =>
        var modules = provider.GetServices<IWebModule>();
        return new MyStartup(modules);
    });
}
// should technically create a new DelegateStartup which resolves IStartup through the service provider
builder.Configure(app => {});

Which then results in a NotSupportedException telling that IStartup isn't supported.

Last I did was trying to create my own Configure()-Extension, but that didn't work either, since Configure seems to internally cast the IWebHostBuilder to ISupportsStartup, which is internal.

Is there a better approach to accomplish this? Or did I completely misunderstand something here? @davidfowl @Tratcher

[Tratcher]

The basic problem here is that while multiple calls to ConfigureServices are supported, only one call to Configure is supported. Services are much less order sensitive, but Configure is responsible for building an order sensitive pipeline.

Your best approach was probably:

builder.Configure(app => {
    var modules = app.ApplicationServices.GetServices<IWebModule>();
    foreach(var module) {
        // service reg left out for readability
        module.Configure(app);
    }));

The MVC errors don't seem to be an issue with your approach, only that you're missing some services that need to be registered. Admittedly IWebModule can't register additional services. The only module system designed for that is HostingStartup. https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/platform-specific-configuration?view=aspnetcore-3.1

[maelscuttle]

@Tratcher

Thank you. I wasn't aware of HostingStartup, which however doesn't fit my use case as it works on an assembly level, not a type level it seems.

I'm not really trying to call Configure multiple times. What I am essentially trying to do is configure both the IServiceCollection as well as the IApplicationBuilder using a type-based module approach, so I can add entire sets of "features" to my web app (e.g. IWebHostBuilder.AddModule<SitemapModule>()) without having to write explicit extension methods, for both the services and the app builder, and also that I can scan for them by convention within an assembly.

What I kinda fail to understand is why my modules technically can't register additional services. I know, the service provider is already built when I would resolve them, so that doesn't make much sense, however when browsing through the AspNetCore hosting code (especially the GenericWebHostBuilder), it seems to me that when using IWebHostBuilder.Configure(), it internally registers a IStartup (DelegateStartup) which is then resolved using an internal IServiceProvider, that THEN calls ConfigureServices, and later on a new IServiceProvider is being built. So that could technically work?

My MVC errors directly relate to that issue. The exception occurs when I try to resolve my modules from IApplicationBuilder.ApplicationServices, because I'm using RazorPages and I haven't called IServiceCollection.AddRazorPages() yet. The reason: I'd like to have a RazorPagesWebModule like this:

public class RazorPagesWebModule : IWebModule {

    public void ConfigureServices(IServiceCollection services) {
        services.AddRazorPages();
    }

    public void Configure(IApplicationBuilder app) {
    
    }
}

But I feel like something like that just isn't possible?

[Tratcher]

Don't pay too much attention to what's going on inside WebHostBuilder. Yes that code did have two service containers, but it caused multiple issues with service lifetimes and we had to deprecate that model. The new model in 3.0 uses HostBuilder and a single service container. Startup is no longer resolved from the container, it's activated directly using only services that the host owns (IConfiguration, IHostEnvironment, etc.).

Because there's only one container now you can't register services, resolve them, and then register additional ones. To build a dynamic module system like this you need to provide your own container or other way of discovering, registering, and activating the modules in advance such that they can then register their own services on the apps container before it gets built.

[davidfowl]

Orchard supports this natively, I'd recommend starting there
cc @sebastienros