FallbackSpaProxyingMiddleware 1.0.0

Fallback SPA proxying middleware

This proxy is only intended for use in development, to support HMR and the other features that frontend SPA frameworks support with their built-in web servers. It must not be used in production.

The default SpaServices Microsoft.AspNetCore.SpaProxy is broken and the development-mode SPA proxy tramples all over any defined MVC-style endpoints, directing all requests to the SPA development server, even those you might like to be served by an API controller.

Microsoft claim that this is intentional and that it should be up to the SPA to determine which endpoints to proxy back to the backend. This behaviour is illogical, poorly-documented and differs completely between development and production (where we use UseSpaStaticFiles / endpoints.MapFallbackToFile).

Microsoft will not discuss or even properly document this logic and have locked numerous issues on the matter:

• https://github.com/dotnet/AspNetCore.Docs/issues/18405
• https://github.com/dotnet/aspnetcore/issues/45130#issuecomment-1326595534

This middleware plays nicely with other MVC endpoints and allows them to serve requests first where a route has been defined. It can be added as follows:

In Program.cs:

// ....
app.UseRouting();
app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.UseSpa(s =>
{
    if (app.Environment.IsDevelopment())
    {
        s.ApplicationBuilder.UseFallbackSpaProxying("http://localhost:9999");
    }
    // .....
});

The implementation is a bit janky, relying on reflection because the classes we need are marked internal, but it works.

FAQ

You should be using the NPM http-proxy-middleware package and running it under Node.js, not doing this!

No. The server is Kestrel. That's what we are using to do the proxying. The frontend is not in charge here.