Nested integration testing

[benmccallum]

I have been trying to setup (somewhat successfully) an integration test that covers our gateway service + x number of downstream services that the gateway calls via named HttpClients; by using WebApplicationFactory inception.

Basically I've:

1. Used WebApplicationFactory<TGatewayStartup> and configured it to swap out its IHttpClientFactory with my own implementation
2. Have an IHttpClientFactory mock which is just a lookup by client name into a built-up dictionary of WebApplicationFactory-spawned HttpClient instances for each downstream service.

This all works well until it come to auth, as we have a delegating handler that passes on the JWT in the header.

The problem I have though is that I need to create the HttpClient instances for the downstream services upfront, and although I can create the handler lazily off the WebApplicationFactory<TGatewayStartup> instance's .Services, it's then singleton, unlike how I would've registered it in my TGatewayStartup as transient and then configured with the IHttpClientBuilder extension .AddHttpHandler<AuthenticationDelegatingHandler>().

Our handler has this ctor:

        public AuthenticationDelegatingHandler(
            IHttpContextAccessor httpContextAccessor,
            ILogger<AuthenticationDelegatingHandler> logger,
            IConfiguration configuration)

I believe IHttpContextAccessor is registered singleton, so potentially AuthenticationDelegatingHandler could be singleton too and I shouldn't worry?

I guess what I'm wondering is whether there's anything at the framework level that could be introduced that would facilitate this nesting integration testing. i.e. Something that let's me "take-over/swap-out" an existing named httpclient (keeping intact its handlers but pointing it to the relevant downstream in-memory TestServer) rather than swapping out IHttpClientFactory entirely, which would also be better as I have a feeling in future that my IHttpClientFactory mock will need to start handling other clients that the gateway may use for other http calls.

Thanks for anyone who joins in on this discussion!