Problem with python + Scalar Integration - Service Discovery broken

[johanndev]

I created an app from the fastapi template and added scalar to it:

#:sdk Aspire.AppHost.Sdk@13.2.0-preview.1.26105.2
#:package Aspire.Hosting.Python@13.2.0-preview.1.26105.2
#:package Scalar.Aspire@0.8.43
using Scalar.Aspire;
var builder = DistributedApplication.CreateBuilder(args);
var app = builder.AddUvicornApp("app", "./app", "main:app")
    .WithUv()
    .WithExternalHttpEndpoints()
    .WithHttpHealthCheck("/health");
var scalar = builder.AddScalarApiReference();
scalar
    .WithApiReference(app, options =>
    {
        options
            .AddDocument("openapi", "FastApi OpenAPI")
            .WithOpenApiRoutePattern("/openapi.json");
    });

builder.Build().Run();

When I access the openapi document directly (via https://app-aspire-py-scalar.dev.localhost:53309/openapi.json), the document is rendered, so the OpenApi integration in FastApi works.

When I open the scalar api reference, I get an error: Document 'app--fastapi-openapi' could not be loaded
Looking at the network tab, I see a failed (Statuscode 500) request to http://localhost:53308/scalar-proxy?scalar_url=https%3A%2F%2Fapp%2Fopenapi.json

Logs from the scalar resource attached, key line:

warn: Yarp.ReverseProxy.Forwarder.HttpForwarder[48]
      Request: An error was encountered before receiving a response.
      System.Net.Http.HttpRequestException: Name does not resolve (app:443)

So somehow service discovery seems to be broken :/

Does anyone have any idea what I should do differently? Thanks!

d4077c9b2a71be5f0cd59d50b71b560fba507b401fe8c2b832da60c7e59778a8
info: Scalar.Aspire.Service[825120733]
      Proxy request received for target URL: https://app/openapi.json
dbug: Scalar.Aspire.Service[1099408831]
      Forwarding request to target host: https://app, URI: https://app/openapi.json
warn: Yarp.ReverseProxy.Forwarder.HttpForwarder[48]
      Request: An error was encountered before receiving a response.
      System.Net.Http.HttpRequestException: Name does not resolve (app:443)
       ---> System.Net.Sockets.SocketException (0xFFFDFFFF): Name does not resolve
         at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
         at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
         at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
         --- End of inner exception stack trace ---
         at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
         at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
         at System.Net.Http.HttpConnectionPool.InjectNewHttp2ConnectionAsync(QueueItem queueItem)
         at System.Threading.Tasks.TaskCompletionSourceWithCancellation`1.WaitWithCancellationAsync(CancellationToken cancellationToken)
         at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
         at System.Net.Http.DiagnosticsHandler.SendAsyncCore(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
         at System.Net.Http.SocketsHttpHandler.<SendAsync>g__CreateHandlerAndSendAsync|115_0(HttpRequestMessage request, CancellationToken cancellationToken)
         at Microsoft.Extensions.ServiceDiscovery.Http.ResolvingHttpDelegatingHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
         at Yarp.ReverseProxy.Forwarder.HttpForwarder.SendAsync(HttpContext context, String destinationPrefix, HttpMessageInvoker httpClient, ForwarderRequestConfig requestConfig, HttpTransformer transformer, CancellationToken cancellationToken)
fail: Scalar.Aspire.Service[84655849]
      Proxy error occurred: Request for target URL: https://app/openapi.json
info: Scalar.Aspire.Service[825120733]
      Proxy request received for target URL: https://app/openapi.json
dbug: Scalar.Aspire.Service[1099408831]
      Forwarding request to target host: https://app, URI: https://app/openapi.json
warn: Yarp.ReverseProxy.Forwarder.HttpForwarder[48]
      Request: An error was encountered before receiving a response.
      System.Net.Http.HttpRequestException: Name does not resolve (app:443)
       ---> System.Net.Sockets.SocketException (0xFFFDFFFF): Name does not resolve
         at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
         at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
         at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
         --- End of inner exception stack trace ---
         at System.Net.Http.HttpConnectionPool.ConnectToTcpHostAsync(String host, Int32 port, HttpRequestMessage initialRequest, Boolean async, CancellationToken cancellationToken)
         at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
         at System.Net.Http.HttpConnectionPool.InjectNewHttp2ConnectionAsync(QueueItem queueItem)
         at System.Threading.Tasks.TaskCompletionSourceWithCancellation`1.WaitWithCancellationAsync(CancellationToken cancellationToken)
         at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
         at System.Net.Http.DiagnosticsHandler.SendAsyncCore(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
         at Microsoft.Extensions.ServiceDiscovery.Http.ResolvingHttpDelegatingHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
         at Yarp.ReverseProxy.Forwarder.HttpForwarder.SendAsync(HttpContext context, String destinationPrefix, HttpMessageInvoker httpClient, ForwarderRequestConfig requestConfig, HttpTransformer transformer, CancellationToken cancellationToken)
fail: Scalar.Aspire.Service[84655849]
      Proxy error occurred: Request for target URL: https://app/openapi.json

[hanspagel]

Is the app host accessible from your aspire container? And does it use https? Because the message says it’s not able to reach it.

The request goes to https://app

[johanndev]

I tried to override the request to force Scalar to use the http Endpoint of the FastApi resource:
http://localhost:53308/scalar-proxy?scalar_url=http%3A%2F%2Fapp%2Fopenapi.json (changed the scalar_url param https->http)

info: Scalar.Aspire.Service[825120733]
      Proxy request received for target URL: http://app/openapi.json
dbug: Scalar.Aspire.Service[1099408831]
      Forwarding request to target host: http://app, URI: http://app/openapi.json
warn: Yarp.ReverseProxy.Forwarder.HttpForwarder[48]
      Request: An error was encountered before receiving a response.

So the error is identical between http and https.