Why does Azure's HealtchCheck service's request give a different result? - asp.net-core

Question

My ASP.NET application has the healthcheck service enabled with the default options, so a request to /hc returns "healthy" and that's all. The application itself requires an authenticated user.
services.AddHealthChecks();
app.UseHealthChecks("/hc");
In Azure, the webapp's health check is configured to /hc. This works fine for other webapps but on this particular one, I get a weird behavior as shown in these logs:
2022-12-08T15:38:32.753925985Z: [INFO] my-app : [ff3978bc-25c6-45f3-8d5e-134d7501ae3f] Incoming request on /hc
2022-12-08T15:38:32.753952886Z: [INFO] my-app : [ff3978bc-25c6-45f3-8d5e-134d7501ae3f] Request to TokenService: Endpoint my-app.azurewebsites.net, Port (null), Path /hc, Query , Method GET, UserAgent HealthCheck/1.0
2022-12-08T15:38:32.754763126Z: [INFO] my-app : [ff3978bc-25c6-45f3-8d5e-134d7501ae3f] Returning response for Site , Endpoint my-app.azurewebsites.net, Port (null), Path /hc, Method GET, Result = 404
2022-12-08T15:38:45.032379314Z: [INFO] info: Microsoft.AspNetCore.Hosting.Diagnostics[1]
2022-12-08T15:38:45.033372564Z: [INFO] Request starting HTTP/1.1 GET http://my-app.mycompany.net/hc?manualtest - -
2022-12-08T15:38:45.033419666Z: [INFO] info: Microsoft.AspNetCore.Hosting.Diagnostics[2]
2022-12-08T15:38:45.033428067Z: [INFO] Request finished HTTP/1.1 GET http://my-app.mycompany.net/hc?manualtest - - - 200 - text/plain 0.3773ms
Why does the request from UserAgent HealthCheck/1.0 involves the TokenService and then fails with a 404 status? If I manually call the /hc endpoint with a browser (for example the request with ?manualtest), curl or another application, it answers properly with a 200 status and "Healthy" body.
It's not a big deal but it's annoying because Azure thinks my webapp is unhealthy.

Answer 1

Just to highlight on what App Service does with Health checks
Further App Service Always ON also generate periodic (every 5min) probes to app instances and causing failed requests with error 404, and this can be found in Application Insights.
You may try rewriting the Always on path by referring this blog:
404 response code caused by App Services– AlwaysOn feature might be helpful
Also check this official document on Health checks in ASP.NET Core for relevant coding
You might refer to Azure App Service Health check details