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Service Discovery

.NET Aspire includes functionality for configuring service discovery at development and testing time. This allows resources to refer to other resources by their name and have the concrete address details resolved at runtime. Service discovery functionality works by providing configuration in the format expected by the configuration-based endpoint resolver from the .NET Aspire App Host project to the individual service projects added to the application model.

Service Discovery Configuration

Currently, the MyWeatherHub is using static configuration in its appsettings.json file to connect to the Api. This is not ideal for several reasons including:

  • The port number of the Api service may change.
  • The IP address of the Api service may change.
  • Multiple configuration settings would need to be defined for http and https settings.
  • As we add more services, the configuration would become more complex.

To address these issues, we will use the service discovery functionality provided by the .NET Aspire App Host project. This will allow the MyWeatherHub service to discover the Api service at runtime.

  1. Open the Program.cs file in the AppHost project.

  2. Earlier we added orchestration to include several projects by using the builder.AddProject method. This returned an IResourceBuilder<TResource> that can be used to reference other resources. Let's reference the Api project in the MyWeatherHub project by updating the code:

    var api = builder.AddProject<Projects.Api>("api");

var web = builder.AddProject<Projects.MyWeatherHub>("myweatherhub")
    .WithReference(api)
    .WithExternalHttpEndpoints();

  3. The WithReference method is used to reference the Api project. This will inject the required configuration to allow the MyWeatherHub project to discover the Api project at runtime via its resource name, api.

  4. If you later choose to deploy this app, you'd need the call to WithExternalHttpEndpoints to ensure that it's public to the outside world.

Enabling Service Discovery in MyWeatherHub

When we added ServiceDefaults to the projects we automatically enrolled them in the service discovery system. This means that the MyWeatherHub project is already configured to use service discovery.

Some services expose multiple, named endpoints. By default, the scheme portion of the URI is used to refer to the name of the endpoint being resolved, e.g. the URI https://basket will resolve an endpoint named https on the basket service. By default in Aspire, project resources declare their endpoints according to the contents of their launchSettings.json file, so most projects will by default receive https and http named endpoints. The scheme portion of the referring URI can include multiple names separated by a + character and in preference order, e.g. https+http://basket will attempt to resolve the https named endpoint, and if not found it will resolve the http endpoint, of the basket service.

For cases when the named endpoint does not match the intended scheme, they can also be resolved explicitly by specifying the endpoint name in the first sub-domain section of the host portion of the request URI, when the first section is prefixed with an underscore (_), following the format scheme://_endpointName.serviceName. For example, if a service named "basket" exposes an HTTPS endpoint named "dashboard", then the URI https+http://_dashboard.basket can be used to specify this endpoint, for example:

builder.Services.AddHttpClient<BasketServiceClient>(
    static client => client.BaseAddress = new("https+http://basket"));

builder.Services.AddHttpClient<BasketServiceDashboardClient>(
    static client => client.BaseAddress = new("https+http://_dashboard.basket"));

In the above example, the BasketServiceClient will use the https or http endpoint of the basket service, while the BasketServiceDashboardClient will use the dashboard endpoint of the basket service, via either the HTTPS or HTTP schemes, depending on which is available.

Now, let's update the MyWeatherHub project to use service discovery to connect to the Api service. This can be accomplished by updating the existing WeatherEndpoint configuration settings in the appsettings.json. This is convenient when enabling .NET Aspire in an existing deployed application as you can continue to use your existing configuration settings.

  1. Open the appsettings.json file in the MyWeatherHub project.

  2. Update the WeatherEndpoint configuration settings to use service discovery:

    "WeatherEndpoint": "https+http://api"

  3. The WeatherEndpoint configuration setting is now using service discovery to connect to the Api service.

Alternatively, we can update the url to not use the WeatherEndpoint configuration settings and instead set the URI directly in the NwsManager class:

  1. Open the Program.cs file in the MyWeatherHub project.

  2. Update the WeatherEndpoint configuration settings to use service discovery:

    builder.Services.AddHttpClient<NwsManager>(
    static client => client.BaseAddress = new("https+http://api"));

Run the Application

  1. Run the application by pressing F5 or selecting the Start Debugging option.

  2. Open the MyWeatheApp by selecting the endpoint in the dashboard.

  3. Notice that the MyWeatherHub app still works and is now using service discovery to connect to the Api service.

  4. In the dashboard click on the Details for the MyWeatherHub project. This will bring up all of the settings that .NET Aspire configured when running the app from the App Host.

  5. Click on the eye icon to reveal the values and scroll to the bottom where you will see services__api__http__0 and services__api__https__0 configured with the correct values of the Api service.`

    Service discovery settings in the dashboard

Conclusion

This was just the start of what we can do with service discovery and .NET Aspire. As our application grows and we add more services, we can continue to use service discovery to connect services at runtime. This will allow us to easily scale our application and make it more resilient to changes in the environment.

Learn More

You can learn more about advanced usage and configuration of service discovery in the .NET Aspire Service Discovery documentation.