Service discovery

Service discovery is how your services find each other. Instead of hardcoding URLs like http://localhost:5001, your frontend can simply reference the catalog service by name—and Aspire handles resolving the actual address.

How service discovery works

When you connect services using WithReference(), Aspire sets up automatic service discovery:

1. AppHost declares resources and allocates endpoints
2. Configuration is injected into each service via environment variables
3. Your code uses logical names like https+http://catalog
4. Aspire's resolver translates names to actual addresses at runtime

The key insight: Your code uses logical service names (like catalog), and Aspire's configuration-based endpoint resolver translates those to actual addresses at runtime.

[!TIP] Why this matters Service discovery eliminates "works on my machine" problems. The same code works locally (where catalog might be localhost:5001) and in production (where it might be catalog.internal.cloudapp.net:443)—without any code changes.

Implicit service discovery by reference

Configuration for service discovery is only added for services that are referenced by a given project. For example, consider the following AppHost program:

var builder = DistributedApplication.CreateBuilder(args);
  
var catalog = builder.AddProject<Projects.CatalogService>("catalog");
var basket = builder.AddProject<Projects.BasketService>("basket");
  
var frontend = builder.AddProject<Projects.MyFrontend>("frontend")
                      .WithReference(basket)
                      .WithReference(catalog);

In the preceding example, the frontend project references the catalog project and the basket project. The two WithReference calls instruct Aspire to:

1. Inject configuration - Pass service discovery information for catalog and basket into frontend
2. Enable resolution - Allow frontend to use URIs like https+http://catalog in HttpClient

Named endpoints

Some services expose multiple, named endpoints. Named endpoints can be resolved by specifying the endpoint name in the host portion of the HTTP request URI, following the format scheme://_endpointName.serviceName. For example, if a service named "basket" exposes an 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 preceding example, two HttpClient classes are added, one for the core basket service and one for the basket service's dashboard.

Named endpoints using configuration

With the configuration-based endpoint resolver, named endpoints can be specified in configuration by prefixing the endpoint value with _endpointName., where endpointName is the endpoint name. For example, consider this appsettings.json configuration which defined a default endpoint (with no name) and an endpoint named "dashboard":

{
  "Services": {
    "basket": {
      "https": "https://10.2.3.4:8080" /* the https endpoint, requested via https://basket */,
      "dashboard": "https://10.2.3.4:9999" /* the "dashboard" endpoint, requested via https://_dashboard.basket */
    }
  }
}

In the preceding JSON:

• The default endpoint, when resolving https://basket is 10.2.3.4:8080.
• The "dashboard" endpoint, resolved via https://_dashboard.basket is 10.2.3.4:9999.

Named endpoints in Aspire

Named endpoints can also be exposed by code in the App Host. For instance the previous example can be modeled as:

var basket = builder.AddProject<Projects.BasketService>("basket")
    .WithHttpsEndpoint(port: 9999, name: "dashboard");

Named endpoints in Kubernetes using DNS SRV

When deploying to Kubernetes, the DNS SRV service endpoint resolver can be used to resolve named endpoints. For example, the following resource definition will result in a DNS SRV record being created for an endpoint named "default" and an endpoint named "dashboard", both on the service named "basket".

apiVersion: v1
kind: Service
metadata:
  name: basket
spec:
  selector:
    name: basket-service
  clusterIP: None
  ports:
    - name: default
      port: 8080
    - name: dashboard
      port: 9999

To configure a service to resolve the "dashboard" endpoint on the "basket" service, add the DNS SRV service endpoint resolver to the host builder as follows:

builder.Services.AddServiceDiscoveryCore();
builder.Services.AddDnsSrvServiceEndpointProvider();

For more information, see AddServiceDiscoveryCore and AddDnsSrvServiceEndpointProvider.

The special port name "default" is used to specify the default endpoint, resolved using the URI https://basket.

As in the previous example, add service discovery to an HttpClient for the basket service:

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

Similarly, the "dashboard" endpoint can be targeted as follows:

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

Troubleshooting

Service not found or "No endpoints resolved"

Symptoms: HttpClient throws an exception like "No endpoints resolved for service 'myservice'" or connection is refused.

Common causes:

1. Missing WithReference() - The calling service wasn't connected to the target service in AppHost
2. Wrong service name - The name in your URI doesn't match the name in AddProject() or AddContainer()
3. Service not started - The target service failed to start or is still starting

Solution: Verify your AppHost connects the services:

var api = builder.AddProject<Projects.Api>("api");
var frontend = builder.AddProject<Projects.Frontend>("frontend")
    .WithReference(api);  // ← This is required!

Named endpoint doesn't resolve

Symptoms: Using https://_endpointName.serviceName returns an error.

Solution: Verify the endpoint is named in your AppHost:

var api = builder.AddProject<Projects.Api>("api")
    .WithHttpEndpoint(port: 9999, name: "dashboard");  // ← Name must match

Then use the exact name with underscore prefix:

client.BaseAddress = new("https://_dashboard.api");

Understanding the URI format

The https+http:// prefix means "prefer HTTPS, fall back to HTTP". Here's the breakdown:

[!NOTE] The underscore prefix The _ prefix before an endpoint name is not arbitrary—it's required syntax that tells the resolver you're specifying a named endpoint rather than a subdomain.