Service Discovery for Inter-Service HTTP Calls

The Standard

When one service calls another over HTTP inside the platform, it MUST address the target by a logical service name resolved through a service-discovery client (e.g. Steeltoe + Consul) with client-side load balancing, not by a hardcoded host/port or static DNS name baked into configuration.

Why

The "before" sample has no HTTP client for the reporting service at all in Newsletter.Api — any inter-service call would have had to hardcode a URL (as MessageBroker:Host does for RabbitMQ: "amqp://rabbitmq:5672"), which breaks the moment the target service moves, scales to multiple instances, or changes port/host in a different environment. The "after" sample instead registers AddServiceDiscovery(o => o.UseConsul()) and configures the typed client with AddHttpClient<GetReportingArticle.Client>(client => client.BaseAddress = new Uri("http://reporting-service")).AddServiceDiscovery().AddRoundRobinLoadBalancer(). The base address uses the service's logical name (reporting-service), and Steeltoe's discovery client resolves it against Consul's service registry at call time, load-balancing round-robin across available instances. This decouples the caller from any specific host/port and supports horizontal scaling without config changes.

Before (Anti-pattern)

// No service-discovery-aware client - an inter-service call would have to
// hardcode a host, exactly like the message broker config here does:
configurator.Host(new Uri(builder.Configuration["MessageBroker:Host"]!), h => { /* ... */ });
// (No HTTP client existed yet to call the reporting service.)

After (Standard)

builder.Services.AddServiceDiscovery(o => o.UseConsul());

builder.Services
    .AddHttpClient<GetReportingArticle.Client>(client =>
        client.BaseAddress = new Uri("http://reporting-service"))
    .AddServiceDiscovery()
    .AddRoundRobinLoadBalancer();

// Client just calls the logical name - resolution/load balancing happens underneath.
public sealed class Client(HttpClient httpClient)
{
    public async Task<Response?> GetAsync(Guid id) =>
        await httpClient.GetFromJsonAsync<Response>($"api/articles/{id}");
}

Rules for LLMs / Agents

• Register AddServiceDiscovery(...) (with the platform's chosen registry, e.g. Consul) once per service that needs to call other internal services over HTTP.
• Configure typed HttpClients for internal services with a BaseAddress using the target's logical service name (http://<service-name>), never a hardcoded host/IP/port.
• Chain .AddServiceDiscovery().AddRoundRobinLoadBalancer() (or the equivalent load-balancing strategy) onto every internal typed HTTP client registration.
• Keep the actual HTTP call code (the typed client's methods) unaware of discovery — it should just call relative paths against HttpClient; discovery/load-balancing is purely a registration-time concern.
• Do not put internal service URLs in appsettings.json/environment variables as literal hosts — externally-facing/third-party APIs (e.g. https://www.alphavantage.co) are the exception and may remain as configured absolute URLs.

When NOT to apply

External third-party APIs (payment providers, market data feeds, etc.) are addressed by their real DNS name/URL as normal — service discovery only applies to calls between services owned and deployed within the same platform/registry.