Why should you care?

ShopNest order_placed fans out to inventory, email, and analytics independently.

See it live — copy this example

Paste into a C# console or class library project and run dotnet run.

public sealed class InProcBus
{
    private readonly Dictionary<string, List<Action<string>>> _subs = new();
    public void Subscribe(string topic, Action<string> handler)
    {
        if (!_subs.TryGetValue(topic, out var list)) _subs[topic] = list = new();
        list.Add(handler);
    }
    public void Publish(string topic, string message)
    {
        if (_subs.TryGetValue(topic, out var list))
            foreach (var h in list) h(message);
    }
}

var bus = new InProcBus();
bus.Subscribe("order_placed", m => Console.WriteLine($"email {m}"));
bus.Subscribe("order_placed", m => Console.WriteLine($"index {m}"));
bus.Publish("order_placed", "o-9");

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• One publish notifies all subscribers.
• Production uses Rabbit/Kafka; the idea stays the same.
• Failures in one subscriber should not erase others (isolate).

Practice next

1. Publish and see two handlers.
2. Add analytics subscriber.
3. Move to a real broker for cross-process.
4. Unsubscribe a handler.
5. Topic per event type.

Remember

One-to-many messaging. Loose coupling. Broker for distribution.

ShopNest order_placed fan-out

Bus notifies email + search.

Outcome: Email outage does not block indexing if isolated.