Queue Groups

When subscribers register themselves to receive messages from a publisher, the 1:N fan-out pattern of messaging ensures that any message sent by a publisher, reaches all subscribers that have registered. NATS provides an additional feature named "queue", which allows subscribers to register themselves as part of a queue. Subscribers that are part of a queue, form the "queue group".

Code below demonstrates multiple subscriptions on the same queue group, receiving messages randomly distributed among them. This example also shows how queue groups can be used to load balance responders:

await using var nats = new NatsConnection();

var replyTasks = new List<Task>();
var cts = new CancellationTokenSource();

for (var i = 0; i < 3; i++)
{
    // Create three subscriptions all on the same queue group
    // Create a background message loop for every subscription
    var replyTaskId = i;
    replyTasks.Add(Task.Run(async () =>
    {
        // Retrieve messages until unsubscribed
        await foreach (var msg in nats.SubscribeAsync<int>("math.double", queueGroup: "maths-service", cancellationToken: cts.Token))
        {
            Console.WriteLine($"[{replyTaskId}] Received request: {msg.Data}");
            await msg.ReplyAsync($"Answer is: {2 * msg.Data}");
        }

        Console.WriteLine($"[{replyTaskId}] Done");
    }));
}

// Give subscriptions time to start
await Task.Delay(1000);

// Send a few requests
for (var i = 0; i < 10; i++)
{
    var reply = await nats.RequestAsync<int, string>("math.double", i);
    Console.WriteLine($"Reply: '{reply.Data}'");
}

Console.WriteLine("Stopping...");

// Cancellation token will unsubscribe and complete the message loops
cts.Cancel();

// Make sure all tasks finished cleanly
await Task.WhenAll(replyTasks);

Console.WriteLine("All done");

Output should look similar to this:

[1] Received request: 0
Reply: 'Answer is: 0'
[2] Received request: 1
Reply: 'Answer is: 2'
[2] Received request: 2
Reply: 'Answer is: 4'
[2] Received request: 3
Reply: 'Answer is: 6'
[2] Received request: 4
Reply: 'Answer is: 8'
[0] Received request: 5
[0] Received request: 7
Reply: 'Answer is: 14'
[1] Received request: 8
Reply: 'Answer is: 16'
[1] Received request: 9
Reply: 'Answer is: 18'
Stopping...
[2] Done
[0] Done
[1] Done
All done

Table of Contents

Queue Groups