Queue Subscriptions

Subscribing to a queue group is only slightly different than subscribing to a subject alone. The application simply includes a queue name with the subscription. The server will load balance between all members of the queue group. In a cluster setup, every member has the same chance of receiving a particular message.

Keep in mind that queue groups in NATS are dynamic and do not require any server configuration.

As an example, to subscribe to the queue workers with the subject updates:

Go

nc, err := nats.Connect("demo.nats.io")
if err != nil {
    log.Fatal(err)
}
defer nc.Close()

// Use a WaitGroup to wait for 10 messages to arrive
wg := sync.WaitGroup{}
wg.Add(10)

// Create a queue subscription on "updates" with queue name "workers"
if _, err := nc.QueueSubscribe("updates", "workers", func(m *nats.Msg) {
    wg.Done()
}); err != nil {
    log.Fatal(err)
}

// Wait for messages to come in
wg.Wait()

Java

Connection nc = Nats.connect("nats://demo.nats.io:4222");

// Use a latch to wait for 10 messages to arrive
CountDownLatch latch = new CountDownLatch(10);

// Create a dispatcher and inline message handler
Dispatcher d = nc.createDispatcher((msg) -> {
    String str = new String(msg.getData(), StandardCharsets.UTF_8);
    System.out.println(str);
    latch.countDown();
});

// Subscribe to the "updates" subject with a queue group named "workers"
d.subscribe("updates", "workers");

// Wait for a message to come in
latch.await(); 

// Close the connection
nc.close();

JavaScript

nc.subscribe(subj, {
    queue: "workers",
    callback: (_err, _msg) => {
      t.log("worker1 got message");
    },
});

nc.subscribe(subj, {
    queue: "workers",
    callback: (_err, _msg) => {
      t.log("worker2 got message");
    },
});

Python

nc = NATS()

await nc.connect(servers=["nats://demo.nats.io:4222"])

future = asyncio.Future()

async def cb(msg):
  nonlocal future
  future.set_result(msg)

await nc.subscribe("updates", queue="workers", cb=cb)
await nc.publish("updates", b'All is Well')

msg = await asyncio.wait_for(future, 1)
print("Msg", msg)

C#

// dotnet add package NATS.Net
using NATS.Net;

await using var client = new NatsClient();

var count = 0;

// Subscribe to the "updates" subject with a queue group named "workers"
await foreach (var msg in client.SubscribeAsync<string>(subject: "updates", queueGroup: "workers"))
{
    Console.WriteLine($"Received {++count}: {msg.Subject}: {msg.Data}");
    
    // Break after 10 messages
    if (count == 10)
    {
        break;
    }
}

Console.WriteLine("Done");

Ruby

require 'nats/client'
require 'fiber'

NATS.start(servers:["nats://127.0.0.1:4222"]) do |nc|
  Fiber.new do
    f = Fiber.current

    nc.subscribe("updates", queue: "worker") do |msg, reply|
      f.resume Time.now
    end

    nc.publish("updates", "A")

    # Use the response
    msg = Fiber.yield
    puts "Msg: #{msg}"
  end.resume
end

C

static void
onMsg(natsConnection *conn, natsSubscription *sub, natsMsg *msg, void *closure)
{
    printf("Received msg: %s - %.*s\n",
           natsMsg_GetSubject(msg),
           natsMsg_GetDataLength(msg),
           natsMsg_GetData(msg));

    // Need to destroy the message!
    natsMsg_Destroy(msg);
}


(...)

natsConnection      *conn = NULL;
natsSubscription    *sub  = NULL;
natsStatus          s;

s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);

// Create a queue subscription on "updates" with queue name "workers"
if (s == NATS_OK)
    s = natsConnection_QueueSubscribe(&sub, conn, "updates", "workers", onMsg, NULL);

(...)


// Destroy objects that were created
natsSubscription_Destroy(sub);
natsConnection_Destroy(conn);

If you run this example with the publish examples that send to updates, you will see that one of the instances gets a message while the others you run won't. But the instance that receives the message will change.