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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 effect of including the group is fairly major, since the server will now load balance messages between the members of the queue group, but the code differences are minimal.

Keep in mind that the queue groups in NATS are dynamic and do not require any server configuration. You can almost think of a regular subscription as a queue group of 1, but it is probably not worth thinking too much about that.

digraph g { rankdir=LR publisher [shape=box, style="rounded", label="PUB updates"]; subject [shape=circle, label="nats-server"]; sub1 [shape=box, style="rounded", label="SUB updates workers"]; sub2 [shape=box, style="rounded", label="SUB updates workers"]; sub3 [shape=box, style="rounded", label="SUB updates workers"]; publisher -> subject [label="msgs 1,2,3"]; subject -> sub1 [label="msg 2"]; subject -> sub2 [label="msg 1"]; subject -> sub3 [label="msg 3"]; }

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

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", "worker", func(m *nats.Msg) {
	wg.Done()
}); err != nil {
	log.Fatal(err)
}

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


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
d.subscribe("updates", "workers");

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

// Close the connection
nc.close();

let nc = NATS.connect({
    url: "nats://demo.nats.io:4222"});

nc.subscribe('updates', {queue: "workers"}, (msg) => {
    t.log('worker got message', msg);
});

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)


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


await nc.subscribe('updates', (err, msg) => {
    t.log('worker got message', msg.data);
}, {queue: "workers"});

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.

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