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
Java
JavaScript
Python
Ruby
TypeScript
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
d.subscribe("updates", "workers");
// Wait for a message to come in
latch.await();
// Close the connection
nc.close();
JavaScript
let nc = NATS.connect({
url: "nats://demo.nats.io:4222"});
nc.subscribe('updates', {queue: "workers"}, (msg) => {
t.log('worker got message', msg);
});
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)
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
TypeScript
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.