Skip to content

Latest commit

 

History

History
160 lines (121 loc) · 3.66 KB

queues.md

File metadata and controls

160 lines (121 loc) · 3.66 KB
Raw

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:

{% tabs %} {% tab title="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()

{% endtab %}

{% tab title="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();

{% endtab %}

{% tab title="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");
    },
});

{% endtab %}

{% tab title="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)

{% endtab %}

{% tab title="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

{% endtab %}

{% tab title="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);

{% endtab %} {% endtabs %}

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.