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
C
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()
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();
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");
},
});
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
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.Last modified 1yr ago