订阅队列#
订阅一个 队列组 与单独订阅主题只有一点点不同。应用程序订阅时只需包含一个队列名称即可。而服务器会在队列组的所有成员之间做负载均衡。在 NATS 集群中，每个成员都有相同的机会接收到特定的消息。
请注意，NATS 中的队列组是动态的，无需任何服务器配置。
例如，要订阅名为 workers 的队列，并监听主题 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 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();{% 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="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");{% 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 %}
如果你运行此示例，并同时运行发送到 updates 的发布示例，你会看到其中一个实例会收到一条消息，而其他实例则不会。但接收消息的实例可能会发生变化。