Release Notes
NATS Concepts
Using NATS
Running a NATS service
Reference
Unsubscribing After N Messages
NATS provides a special form of unsubscribe that is configured with a message count and takes effect when that many messages are sent to a subscriber. This mechanism is very useful if only a single message is expected.
The message count you provide is the total message count for a subscriber. So if you unsubscribe with a count of 1, the server will stop sending messages to that subscription after it has received one message. If the subscriber has already received one or more messages, the unsubscribe will be immediate. This action based on history can be confusing if you try to auto-unsubscribe on a long running subscription, but is logical for a new one.
Auto-unsubscribe is based on the total messages sent to a subscriber, not just the new ones. Most of the client libraries also track the max message count after an auto-unsubscribe request. On reconnect, this enables clients to resend the unsubscribe with an updated total.
The following example shows unsubscribe after a single message:
Go
Java
JavaScript
Python
Ruby
C
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nc, err := nats.Connect("demo.nats.io")
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if err != nil {
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log.Fatal(err)
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}
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defer nc.Close()
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​
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// Sync Subscription
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sub, err := nc.SubscribeSync("updates")
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if err != nil {
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log.Fatal(err)
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}
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if err := sub.AutoUnsubscribe(1); err != nil {
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log.Fatal(err)
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}
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​
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// Async Subscription
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sub, err = nc.Subscribe("updates", func(_ *nats.Msg) {})
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if err != nil {
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log.Fatal(err)
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}
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if err := sub.AutoUnsubscribe(1); err != nil {
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log.Fatal(err)
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}
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Connection nc = Nats.connect("nats://demo.nats.io:4222");
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Dispatcher d = nc.createDispatcher((msg) -> {
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String str = new String(msg.getData(), StandardCharsets.UTF_8);
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System.out.println(str);
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});
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​
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// Sync Subscription
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Subscription sub = nc.subscribe("updates");
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sub.unsubscribe(1);
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​
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// Async Subscription
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d.subscribe("updates");
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d.unsubscribe("updates", 1);
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​
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// Close the connection
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nc.close();
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const sc = StringCodec();
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// `max` specifies the number of messages that the server will forward.
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// The server will auto-cancel.
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const subj = createInbox();
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const sub1 = nc.subscribe(subj, {
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callback: (_err, msg) => {
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t.log(`sub1 ${sc.decode(msg.data)}`);
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},
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max: 10,
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});
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​
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// another way after 10 messages
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const sub2 = nc.subscribe(subj, {
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callback: (_err, msg) => {
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t.log(`sub2 ${sc.decode(msg.data)}`);
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},
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});
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// if the subscription already received 10 messages, the handler
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// won't get any more messages
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sub2.unsubscribe(10);
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nc = NATS()
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​
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await nc.connect(servers=["nats://demo.nats.io:4222"])
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​
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async def cb(msg):
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print(msg)
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​
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sid = await nc.subscribe("updates", cb=cb)
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await nc.auto_unsubscribe(sid, 1)
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await nc.publish("updates", b'All is Well')
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​
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# Won't be received...
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await nc.publish("updates", b'...')
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require 'nats/client'
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require 'fiber'
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​
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NATS.start(servers:["nats://127.0.0.1:4222"]) do |nc|
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Fiber.new do
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f = Fiber.current
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​
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nc.subscribe("time", max: 1) do |msg, reply|
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f.resume Time.now
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end
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​
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nc.publish("time", 'What is the time?', NATS.create_inbox)
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​
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# Use the response
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msg = Fiber.yield
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puts "Reply: #{msg}"
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​
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# Won't be received
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nc.publish("time", 'What is the time?', NATS.create_inbox)
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​
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end.resume
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end
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natsConnection *conn = NULL;
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natsSubscription *sub = NULL;
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natsMsg *msg = NULL;
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natsStatus s = NATS_OK;
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​
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s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);
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​
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// Subscribe
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if (s == NATS_OK)
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s = natsConnection_SubscribeSync(&sub, conn, "updates");
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​
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// Unsubscribe after 1 message is received
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if (s == NATS_OK)
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s = natsSubscription_AutoUnsubscribe(sub, 1);
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​
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// Wait for messages
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if (s == NATS_OK)
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s = natsSubscription_NextMsg(&msg, sub, 10000);
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​
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if (s == NATS_OK)
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{
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printf("Received msg: %s - %.*s\n",
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natsMsg_GetSubject(msg),
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natsMsg_GetDataLength(msg),
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natsMsg_GetData(msg));
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​
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// Destroy message that was received
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natsMsg_Destroy(msg);
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}
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​
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(...)
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​
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// Destroy objects that were created
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natsSubscription_Destroy(sub);
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natsConnection_Destroy(conn);
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