Including a Reply Subject

The optional reply-to field when publishing a message can be used on the receiving side to respond. The reply-to subject is often called an inbox, and most libraries may provide a method for generating unique inbox subjects. Most libraries also provide for the request-reply pattern with a single call. For example to send a request to the subject time, with no content for the messages, you might:

Go

nc, err := nats.Connect("demo.nats.io")
if err != nil {
    log.Fatal(err)
}
defer nc.Close()

// Create a unique subject name for replies.
uniqueReplyTo := nats.NewInbox()

// Listen for a single response
sub, err := nc.SubscribeSync(uniqueReplyTo)
if err != nil {
    log.Fatal(err)
}

// Send the request.
// If processing is synchronous, use Request() which returns the response message.
if err := nc.PublishRequest("time", uniqueReplyTo, nil); err != nil {
    log.Fatal(err)
}

// Read the reply
msg, err := sub.NextMsg(time.Second)
if err != nil {
    log.Fatal(err)
}

// Use the response
log.Printf("Reply: %s", msg.Data)

Java

Connection nc = Nats.connect("nats://demo.nats.io:4222");

// Create a unique subject name
String uniqueReplyTo = NUID.nextGlobal();

// Listen for a single response
Subscription sub = nc.subscribe(uniqueReplyTo);
sub.unsubscribe(1);

// Send the request
nc.publish("time", uniqueReplyTo, null);

// Read the reply
Message msg = sub.nextMessage(Duration.ofSeconds(1));

// Use the response
System.out.println(new String(msg.getData(), StandardCharsets.UTF_8));

// Close the connection
nc.close();

JavaScript

// set up a subscription to process the request
const sc = StringCodec();
nc.subscribe("time", {
  callback: (_err, msg) => {
    msg.respond(sc.encode(new Date().toLocaleTimeString()));
  },
});

// create a subscription subject that the responding send replies to
const inbox = createInbox();
const sub = nc.subscribe(inbox, {
  max: 1,
  callback: (_err, msg) => {
    t.log(`the time is ${sc.decode(msg.data)}`);
  },
});

nc.publish("time", Empty, { reply: inbox });

Python

nc = NATS()

future = asyncio.Future()

async def sub(msg):
  nonlocal future
  future.set_result(msg)

await nc.connect(servers=["nats://demo.nats.io:4222"])
await nc.subscribe("time", cb=sub)

unique_reply_to = nc.new_inbox()
await nc.publish("time", b'', unique_reply_to)

# Use the response
msg = await asyncio.wait_for(future, 1)
print("Reply:", 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("time") do |msg, reply|
      f.resume msg
    end

    nc.publish("time", 'example', NATS.create_inbox)

    # Use the response
    msg = Fiber.yield
    puts "Reply: #{msg}"

  end.resume
end

C

natsConnection      *conn      = NULL;
natsStatus          s          = NATS_OK;

s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);
// Publish a message and provide a reply subject
if (s == NATS_OK)
    s = natsConnection_PublishRequestString(conn, "request", "reply", "this is the request");

(...)

// Destroy objects that were created
natsConnection_Destroy(conn);