request_reply.md

Request-Reply Semantics

The pattern of sending a message and receiving a response is encapsulated in most client libraries into a request method. Under the covers this method will publish a message with a unique reply-to subject and wait for the response before returning.

In the older versions of some libraries a completely new reply-to subject is created each time. In newer versions, a subject hierarchy is used so that a single subscriber in the client library listens for a wildcard, and requests are sent with a unique child subject of a single subject.

The primary difference between the request method and publishing with a reply-to is that the library is only going to accept one response, and in most libraries the request will be treated as a synchronous action. The library may even provide a way to set the timeout.

For example, updating the previous publish example we may request time with a one second timeout:

{% tabs %} {% tab title="Go" %}

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

// Send the request
msg, err := nc.Request("time", nil, time.Second)
if err != nil {
    log.Fatal(err)
}

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

// Close the connection
nc.Close()

{% endtab %}

{% tab title="Java" %}

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

// Send the request
Message msg = nc.request("time", null, Duration.ofSeconds(1));

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

// Close the connection
nc.close();

{% endtab %}

{% tab title="JavaScript" %}

let nc = NATS.connect({url: "nats://demo.nats.io:4222"});

// set up a subscription to process the request
nc.subscribe('time', (msg, reply) => {
    if(reply) {
        nc.publish(reply, new Date().toLocaleTimeString());
    }
});

nc.requestOne('time', (msg) => {
    t.log('the time is', msg);
    nc.close();
});

{% endtab %}

{% tab title="Python" %}

nc = NATS()

async def sub(msg):
  await nc.publish(msg.reply, b'response')

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

# Send the request
try:
  msg = await nc.request("time", b'', timeout=1)
  # Use the response
  print("Reply:", msg)
except asyncio.TimeoutError:
  print("Timed out waiting for response")

{% endtab %}

{% tab title="Ruby" %}

require 'nats/client'
require 'fiber'

NATS.start(servers:["nats://127.0.0.1:4222"]) do |nc|
  nc.subscribe("time") do |msg, reply|
    nc.publish(reply, "response")
  end

  Fiber.new do
    # Use the response
    msg = nc.request("time", "")
    puts "Reply: #{msg}"
  end.resume
end

{% endtab %}

{% tab title="TypeScript" %}

let msg = await nc.request('time', 1000);
t.log('the time is', msg.data);
nc.close();

{% endtab %} {% endtabs %}

You can think of request-reply in the library as a subscribe, get one message, unsubscribe pattern. In Go this might look something like:

sub, err := nc.SubscribeSync(replyTo)
if err != nil {
    log.Fatal(err)
}
nc.Flush()

// Send the request
nc.PublishRequest(subject, replyTo, []byte(input))

// Wait for a single response
for {
    msg, err := sub.NextMsg(1 * time.Second)
    if err != nil {
        log.Fatal(err)
    }

    response = string(msg.Data)
    break
}
sub.Unsubscribe()

Scatter-Gather

You can expand the request-reply pattern into something often called scatter-gather. To receive multiple messages, with a timeout, you could do something like the following, where the loop getting messages is using time as the limitation, not the receipt of a single message:

sub, err := nc.SubscribeSync(replyTo)
if err != nil {
    log.Fatal(err)
}
nc.Flush()

// Send the request
nc.PublishRequest(subject, replyTo, []byte(input))

// Wait for a single response
max := 100 * time.Millisecond
start := time.Now()
for time.Now().Sub(start) < max {
    msg, err := sub.NextMsg(1 * time.Second)
    if err != nil {
        break
    }

    responses = append(responses, string(msg.Data))
}
sub.Unsubscribe()

Or, you can loop on a counter and a timeout to try to get at least N responses:

sub, err := nc.SubscribeSync(replyTo)
if err != nil {
    log.Fatal(err)
}
nc.Flush()

// Send the request
nc.PublishRequest(subject, replyTo, []byte(input))

// Wait for a single response
max := 500 * time.Millisecond
start := time.Now()
for time.Now().Sub(start) < max {
    msg, err := sub.NextMsg(1 * time.Second)
    if err != nil {
        break
    }

    responses = append(responses, string(msg.Data))

    if len(responses) >= minResponses {
        break
    }
}
sub.Unsubscribe()