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:
Go
Java
JavaScript
Python
Ruby
TypeScript
C
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()
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();
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();
});
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")
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
TypeScript
let msg = await nc.request('time', 1000);
t.log('the time is', msg.data);
nc.close();
C
natsConnection      *conn      = NULL;
natsMsg             *msg       = NULL;
natsStatus          s          = NATS_OK;
​
s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);
​
// Send a request and wait for up to 1 second
if (s == NATS_OK)
    s = natsConnection_RequestString(&msg, conn, "request", "this is the request", 1000);
​
if (s == NATS_OK)
{
    printf("Received msg: %s - %.*s\n",
           natsMsg_GetSubject(msg),
           natsMsg_GetDataLength(msg),
           natsMsg_GetData(msg));
​
    // Destroy the message that was received
    natsMsg_Destroy(msg);
}
​
(...)
​
// Destroy objects that were created
natsConnection_Destroy(conn);
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)
}
​
// Send the request immediately
nc.PublishRequest(subject, replyTo, []byte(input))
nc.Flush()
​
// 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()
Including a Reply Subject
Caches, Flush and Ping
Last updated 2 months ago