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

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

JavaScript

Python

Ruby

C

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()