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()
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