Replying to a Message

Incoming messages have an optional reply-to field. If that field is set, it will contain a subject to which a reply is expected.

For example, the following code will listen for that request and respond with the time.

Go
Java
JavaScript
Python
Ruby
TypeScript
C
Go
nc, err := nats.Connect("demo.nats.io")
if err != nil {
log.Fatal(err)
}
defer nc.Close()
// Subscribe
sub, err := nc.SubscribeSync("time")
if err != nil {
log.Fatal(err)
}
// Read a message
msg, err := sub.NextMsg(10 * time.Second)
if err != nil {
log.Fatal(err)
}
// Get the time
timeAsBytes := []byte(time.Now().String())
// Send the time as the response.
msg.Respond(timeAsBytes)
Java
Connection nc = Nats.connect("nats://demo.nats.io:4222");
// Subscribe
Subscription sub = nc.subscribe("time");
// Read a message
Message msg = sub.nextMessage(Duration.ZERO);
// Get the time
Calendar cal = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
byte[] timeAsBytes = sdf.format(cal.getTime()).getBytes(StandardCharsets.UTF_8);
// Send the time
nc.publish(msg.getReplyTo(), timeAsBytes);
// Flush and close the connection
nc.flush(Duration.ZERO);
nc.close();
JavaScript
let nc = NATS.connect({
url: "nats://demo.nats.io:4222"
});
// set up a subscription to process a request
nc.subscribe('time', (msg, reply) => {
if (msg.reply) {
nc.publish(msg.reply, new Date().toLocaleTimeString());
}
});
Python
nc = NATS()
await nc.connect(servers=["nats://demo.nats.io:4222"])
future = asyncio.Future()
async def cb(msg):
nonlocal future
future.set_result(msg)
await nc.subscribe("time", cb=cb)
await nc.publish_request("time", new_inbox(), b'What is the time?')
await nc.flush()
# Read the message
msg = await asyncio.wait_for(future, 1)
# Send the time
time_as_bytes = "{}".format(datetime.now()).encode()
await nc.publish(msg.reply, time_as_bytes)
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 Time.now
end
nc.publish("time", 'What is the time?', NATS.create_inbox)
# Use the response
msg = Fiber.yield
puts "Reply: #{msg}"
end.resume
end
TypeScript
// set up a subscription to process a request
await nc.subscribe('time', (err, msg) => {
if (msg.reply) {
nc.publish(msg.reply, new Date().toLocaleTimeString());
} else {
t.log('got a request for the time, but no reply subject was set.');
}
});
C
natsConnection *conn = NULL;
natsSubscription *sub = NULL;
natsMsg *msg = NULL;
natsStatus s = NATS_OK;
s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);
// Subscribe
if (s == NATS_OK)
s = natsConnection_SubscribeSync(&sub, conn, "time");
// Wait for messages
if (s == NATS_OK)
s = natsSubscription_NextMsg(&msg, sub, 10000);
if (s == NATS_OK)
{
char buf[64];
snprintf(buf, sizeof(buf), "%lld", nats_Now());
// Send the time as a response
s = natsConnection_Publish(conn, natsMsg_GetReply(msg), buf, (int) strlen(buf));
// Destroy message that was received
natsMsg_Destroy(msg);
}
(...)
// Destroy objects that were created
natsSubscription_Destroy(sub);
natsConnection_Destroy(conn);
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