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2. Request and reply

In Hello NATS a publisher sent a message and never heard back. This time you'll build a two-way conversation: a small responder that answers questions on a subject, and a request that calls it and gets exactly one reply. By the end you'll start a time service in one terminal and ask it for the time from another.

What you'll need​

  • The nats CLI and a running nats-server from Hello NATS. If the server isn't running, start it:

    nats-server

  • Two terminals: one for the responder, one for the request.

Step 1: Start a responder​

In your first terminal, start a responder on the subject time. The --command flag tells nats reply to run a command for each request and send its output back as the reply.

#!/bin/bash
# Start a responder on the subject "time". `nats reply` subscribes to the
# subject and, for every request it receives, runs the command and sends its
# output back on the request's private reply subject. Leave this running.
nats reply time --command "date"

You should see it start up and wait:

17:42:10 Listening on "time" in group "NATS-RPLY-22"

Leave this terminal running. It's now a service waiting to answer.

Step 2: Send a request​

Open your second terminal and call the service. nats request publishes one message on time and waits for a single reply, up to the global --timeout (five seconds by default).

#!/bin/bash
# Call the responder once. `nats request` creates a private inbox, subscribes
# to it, publishes an (empty) request on "time" with the inbox attached, and
# prints the first reply it gets back. --timeout is the longest it waits.
nats request time "" --timeout 2s

You should see one reply printed, the current date and time from the responder:

17:42:31 Sending request on "time"
17:42:31 Received with rtt 612µs
Mon Jun  9 17:42:31 CEST 2026

That date came from the responder's date command, sent back to your request.

Step 3: Watch the responder log the call​

Switch back to your first terminal. You should see the responder log the call:

17:42:31 [#0] Received on subject "time":

Each time you rerun the request in your second terminal, the counter goes up by one ([#1], [#2], …) and a fresh reply comes back. Try it a couple of times.

Step 4: See what happens with no responder​

Stop the responder in your first terminal with Ctrl+C, then send the request again from your second terminal:

nats request time "" --timeout 2s

Instead of waiting out the timeout, the request returns right away, because the server knows nobody is subscribed to time:

17:43:02 No responders are available

Start the responder again and the same request succeeds.

What you built​

You ran a time responder and called it with a request, getting exactly one reply per call, all over NATS with no shared address between the two sides.

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