Send Command

Overview

A command is KubeMQ's fire-and-confirm RPC pattern: you reach for it when you need to know that an action actually ran on the other end — "restart-service" — but you don't need any data back, just a yes/no on execution. It's the middle ground between one-way pub/sub, where you get no confirmation at all, and a query, where the handler returns a result payload. Commands turn "I hope that worked" into a definite outcome your caller can branch on.

This sample builds that lesson: client.subscribe_to_commands(sub, ...) registers the handler block, and client.send_command(msg) blocks until a response arrives or the timeout (in milliseconds) on CommandMessage expires. The handler builds its reply with CommandResponseMessage.new(request_id: cmd.id, reply_channel: cmd.reply_channel, executed: true) and sends it with client.send_response — that correlation is what routes the response back to the exact caller waiting on it.

Gotchas: if no handler is subscribed (or it's still starting up), send_command waits out the full timeout before failing — there's no fast "nobody's listening" error. A handler that omits request_id/reply_channel on the response leaves the caller hanging until timeout. And a command's response carries no business data — if you need the handler to return a value, use a query instead.

Prerequisites

• KubeMQ server running on localhost:50000
• Ruby SDK installed (gem install kubemq)

Code

require 'kubemq'

address = ENV.fetch('KUBEMQ_ADDRESS', 'localhost:50000')
channel = 'ruby-rpc.command-send'

begin
  client = KubeMQ::CQClient.new(address: address, client_id: 'cmd-sender')
  puts "Connected to #{address}"

  cancel = KubeMQ::CancellationToken.new
  sub = KubeMQ::CQ::CommandsSubscription.new(channel: channel)
  client.subscribe_to_commands(sub, cancellation_token: cancel, on_error: ->(e) { puts "Error: #{e.message}" }) do |cmd|
    puts "Handler received command: #{cmd.metadata}"
    response = KubeMQ::CQ::CommandResponseMessage.new(
      request_id: cmd.id,
      reply_channel: cmd.reply_channel,
      executed: true
    )
    client.send_response(response)
  end
  sleep 1

  msg = KubeMQ::CQ::CommandMessage.new(
    channel: channel,
    timeout: 10_000,
    metadata: 'restart-service',
    body: 'service-name'
  )
  result = client.send_command(msg)
  puts "Command result: executed=#{result.executed}, error=#{result.error}"
rescue KubeMQ::Error => e
  puts "KubeMQ error: #{e.message}"
ensure
  cancel&.cancel
  client&.close
  puts 'Done'
end

How It Works

• Commands follow a fire-and-confirm pattern — the handler processes and responds with execution status.
• The timeout field (in milliseconds) controls how long the sender waits for a response.
• The handler must call send_response with the matching request_id and reply_channel.
• Review timeouts, channel names, and client IDs before running against shared environments.

Related

• Pattern overview
• Ruby SDK Reference
• Handle Command
• Command Timeout