Handle Command

Overview

A command handler is the receiving side of KubeMQ's Commands pattern — the code that actually does the work a caller is blocked waiting on. Instead of building your own request-routing layer on top of a queue, you register a handler once with subscribe_to_commands, and KubeMQ delivers every matching command on that channel to it as a long-lived subscription running on a background thread, turning the channel into a synchronous RPC endpoint.

Handling happens inside the block: you read the command's id, metadata, and body, run your business logic, then build a reply with KubeMQ::CQ::CommandResponseMessage.new(request_id: cmd.id, reply_channel: cmd.reply_channel, executed: ...) and send it with send_response. Copying request_id and reply_channel from the received command is what lets the broker correlate the reply back to the exact caller blocked on the send — nothing else identifies which request the response belongs to.

Gotchas: you must call send_response for every command or the sender times out waiting; the block runs on the subscription's background thread, so slow or blocking business logic head-of-line blocks the next command; and cancel.wait only unblocks the main thread — it doesn't stop the handler thread, which is why cleanup happens in ensure.

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-handle'

begin
  client = KubeMQ::CQClient.new(address: address, client_id: 'cmd-handler')
  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 "Received command: id=#{cmd.id}, metadata=#{cmd.metadata}, body=#{cmd.body}"

    executed = cmd.metadata != 'fail'
    response = KubeMQ::CQ::CommandResponseMessage.new(
      request_id: cmd.id,
      reply_channel: cmd.reply_channel,
      executed: executed,
      error: executed ? nil : 'Simulated failure'
    )
    client.send_response(response)
    puts "Sent response: executed=#{executed}"
  end

  puts "Listening for commands on '#{channel}'. Press Ctrl+C to stop."
  cancel.wait
rescue Interrupt
  puts "\nShutting down..."
rescue KubeMQ::Error => e
  puts "KubeMQ error: #{e.message}"
ensure
  cancel&.cancel
  client&.close
  puts 'Done'
end

How It Works

• subscribe_to_commands runs a background thread that delivers incoming commands to the block.
• The handler must call send_response for each command — otherwise the sender times out.
• cancel.wait blocks the main thread until the token is cancelled (e.g., by Ctrl+C).
• Review timeouts, channel names, and client IDs before running against shared environments.

Related

• Pattern overview
• Ruby SDK Reference
• Send Command
• Command Group