Handle Query

Overview

A query handler is the answering side of KubeMQ's request/response RPC pattern — the code that does real work and sends back data, unlike a Command handler, which only acknowledges receipt. Reach for it whenever a caller needs an actual answer — a lookup result, a computed value, a status object — not just confirmation that a message arrived.

Registering a handler with subscribe_to_queries opens a subscription; the broker delivers every matching query to your block as it arrives. The block builds a QueryResponseMessage carrying the original query's correlation id (request_id/reply_channel) back to the broker, so the answer routes to the specific caller blocked waiting, and sets body with the real result before calling send_response.

Gotchas: if the handler never sends a response, the caller blocks until its own timeout elapses and fails with a timeout, not a fast error. An exception inside the block doesn't automatically become a failure reply, so uncaught errors can leave the sender hanging. And because every matching query lands on the same background thread, slow handler code delays every other in-flight caller.

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.query-handle'

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

  cancel = KubeMQ::CancellationToken.new

  sub = KubeMQ::CQ::QueriesSubscription.new(channel: channel)
  client.subscribe_to_queries(sub, cancellation_token: cancel, on_error: ->(e) { puts "Error: #{e.message}" }) do |query|
    puts "Received query: id=#{query.id}, metadata=#{query.metadata}"

    response = KubeMQ::CQ::QueryResponseMessage.new(
      request_id: query.id,
      reply_channel: query.reply_channel,
      executed: true,
      body: "response-to-#{query.metadata}",
      metadata: 'query-result'
    )
    client.send_response(response)
    puts "Sent response for query #{query.id}"
  end

  puts "Listening for queries 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_queries delivers incoming queries to the block on a background thread.
• The handler must call send_response for each query with data in the body and metadata.
• Review timeouts, channel names, and client IDs before running against shared environments.

Related

• Pattern overview
• Ruby SDK Reference
• Send Query
• Cached Query