Persistent Pub/Sub

Overview

This tutorial builds a publisher and subscriber on a KubeMQ Events Store channel — reach for this pattern when a subscriber can't guarantee it's listening the instant a message is published. Plain events are fire-and-forget: publish with no one subscribed and the message is gone. Events Store persists every event to a durable, ordered log, so a subscriber connecting seconds or a full restart later still catches up — useful for anything needing a complete history, like an audit trail or event-sourced state.

The two calls involved: send_event_store publishes and returns a result confirming storage with a sent flag plus a broker-assigned sequence number, and subscribe_to_events_store takes a subscription with a required start_position telling the broker where to start — new events only (EventStoreStartPosition::START_NEW_ONLY, used here), from the first stored event, or a given sequence or time. Production subscribers usually resume from a saved checkpoint instead of starting fresh.

Gotchas: starting from new events means anything published earlier is silently skipped — this sample papers over that race with a fixed sleep instead of a ready signal, fine for a demo but not production. Replaying from the first event on every restart replays the whole log, which gets costly on a busy channel. Persistence isn't consumer coordination: each independent subscriber gets its own full replay unless grouped with a consumer group.

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-events-store.persistent-pubsub'

begin
  client = KubeMQ::PubSubClient.new(address: address, client_id: 'es-basic-example')
  puts "Connected to #{address}"

  cancel = KubeMQ::CancellationToken.new

  sub = KubeMQ::PubSub::EventsStoreSubscription.new(
    channel: channel,
    start_position: KubeMQ::PubSub::EventStoreStartPosition::START_NEW_ONLY
  )
  client.subscribe_to_events_store(sub, cancellation_token: cancel, on_error: lambda { |e|
    puts "Error: #{e.message}"
  }) do |event|
    puts "Received: channel=#{event.channel}, metadata=#{event.metadata}, body=#{event.body}"
  end
  sleep 1

  3.times do |i|
    msg = KubeMQ::PubSub::EventStoreMessage.new(
      channel: channel,
      metadata: "es event #{i}",
      body: "payload-#{i}"
    )
    result = client.send_event_store(msg)
    puts "Sent event #{i}: sent=#{result.sent}"
  end

  sleep 2
rescue KubeMQ::Error => e
  puts "KubeMQ error: #{e.message}"
ensure
  cancel&.cancel
  client&.close
  puts 'Done'
end

How It Works

• Events store messages are persisted by the broker and can be replayed by future subscribers.
• send_event_store returns a confirmed result — unlike fire-and-forget events, the broker acknowledges persistence.
• The start_position controls where the subscription begins reading from the event log.
• Review timeouts, channel names, and client IDs before running against shared environments.

Related

• Pattern overview
• Ruby SDK Reference
• Start from First
• Replay from Sequence