Redis pub/sub with redis-rb

Implement Redis pub/sub messaging in Ruby with redis-rb

This guide shows you how to implement a Redis-backed pub/sub broadcaster in Ruby with redis-rb. It includes a small local web server built with the Ruby standard library webrick so you can publish messages to named channels, add and remove subscribers live, and watch Redis fan out each message to every interested listener.

Overview

Pub/sub lets your application broadcast events — chat messages, cache invalidation signals, presence updates, notifications — to many consumers without per-pair wiring. The publisher names a channel; every client currently subscribed to that channel receives the message, in publish order, with sub-millisecond fan-out.

That gives you:

  • Many-to-many event delivery with no message storage cost in Redis
  • Exact-match subscriptions (SUBSCRIBE orders:new) for known topics
  • Pattern subscriptions (PSUBSCRIBE notifications:*) for whole topic hierarchies
  • Live server-side introspection through PUBSUB CHANNELS, PUBSUB NUMSUB, and PUBSUB NUMPAT
  • At-most-once delivery: subscribers that are offline when a message is published miss it, so durable state should live in keys or a Stream, not in the pub/sub channel itself

In this example, the publisher side calls PUBLISH with a JSON-encoded body and counts how many subscribers Redis reported delivering to. Each in-process subscriber owns its own Redis connection and a background Ruby Thread that pumps incoming messages into a callback.

How it works

The flow looks like this:

  1. The application calls hub.subscribe(name:, channels:) or hub.psubscribe(name:, patterns:)
  2. The helper creates a fresh Redis client for the subscriber, spawns a Ruby Thread, and inside that thread calls redis.subscribe(*channels) (or redis.psubscribe(*patterns)), which blocks the connection in subscribe-only mode
  3. The application (or another process) calls hub.publish(channel, message) on the hub's plain (non-subscriber) Redis client
  4. Redis fans the message out over every subscribing client's open socket
  5. Each subscriber's worker thread receives the message inside the on.message / on.pmessage callback, wraps it as a ReceivedMessage, and pushes it into a per-subscriber ring buffer
  6. The publisher receives the integer subscriber count back from PUBLISH, which is the number of clients Redis delivered to right then

Pattern subscriptions match channels by glob (*, ?, [abc]). A single message that matches both an exact subscription and a pattern subscription is delivered twice — once as a message and once as a pmessage.

The pub/sub hub helper

The RedisPubSubHub class wraps the publish, subscribe, and introspection operations (source):

require 'redis'
require_relative 'pubsub_hub'

hub = PubSubHub::RedisPubSubHub.new(
  redis_options: { host: 'localhost', port: 6379 }
)

# Exact-match subscriber
hub.subscribe(name: 'orders-listener', channels: ['orders:new'])

# Pattern subscriber covering an entire topic hierarchy
hub.psubscribe(name: 'all-notifications', patterns: ['notifications:*'])

# Publish — returns Redis' delivered count for this PUBLISH
delivered = hub.publish('orders:new', { 'order_id' => 42, 'total' => 199.0 })
puts "Redis delivered to #{delivered} subscriber(s)"

# Look at what each subscriber received
hub.subscriptions.each do |sub|
  puts "#{sub.name} #{sub.received_total} messages"
  sub.messages(5).each do |message|
    puts "  #{message.channel} #{message.payload.inspect}"
  end
end

hub.unsubscribe('orders-listener')
hub.shutdown  # closes every remaining subscription

Data model

Pub/sub has no Redis keyspace footprint of its own — channels are server-side routing entries, not stored values. The hub keeps its own bookkeeping in process memory:

RedisPubSubHub                          (in-process)
  subscriptions             Hash{String => Subscription}
  published_total           Integer
  delivered_total           Integer
  channel_published         Hash{channel => count}

Subscription                            (in-process, one per subscriber)
  name                      String
  targets                   Array<channel | pattern>
  is_pattern?               Boolean
  buffer                    Array<ReceivedMessage>     (capped, default 50)
  received_total            Integer
  redis                     Redis                       (its own connection)
  thread                    Thread                       (blocks on subscribe)

The implementation uses:

  • PUBLISH to fan a JSON-encoded message out to every subscriber of a channel
  • SUBSCRIBE for exact-match subscribers
  • PSUBSCRIBE for glob-style pattern subscribers
  • PUBSUB CHANNELS to list the channels with at least one active exact-match subscriber
  • PUBSUB NUMSUB to count subscribers per channel
  • PUBSUB NUMPAT to count active pattern subscriptions server-wide
  • A Ruby Thread per subscriber, each with its own Redis instance, so redis.subscribe can monopolise its connection without blocking the rest of the process

Publishing messages

publish JSON-encodes the message body, calls PUBLISH, and updates the per-channel publish counter:

def publish(channel, message)
  payload = JSON.generate(message)
  delivered = @redis.publish(channel, payload).to_i
  @stats_lock.synchronize do
    @published_total += 1
    @delivered_total += delivered
    @channel_published[channel] += 1
  end
  delivered
end

The integer returned by PUBLISH is what Redis itself reports — the number of subscribers (direct and pattern) that received the message in that call. It's a useful sanity check that the channel name is actually being listened to: a steady stream of 0s means you have a typo somewhere or your subscriber crashed.

Subscribing to channels

subscribe creates a named in-process Subscription that owns its own Redis client and a worker thread. Inside Subscription#initialize (the relevant excerpt):

@redis = Redis.new(redis_options)
@thread = Thread.new do
  @redis.subscribe(*@targets) do |on|
    on.subscribe   { |_channel, _count| @ready_latch.push(true) }
    on.message     { |channel, raw|    dispatch(channel: channel, pattern: nil, raw: raw) }
    on.unsubscribe { |_channel, _count| }
  end
end
@ready_latch.pop  # block until the server acks SUBSCRIBE

A few details matter here:

  • Each Subscription gets its own Redis client. In redis-rb 5.x, Redis#subscribe puts its connection into subscribe-only mode: ordinary commands (GET, HSET) on the same connection will raise. Sharing one client across subscribers would also couple their lifetimes — unsubscribing one would close the channel for the others.
  • The thread blocks inside subscribe. Unlike redis-py's pubsub.run_in_thread(), redis-rb's subscribe loop is the worker — it doesn't return until the server has unsubscribed the connection. That's why the helper spawns one Thread per subscriber and stores it.
  • A Queue latch lets the constructor wait for the first SUBSCRIBE ack. Without it, a publish issued immediately after subscribe could race the subscribe handshake and arrive before Redis knew about the client. Blocking on the on.subscribe callback before returning from initialize removes that race.

Pattern subscriptions with PSUBSCRIBE

psubscribe works the same way but routes messages through PSUBSCRIBE so each binding is a glob, not a literal channel name:

hub.psubscribe(name: 'all-notifications', patterns: ['notifications:*'])
hub.psubscribe(name: 'cache-invalidator', patterns: ['cache:invalidate:*'])

When a published channel matches a pattern, redis-rb invokes the on.pmessage callback with both the matched channel and the original pattern:

@redis.psubscribe(*@targets) do |on|
  on.pmessage do |pattern, channel, raw|
    dispatch(channel: channel, pattern: pattern, raw: raw)
  end
end

That distinction is useful for routing: a pattern subscriber can do one thing for the whole hierarchy (e.g., increment a counter) and dispatch on the specific channel within its callback (e.g., "invalidate this region's cache").

Unsubscribing without sentinel messages

hub.unsubscribe(name) (and hub.shutdown for everything) needs to break the worker thread out of its blocking subscribe call. The cleanest way in redis-rb 5.x is to call unsubscribe (or punsubscribe) on the same subscriber connection from another thread — redis-rb sends the command down the open subscribe socket, Redis responds with the unsubscribe acknowledgement, the subscribe block returns, and the worker thread exits:

def close
  return if @closed
  @closed = true
  if @is_pattern
    @redis.punsubscribe
  else
    @redis.unsubscribe
  end
  @thread.join(2)
  @redis.close
end

That avoids the older "publish a STOP sentinel and unsubscribe from inside the handler" pattern, which pollutes the user-facing channel namespace with control messages.

Inspecting active subscribers

Redis exposes a small set of pub/sub introspection commands that report on subscriber state without traversing any keyspace:

hub.active_channels                       # PUBSUB CHANNELS *
hub.channel_subscriber_counts(channels)   # PUBSUB NUMSUB ch1 ch2 ...
hub.pattern_subscriber_count              # PUBSUB NUMPAT

PUBSUB CHANNELS only reports channels with at least one exact-match subscriber — pattern subscribers do not appear here. That's a deliberate Redis design choice: a glob like * would otherwise show up as a subscriber to every conceivable channel. PUBSUB NUMPAT covers the pattern side as a single global count.

Note that redis-rb returns PUBSUB NUMSUB as an alternating [channel, count, channel, count, …] array; the helper pairs the elements up into a hash before returning to keep the wire shape consistent with the other clients.

Stats and history

stats reports publish and receive counters plus the size of the subscription registry:

def stats
  published_total, delivered_total, channel_published =
    @stats_lock.synchronize do
      [@published_total, @delivered_total, @channel_published.dup]
    end
  subs = subscriptions
  {
    'published_total'       => published_total,
    'delivered_total'       => delivered_total,           # sum of PUBLISH return values
    'received_total'        => subs.sum(&:received_total),
    'active_subscriptions'  => subs.length,
    'channel_published'     => channel_published,
    'pattern_subscriptions' => pattern_subscriber_count
  }
end

delivered_total is what Redis itself counted; received_total is what this process's in-memory subscribers saw. In a single-process demo they should track each other closely — a sustained divergence usually means a callback raised, or a subscriber crashed while a publisher kept publishing. (Pub/sub is at-most-once: if your subscriber wasn't connected at publish time, the message is gone.)

Prerequisites

  • Redis 6.2 or later running locally on the default port (6379). Earlier versions still work for plain PUBLISH/SUBSCRIBE; PUBSUB NUMPAT is older than that.

  • Ruby 3.0 or later.

  • The redis-rb gem (5.x). Install it with:

    gem install redis

  • The webrick gem (it was extracted from the Ruby standard library in Ruby 3.0):

    gem install webrick

Running the demo

Get the source files

The demo consists of two Ruby files. Download them from the ruby source folder on GitHub, or grab them with curl:

mkdir pub-sub-demo && cd pub-sub-demo
BASE=https://raw.githubusercontent.com/redis/docs/main/content/develop/use-cases/pub-sub/ruby
curl -O $BASE/pubsub_hub.rb
curl -O $BASE/demo_server.rb

Start the demo server

From that directory:

gem install redis webrick
ruby demo_server.rb

You should see:

Redis pub/sub demo server listening on http://127.0.0.1:8102
Using Redis at localhost:6379
Seeded 3 default subscription(s)

Open http://127.0.0.1:8102 in a browser. You can:

  • Publish messages of any text to any channel name in any batch size.
  • Add named subscribers that listen on either a specific channel (orders:new) or a glob pattern (notifications:*). A single subscriber can listen on multiple targets — enter them comma-separated.
  • Watch each subscriber's incoming-message panel update every 800 ms.
  • See the server-side view: PUBSUB CHANNELS lists exact-match channels with subscribers, PUBSUB NUMSUB gives per-channel counts, and PUBSUB NUMPAT counts active pattern subscriptions.
  • Click Reset to drop every subscription, zero the counters, and re-seed the three default subscribers.

If your Redis server is running elsewhere, start the demo with --redis-host and --redis-port. The HTTP bind host and port can be overridden with --host and --port.

Production usage

Pub/sub is at-most-once — pair it with durable state if you need replay

A subscriber that's offline when a message is published misses it permanently. For events you can't afford to lose, write the durable record (the order row, the cache key version, the audit log entry) to its primary store, then PUBLISH a notification so live consumers can pick it up immediately. On reconnect, consumers reconcile by reading the durable store, not by waiting for missed pub/sub messages. If you actually need replay or at-least-once delivery, switch to Redis Streams with consumer groups.

Give every subscriber its own Redis connection

In redis-rb 5.x, Redis#subscribe blocks the calling thread and monopolises the underlying connection — any non-pub/sub command issued on the same client will raise. The helper sidesteps that by creating a fresh Redis client for every Subscription, and using a separate plain client on the hub for PUBLISH and the PUBSUB CHANNELS / NUMSUB / NUMPAT introspection commands. In a long-running service, treat each subscriber as owning its connection for the duration of its subscription, and pool connections separately for command-mode work.

Choose a topic naming convention up front

A flat namespace gets ugly fast — email, email_high_priority, email_high_priority_billing. Pick a colon-separated hierarchy (notifications:billing:invoice, cache:invalidate:products:p-001) so consumers can subscribe at the right level: a billing service uses notifications:billing:*, the audit logger uses notifications:*. Glob patterns are evaluated for every published message, so don't go wild with multiple wildcards on hot paths — *:*:* matches everything and costs more than a flat notifications:* would.

Don't do heavy work in the subscribe block

The worker thread reads messages from a single socket inside the on.message / on.pmessage callback. If the callback blocks (synchronous HTTP call, big computation, slow DB write), the next message waits behind it and the subscriber's effective throughput drops to whatever the callback's latency is. For heavier work, the callback should hand the message off to a worker pool (a Concurrent::ThreadPoolExecutor, a Sidekiq job, …) or — for true durable handoff — a Redis Streams consumer group.

Tune the subscriber buffer for your traffic shape

The demo caps each subscriber's in-memory message buffer at 50. That's right for showing the recent activity in a UI, but a real subscriber typically processes each message and discards it — the buffer is only there for human inspection. If you keep a buffer, make sure it's bounded; an unbounded ring on a chatty pattern subscriber will eventually OOM the process.

Use the GIL-friendly cooperative concurrency model

Ruby (MRI) threads release the GIL on blocking I/O, so a per-subscriber thread sleeping inside a socket read does not stall the rest of the process. That's why one Ruby thread per active subscriber is a workable model even at hundreds of subscribers — you're not consuming a CPU thread, just a Redis connection. If you instead need thousands of subscribers per process, consider switching to one of the fibre-based async drivers (e.g. async-redis) so subscriber sockets can multiplex inside a single reactor.

Sharded pub/sub on a Redis Cluster

On a Redis Cluster, plain PUBLISH fans every message out to every node via the cluster bus, which becomes a hotspot at high throughput. Redis 7.0 added sharded pub/sub: channels are hashed to slots, and SPUBLISH / SSUBSCRIBE only touch the shard that owns the slot. If you're scaling pub/sub on a cluster, prefer the sharded commands and pick channel names whose hash distribution matches your traffic.

Inspect pub/sub state directly in Redis

Because pub/sub has no keyspace, KEYS/SCAN won't show you anything. Use the introspection commands instead:

# Which channels currently have at least one exact-match subscriber?
redis-cli pubsub channels '*'

# How many subscribers does each channel have?
redis-cli pubsub numsub orders:new notifications:billing chat:lobby

# How many active pattern subscriptions across the whole server?
redis-cli pubsub numpat

# Subscribe interactively from the CLI to watch traffic on a pattern
redis-cli psubscribe 'orders:*'

redis-cli in subscribe mode only exits with Ctrl-C — it can't issue any other commands while subscribed.

Learn more

This example uses the following Redis commands:

See the redis-rb documentation for full client reference, including the Redis#subscribe and Redis#psubscribe helpers.

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