Redis pub/sub with redis-py

Implement Redis pub/sub messaging in Python with redis-py

This guide shows you how to implement a Redis-backed pub/sub broadcaster in Python with redis-py. It includes a small local web server built with the Python standard library 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 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 redis-py PubSub object, binds each target to a dispatch callback, and starts a background thread on run_in_thread()
  3. The application (or another process) calls hub.publish(channel, message)
  4. Redis fans the message out over every subscribing client's open socket
  5. Each subscriber's dispatch thread wraps the raw message as a ReceivedMessage, appends it to a per-subscriber ring buffer, and invokes the optional callback
  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):

import redis
from pubsub_hub import RedisPubSubHub

r = redis.Redis(host="localhost", port=6379, decode_responses=True)
hub = RedisPubSubHub(redis_client=r)

# 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})
print(f"Redis delivered to {delivered} subscriber(s)")

# Look at what each subscriber received
for sub in hub.subscriptions():
    print(sub.name, sub.received_total(), "messages")
    for message in sub.messages(limit=5):
        print("  ", message.channel, message.payload)

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             dict[str, Subscription]
  published_total           int
  delivered_total           int
  channel_published         dict[channel -> count]

Subscription                            (in-process, one per subscriber)
  name                      str
  targets                   list[channel | pattern]
  is_pattern                bool
  buffer                    deque[ReceivedMessage]      (capped, default 50)
  received_total            int
  pubsub                    redis.client.PubSub          (owns one connection)
  thread                    PubSubWorkerThread           (run_in_thread)

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
  • The redis-py pubsub.run_in_thread() helper to dispatch messages from each subscriber's own connection without writing the thread loop by hand

Publishing messages

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

def publish(self, channel: str, message: object) -> int:
    payload = json.dumps(message, default=str)
    delivered = int(self.redis.publish(channel, payload))
    with self._stats_lock:
        self._published_total += 1
        self._delivered_total += delivered
        self._channel_published[channel] += 1
    return delivered

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 PubSub object and dispatch thread:

def subscribe(self, name: str, channels: list[str], on_message=None) -> Subscription:
    return self._register(name, channels, is_pattern=False, on_message=on_message)

Inside Subscription.__init__, the helper binds each target to a callback and starts a background thread:

self._pubsub = hub.redis.pubsub(ignore_subscribe_messages=True)
bindings = {target: self._dispatch for target in self.targets}
if is_pattern:
    self._pubsub.psubscribe(**bindings)
else:
    self._pubsub.subscribe(**bindings)
self._thread = self._pubsub.run_in_thread(sleep_time=0.01, daemon=True)

A few details matter here:

  • pubsub(ignore_subscribe_messages=True) skips the subscribe/unsubscribe acknowledgements Redis sends back on the same socket, so the dispatch callback only ever sees real published payloads.
  • Each Subscription gets its own PubSub (and therefore its own Redis connection). Sharing one connection across subscribers would couple their lifetimes — closing one would close the channel for the others.
  • run_in_thread runs a short poll loop (sleep_time=0.01) rather than blocking on listen(). That's the cleanest way to support subscription.close() without leaving a thread stuck inside a blocking socket read.

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, the dispatch callback receives both the matched channel and the original pattern:

def _dispatch(self, raw: dict) -> None:
    channel = raw.get("channel")   # the actual channel the message was sent to
    pattern = raw.get("pattern")   # the pattern that matched (None for exact subs)
    data    = raw.get("data")
    ...

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

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(chs)     # 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.

Stats and history

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

def stats(self) -> dict:
    return {
        "published_total": self._published_total,
        "delivered_total": self._delivered_total,         # sum of PUBLISH return values
        "received_total": sum(s.received_total() for s in self.subscriptions()),
        "active_subscriptions": len(self.subscriptions()),
        "channel_published": dict(self._channel_published),
        "pattern_subscriptions": self.pattern_subscriber_count(),
    }

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.

  • Python 3.9 or later.

  • The redis-py client. Install it with:

    pip install "redis>=5.0"

Running the demo

Get the source files

The demo consists of two Python files. Download them from the redis-py 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/redis-py
curl -O $BASE/pubsub_hub.py
curl -O $BASE/demo_server.py

Start the demo server

From that directory:

python3 demo_server.py

You should see:

Redis pub/sub demo server listening on http://127.0.0.1:8095
Using Redis at localhost:6379
Seeded 3 default subscription(s)

Open http://127.0.0.1:8095 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.

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.

Use a separate connection (or PubSub object) per subscriber

A redis-py PubSub puts its connection into subscribe-only mode: ordinary commands (GET, HSET, etc.) on the same connection will hang. Always create the PubSub from a client whose pool can spare a connection, or — as the helper does — give every subscriber its own PubSub. Sharing one PubSub across business-logic subscribers couples their lifetimes (closing one closes the channel for the others) and serialises their callbacks on a single dispatch thread.

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 dispatch callback

The dispatch thread reads messages from a single socket. 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 queue, 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.

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-py documentation for full client reference, including the PubSub class and the run_in_thread() dispatch helper.

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