Redis Pub/Sub

Redis Pub/Sub implements the publish/subscribe messaging paradigm where publishers send messages to channels, and subscribers receive them in real time. It is a fire-and-forget system with no message persistence.

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Core Commands

# SUBSCRIBE — listen to one or more channels
SUBSCRIBE news:breaking news:sports

# PSUBSCRIBE — pattern subscribe (glob patterns)
PSUBSCRIBE news:*          # All news channels
PSUBSCRIBE user:*.events   # All user event channels

# PUBLISH — send a message to a channel
PUBLISH news:breaking "Redis 8.0 released"
# Returns: number of subscribers who received the message

# UNSUBSCRIBE
UNSUBSCRIBE news:sports     # Unsubscribe from specific channel
UNSUBSCRIBE                 # Unsubscribe from all channels

PUNSUBSCRIBE news:*         # Pattern unsubscribe

Subscription Lifecycle

Subscriber A:  SUBSCRIBE chat:room1 chat:room2
                         → waiting for messages...

Publisher:     PUBLISH chat:room1 "Hello everyone!"
                         → Subscriber A receives:
                            ["message", "chat:room1", "Hello everyone!"]

Pattern sub:   PSUBSCRIBE chat:*
               PUBLISH chat:room2 "New user joined"
                         → Pattern subscriber receives:
                            ["pmessage", "chat:*", "chat:room2", "New user joined"]

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Delivery Semantics

Property	Behavior
Persistence	❌ Zero — messages not stored
Delivery guarantee	At-most-once — fire-and-forget
Offline subscribers	❌ Miss all messages while disconnected
History/replay	❌ Impossible — no message log
Message ordering	✅ FIFO within a channel
Acknowledgment	❌ No ACK mechanism

Critical: If a subscriber disconnects and reconnects, it will miss all messages published during its absence. Pub/Sub is only appropriate when message loss is acceptable.

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Connection Modes

A subscriber connection enters a blocking subscribe mode — it can only receive messages. It cannot send other commands while subscribed (except SUBSCRIBE, UNSUBSCRIBE, PING, RESET, QUIT).

// Spring Boot Redis Pub/Sub
@Configuration
public class PubSubConfig {

    @Bean
    public RedisMessageListenerContainer listenerContainer(
            RedisConnectionFactory factory,
            MessageListenerAdapter adapter) {
        RedisMessageListenerContainer container = new RedisMessageListenerContainer();
        container.setConnectionFactory(factory);
        container.addMessageListener(adapter, new PatternTopic("user:*.events"));
        return container;
    }

    @Bean
    public MessageListenerAdapter listenerAdapter(UserEventListener listener) {
        return new MessageListenerAdapter(listener, "onMessage");
    }
}

@Component
public class UserEventListener {
    public void onMessage(String message, String channel) {
        log.info("Received on {}: {}", channel, message);
    }
}

// Publisher
@Service
public class EventPublisher {
    private final RedisTemplate<String, String> redisTemplate;

    public void publishUserEvent(Long userId, String event) {
        redisTemplate.convertAndSend("user:" + userId + ".events", event);
    }
}

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Real-World Use Cases

Use Case	Why Pub/Sub Fits
Live chat (in-memory only)	Users online — message loss on disconnect OK
Real-time notifications	Push to connected clients
Cache invalidation broadcast	All nodes invalidate cache entry simultaneously
Dashboard live updates	Emit metrics to connected dashboard (tolerate drops)
Debug events / logging broadcast	Development environment tracing
WebSocket fan-out via Redis	Horizontal scaling of WebSocket servers

Cache Invalidation Pattern

Service A updates product:123
→ PUBLISH cache:invalidate "product:123"

Service B (subscribed to cache:invalidate):
→ evict("product:123") from local in-process cache
→ ensures all nodes' L1 caches are invalidated on write

(Uses Pub/Sub for broadcast — doesn't need persistence)

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Pub/Sub vs Streams

	Pub/Sub	Streams
Persistence	❌ None	✅ Yes (configurable)
Replay history	❌	✅ By ID range
Offline client support	❌ (miss messages)	✅ (reads from last consumed ID)
Consumer groups	❌ (all get all)	✅ (one-to-one distribution within group)
Message acknowledgment	❌	✅ (XACK)
Pattern matching	✅ (PSUBSCRIBE)	❌ (use separate streams)
Throughput (simple fan-out)	Higher (no storage overhead)	Slightly lower
Use case	Real-time ephemeral fanout	Reliable event queues

Production guidance: Unless you specifically need zero-overhead real-time broadcast and can tolerate message loss, prefer Redis Streams for production message passing.

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Sharding Pub/Sub in Redis Cluster

Standard Pub/Sub in Redis Cluster broadcasts to ALL nodes — every PUBLISH is forwarded to all cluster nodes, creating O(N-nodes) overhead.

Redis 7.0+: Sharded Pub/Sub

SSUBSCRIBE channel     # Subscribe to sharded channel
SUNSUBSCRIBE channel   # Unsubscribe from sharded channel
SPUBLISH channel msg   # Publish to specific shard only

Sharded Pub/Sub routes channels to a specific hash slot — messages only go to the node owning that slot. Dramatically reduces cluster-wide broadcast overhead for high-volume apps.

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Production Limitations and Solutions

Limitation	Problem	Solution
No persistence	Message loss on disconnect	Redis Streams for reliability
No ACK	Can't confirm delivery	Streams with XACK
Cluster broadcast overhead	O(N) node fanout	Redis 7 Sharded Pub/Sub
Slow subscriber blocks	Publisher blocked if subscriber is slow (TCP backpressure)	Set client-output-buffer-limit pubsub
Memory pressure	Slow subscriber accumulates messages in send buffer	Limit buffer: client-output-buffer-limit pubsub 8mb 2mb 60

# Redis config: disconnect slow pub/sub subscribers
client-output-buffer-limit pubsub 8mb 2mb 60
# Hard limit: 8mb (immediate disconnect)
# Soft limit: 2mb for 60 seconds → then disconnect
# Prevents one slow subscriber from consuming all server memory