spring-boot-demo/redis/docs/07-pub-sub.md

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7. Pub/Sub

Redis Pub/Sub is a broadcast: PUBLISH channel message sends the message to whoever is subscribed to the channel at that moment, and Redis keeps nothing. The reply to PUBLISH is the number of subscribers that received it.

Boot 4.1 sets up the receiving side

Older articles declare a RedisMessageListenerContainer bean by hand. Boot 4.1 registers one (redisMessageListenerContainer) and enables @RedisListener, so a listener is a method (NewsListener):

@RedisListener(topic = "news")
void onNews(String body) { ... }

11-pub-sub.txt starts with what Boot did:

beans of type RedisMessageListenerContainer: [redisMessageListenerContainer]
running=true listening=true

A topic that contains a glob (alerts.*) subscribes with PSUBSCRIBE; the channel the message came in on is available as a header (@Header(PubSubHeaders.CHANNEL) String channel). The @RedisListener attributes, from the jar, are id, container, value, topic and consumes. A String parameter receives the raw body; a org.springframework.data.redis.connection.Message parameter did not work in this test, because the adapter tried to parse the payload as JSON (a StreamReadException on the text 90% full). This repository does not investigate why.

Sending

stringRedisTemplate.convertAndSend(channel, message) returns the number of receivers:

• to a channel nobody listens to: 0, and the message is gone;
• to news with the annotation listener: 1;
• after a second listener is added to the same channel through the container, still 1, because the container subscribes once to the channel and fans out to its listeners in-process. Both listeners received the message. The number is receivers on the Redis side, not listeners on yours.

PUBSUB NUMSUB alerts.disk is 0 while a pattern subscriber is receiving on alerts.*: NUMSUB counts channel subscribers only, and PUBSUB NUMPAT counts patterns. PUBLISH counts both.

At-most-once

12-pub-sub-lost-message.txt: stop the container, publish, start it again.

template.convertAndSend("news", "while you were away") = 0 receivers
...
messages the listener has: []

Nothing is queued. That is the design, and it is why Pub/Sub is right for cache invalidation hints and live dashboards, and wrong for anything that must not be lost. For that, look at Redis Streams (not covered in this repository).

Going deeper

• PubSubTest
• Redis Pub/Sub
• Spring Data Redis: Pub/Sub messaging

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