Add redis: Boot 4.1 + Spring Data Redis 4.1, the JDK-serialisation default vs Jackson 3 serializers, @RedisHash TTL and keyspace events, @RedisListener, and Redis as the Spring cache

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
2026-09-21 15:46:55 +00:00
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+[Home](../README.md) | Prev: [Hash TTL and keyspace events](06-hash-ttl-and-keyspace-events.md) | Next: [TTL](08-ttl.md)
+
+# 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](../src/main/java/com/ankurm/redis/messaging/NewsListener.java)):
+
+```java
+@RedisListener(topic = "news")
+void onNews(String body) { ... }
+```
+
+[11-pub-sub.txt](output/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](output/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](../src/test/java/com/ankurm/redis/PubSubTest.java)
+- [Redis Pub/Sub](https://redis.io/docs/latest/develop/pubsub/)
+- [Spring Data Redis: Pub/Sub messaging](https://docs.spring.io/spring-data/redis/reference/redis/pubsub.html)
+
+[Home](../README.md) | Prev: [Hash TTL and keyspace events](06-hash-ttl-and-keyspace-events.md) | Next: [TTL](08-ttl.md)