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: [@RedisHash](05-redis-hash.md) | Next: [Pub/Sub](07-pub-sub.md)
+
+# 6. Hash TTL and keyspace events
+
+`@RedisHash(value = "session", timeToLive = 2)` gives every saved [Session](../src/main/java/com/ankurm/redis/hash/Session.java)
+a two-second TTL. Redis will delete the hash when the time is up. The question is what happens to
+everything Spring Data Redis wrote *around* the hash: the id in the `session` set, the
+`session:user:ankur` index, the `session:s1:idx` bookkeeping set (see [chapter 5](05-redis-hash.md)).
+
+## With Boot's defaults: nothing
+
+Boot enables Redis repositories with `enableKeyspaceEvents = OFF`
+([16-serializers-javap.txt](output/16-serializers-javap.txt), "Defaults of @EnableRedisRepositories").
+Redis expires the hash and nobody tells the application
+([09-hash-ttl-events-off.txt](output/09-hash-ttl-events-off.txt)):
+
+```
+$ redis-cli -p 6390 KEYS '*' | sort
+session
+session:s1:idx
+session:user:ankur
+$ redis-cli -p 6390 SMEMBERS session | sort
+s1
+...
+sessions.findById("s1")   = Optional.empty
+sessions.count()          = 1
+```
+
+`findById` is right (there is no hash), `count()` is wrong (the id is still in the set), and
+`session:user:ankur` still lists an id that resolves to nothing. On a busy system this accumulates.
+
+## With ON_STARTUP: cleanup
+
+`@EnableRedisRepositories(enableKeyspaceEvents = ON_STARTUP)` changes three things
+([10-hash-ttl-events-on.txt](output/10-hash-ttl-events-on.txt), test config in
+[HashTtlEventsOnTest](../src/test/java/com/ankurm/redis/HashTtlEventsOnTest.java)):
+
+1. **The server setting.** Redis only publishes expiry notifications if `notify-keyspace-events` is set. It is empty by default, and Spring Data Redis sets it at startup (the annotation's `keyspaceNotificationsConfigParameter` defaults to `Ex`; Redis reports it back as `xE`):
+
+   ```
+   $ redis-cli -p 6390 --no-raw CONFIG GET notify-keyspace-events
+   1) "notify-keyspace-events"
+   2) "xE"
+   ```
+
+   Some managed Redis services restrict `CONFIG SET`; this repository does not run against one, so check yours. The `EnableKeyspaceEvents` enum also has an `ON_DEMAND` value, which is not exercised here.
+
+2. **A phantom key.** Once a hash has expired there is nothing left to read, so Spring Data Redis writes a copy, `session:s1:phantom`, that outlives the hash by five minutes (302 seconds for a 2-second TTL). When the expiry notification arrives, Spring reads the phantom copy, removes the index entries, and deletes the phantom.
+
+3. **An application event.** `RedisKeyExpiredEvent` is published with the deleted entity as its value, so an `@EventListener` can react (audit, cleanup, notify):
+
+   ```
+   RedisKeyExpiredEvent: keyspace=session id=s1 value=Session[id=s1, user=ankur]
+   ```
+
+After it, `DBSIZE` is 0 and `count()` is 0.
+
+## The catch
+
+The expiry notification is delivered over Pub/Sub, which is fire-and-forget: a subscriber that is
+not connected when the message is published never sees it ([chapter 7](07-pub-sub.md) shows that
+with a stopped listener). If your application is down when a session expires, or the connection
+drops, the cleanup for that entry is not guaranteed. This repository does not test a missed notification. If exactness matters, do not build correctness on keyspace events; run a periodic job.
+
+## Going deeper
+
+- [HashTtlEventsOffTest](../src/test/java/com/ankurm/redis/HashTtlEventsOffTest.java)
+- [Redis keyspace notifications](https://redis.io/docs/latest/develop/pubsub/keyspace-notifications/)
+- [Spring Data Redis: Expirations and Time To Live](https://docs.spring.io/spring-data/redis/reference/redis/redis-repositories/expirations.html)
+
+[Home](../README.md) | Prev: [@RedisHash](05-redis-hash.md) | Next: [Pub/Sub](07-pub-sub.md)