Claude
b0c39a8d4c
Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
72 lines
3.7 KiB
Markdown
72 lines
3.7 KiB
Markdown
[Home](../README.md) | Prev: [@RedisHash](05-redis-hash.md) | Next: [Pub/Sub](07-pub-sub.md)
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# 6. Hash TTL and keyspace events
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`@RedisHash(value = "session", timeToLive = 2)` gives every saved [Session](../src/main/java/com/ankurm/redis/hash/Session.java)
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a two-second TTL. Redis will delete the hash when the time is up. The question is what happens to
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everything Spring Data Redis wrote *around* the hash: the id in the `session` set, the
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`session:user:ankur` index, the `session:s1:idx` bookkeeping set (see [chapter 5](05-redis-hash.md)).
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## With Boot's defaults: nothing
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Boot enables Redis repositories with `enableKeyspaceEvents = OFF`
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([16-serializers-javap.txt](output/16-serializers-javap.txt), "Defaults of @EnableRedisRepositories").
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Redis expires the hash and nobody tells the application
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([09-hash-ttl-events-off.txt](output/09-hash-ttl-events-off.txt)):
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```
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$ redis-cli -p 6390 KEYS '*' | sort
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session
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session:s1:idx
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session:user:ankur
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$ redis-cli -p 6390 SMEMBERS session | sort
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s1
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...
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sessions.findById("s1") = Optional.empty
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sessions.count() = 1
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```
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`findById` is right (there is no hash), `count()` is wrong (the id is still in the set), and
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`session:user:ankur` still lists an id that resolves to nothing. On a busy system this accumulates.
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## With ON_STARTUP: cleanup
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`@EnableRedisRepositories(enableKeyspaceEvents = ON_STARTUP)` changes three things
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([10-hash-ttl-events-on.txt](output/10-hash-ttl-events-on.txt), test config in
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[HashTtlEventsOnTest](../src/test/java/com/ankurm/redis/HashTtlEventsOnTest.java)):
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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`):
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```
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$ redis-cli -p 6390 --no-raw CONFIG GET notify-keyspace-events
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1) "notify-keyspace-events"
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2) "xE"
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```
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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.
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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.
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3. **An application event.** `RedisKeyExpiredEvent` is published with the deleted entity as its value, so an `@EventListener` can react (audit, cleanup, notify):
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```
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RedisKeyExpiredEvent: keyspace=session id=s1 value=Session[id=s1, user=ankur]
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```
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After it, `DBSIZE` is 0 and `count()` is 0.
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## The catch
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The expiry notification is delivered over Pub/Sub, which is fire-and-forget: a subscriber that is
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not connected when the message is published never sees it ([chapter 7](07-pub-sub.md) shows that
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with a stopped listener). If your application is down when a session expires, or the connection
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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.
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## Going deeper
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- [HashTtlEventsOffTest](../src/test/java/com/ankurm/redis/HashTtlEventsOffTest.java)
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- [Redis keyspace notifications](https://redis.io/docs/latest/develop/pubsub/keyspace-notifications/)
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- [Spring Data Redis: Expirations and Time To Live](https://docs.spring.io/spring-data/redis/reference/redis/redis-repositories/expirations.html)
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[Home](../README.md) | Prev: [@RedisHash](05-redis-hash.md) | Next: [Pub/Sub](07-pub-sub.md)
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