Claude
b0c39a8d4c
Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
55 lines
2.4 KiB
Markdown
55 lines
2.4 KiB
Markdown
[Home](../README.md) | Prev: [Pub/Sub](07-pub-sub.md) | Next: [Redis as the Spring cache](09-redis-as-cache.md)
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# 8. TTL: setting it, reading it, and losing it
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[13-ttl.txt](output/13-ttl.txt) drives `StringRedisTemplate` and checks the server with `redis-cli`.
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## Reading a TTL
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`getExpire(key)` returns seconds, with two special values that are easy to misread as errors:
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| Value | Meaning |
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|---|---|
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| `-1` | The key exists and has no TTL |
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| `-2` | The key does not exist |
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Redis rounds `TTL` to the nearest second (a fresh 10-minute TTL reads `600`). The template's
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`getExpire(key, TimeUnit)` converts, and the conversion **truncates**: the same key, read a moment
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later, gave `600` seconds and `9` minutes.
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## Setting one
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- `opsForValue().set(key, value, Duration)` sets value and TTL in one command, so there is no window in which the key exists without a TTL.
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- `Duration.ofMillis(1500)` is honoured (`getExpire(key, MILLISECONDS)` is at most 1500); `getExpire(key)` in seconds reads `1`.
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- `expire(key, Duration)` sets a TTL on an existing key; `persist(key)` removes it.
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## The trap: a plain `set` throws the TTL away
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```
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set("session", "v1", Duration.ofMinutes(10)) -> TTL 600
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set("session", "v2") -> TTL -1
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```
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Overwriting a key with a plain `SET` clears its TTL. If you refresh a cached value, pass the TTL again, or
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overwrite with `Expiration.keepTtl()` (the `KEEPTTL` option), which the transcript shows leaving the TTL at 600.
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## setIfAbsent with a TTL
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`setIfAbsent("lock:job", "worker-1", Duration.ofSeconds(30))` is a `SET` with the `NX` option and a 30-second expiry: the first caller gets
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`true`, the second `false`, and the key disappears by itself if the holder dies. It is the smallest useful
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lock. It is not a complete distributed-lock recipe (no owner-checked release, no fencing), and this repository does
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not test one.
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## Expiry is real
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The transcript sets a 300 ms TTL, reads the value, waits until `get` returns `null`, and confirms with
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`EXISTS blink` returning `0`.
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## Going deeper
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- [TtlTest](../src/test/java/com/ankurm/redis/TtlTest.java)
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- [Redis EXPIRE](https://redis.io/docs/latest/commands/expire/) and [SET](https://redis.io/docs/latest/commands/set/)
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- [Chapter 6](06-hash-ttl-and-keyspace-events.md) for TTL on `@RedisHash` entities
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[Home](../README.md) | Prev: [Pub/Sub](07-pub-sub.md) | Next: [Redis as the Spring cache](09-redis-as-cache.md)
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