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# 8. TTL: setting it, reading it, and losing it

[13-ttl.txt](output/13-ttl.txt) drives `StringRedisTemplate` and checks the server with `redis-cli`.

## Reading a TTL

`getExpire(key)` returns seconds, with two special values that are easy to misread as errors:

| Value | Meaning |
|---|---|
| `-1` | The key exists and has no TTL |
| `-2` | The key does not exist |

Redis rounds `TTL` to the nearest second (a fresh 10-minute TTL reads `600`). The template's
`getExpire(key, TimeUnit)` converts, and the conversion **truncates**: the same key, read a moment
later, gave `600` seconds and `9` minutes.

## Setting one

- `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.
- `Duration.ofMillis(1500)` is honoured (`getExpire(key, MILLISECONDS)` is at most 1500); `getExpire(key)` in seconds reads `1`.
- `expire(key, Duration)` sets a TTL on an existing key; `persist(key)` removes it.

## The trap: a plain `set` throws the TTL away

```
set("session", "v1", Duration.ofMinutes(10))   -> TTL 600
set("session", "v2")                           -> TTL -1
```

Overwriting a key with a plain `SET` clears its TTL. If you refresh a cached value, pass the TTL again, or
overwrite with `Expiration.keepTtl()` (the `KEEPTTL` option), which the transcript shows leaving the TTL at 600.

## setIfAbsent with a TTL

`setIfAbsent("lock:job", "worker-1", Duration.ofSeconds(30))` is a `SET` with the `NX` option and a 30-second expiry: the first caller gets
`true`, the second `false`, and the key disappears by itself if the holder dies. It is the smallest useful
lock. It is not a complete distributed-lock recipe (no owner-checked release, no fencing), and this repository does
not test one.

## Expiry is real

The transcript sets a 300 ms TTL, reads the value, waits until `get` returns `null`, and confirms with
`EXISTS blink` returning `0`.

## Going deeper

- [TtlTest](../src/test/java/com/ankurm/redis/TtlTest.java)
- [Redis EXPIRE](https://redis.io/docs/latest/commands/expire/) and [SET](https://redis.io/docs/latest/commands/set/)
- [Chapter 6](06-hash-ttl-and-keyspace-events.md) for TTL on `@RedisHash` entities

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