Name: Redis
Availability: InStock
Author: ivangdavila

Keys without TTL live forever—set expiry on every cache key:
```
SET key value EX 3600
```
Can't add TTL after SET without another command—use
```
SETEX
```
or
```
SET ... EX
```
```
EXPIRE
```
resets on key update by default—
```
SET
```
removes TTL; use
```
SET ... KEEPTTL
```
(Redis 6+)
Lazy expiration: expired keys removed on access—may consume memory until touched
```
SCAN
```
with large database: expired keys still show until cleanup cycle runs

Sorted sets for rate limiting:

ZADD limits:{user} {now} {request_id}

ZREMRANGEBYSCORE

for sliding window

HyperLogLog for unique counts:
```
PFADD visitors {ip}
```
uses 12KB for billions of uniques
Streams for queues:
```
XADD
```
,
```
XREAD
```
,
```
XACK
```
—better than LIST for reliable queues
Hashes for objects:
```
HSET user:1 name "Alice" email "a@b.com"
```
—more memory efficient than JSON string

```
GET
```
then
```
SET
```
is not atomic—another client can modify between; use
```
INCR
```
,
```
SETNX
```
, or Lua
```
SETNX
```
for locks:
```
SET lock:resource {token} NX EX 30
```
—NX = only if not exists
```
WATCH
```
/
```
MULTI
```
/
```
EXEC
```
for optimistic locking—transaction aborts if watched key changed
Lua scripts are atomic—use for complex operations:
```
EVAL "script" keys args
```

RDB (snapshots): fast recovery, but data loss between snapshots—default every 5min
AOF (append log): less data loss, slower recovery—
```
appendfsync everysec
```
is good balance
Both off = pure cache—acceptable if data can be regenerated
```
BGSAVE
```
for manual snapshot—doesn't block but forks process, needs memory headroom

```
maxmemory
```
must be set—without it, Redis uses all RAM, then swap = disaster
Eviction policies:
```
allkeys-lru
```
for cache,
```
volatile-lru
```
for mixed,
```
noeviction
```
for persistent data
```
INFO memory
```
shows usage—monitor
```
used_memory
```
vs
```
maxmemory
```
Large keys hurt eviction—one 1GB key evicts poorly; prefer many small keys

Hash slots: keys distributed by hash—same slot required for multi-key operations
Hash tags:
```
{user:1}:profile
```
and
```
{user:1}:sessions
```
go to same slot—use for related keys
No cross-slot
```
MGET
```
/
```
MSET
```
—error unless all keys in same slot
```
MOVED
```
redirect: client must follow—use cluster-aware client library

Redis