# 🔴 Redis

## Overview

Redis is an in-memory key-value store with advanced data structures, used primarily as a cache, session store, message broker, and real-time database. Runs in RAM with optional disk persistence (RDB/AOF).

## Data structures

| Structure | Description | Use case |
|-----------|-------------|----------|
| **String** | Binary string (max 512 MB) | Cache values, session tokens, counters |
| **Hash** | Map field-value | User profile, cached object |
| **List** | Linked list (push/pop on both ends) | Queue (RPUSH/LPOP), log stream |
| **Set** | Unique values (unordered) | Tags, deduplication, memberships |
| **Sorted Set** | Unique + score (sorted) | Leaderboards, rate limiting, timeouts |
| **Bitmap** | Bit field | Feature flags, daily active users |
| **HyperLogLog** | Approximate cardinality (12 KB = 2^64) | Unique visitors (error < 1%) |
| **Stream** | Append-only log (Kafka-like) | Event store, messaging |
| **Geospatial** | Geo-indexing (GEOADD, GEOSEARCH) | Location queries, proximity search |
| **JSON** | JSON document (RedisJSON module) | Document structures |

## Eviction policies

| Policy | Description | Use case |
|--------|-------------|----------|
| **noeviction** | Error on write when full | Transactional data, must not lose |
| **allkeys-lru** | LRU on all keys | General cache, standard |
| **allkeys-lfu** | LFU on all keys | Frequently accessed data |
| **volatile-lru** | LRU on keys with TTL | Cache with expiration |
| **volatile-ttl** | Closest to expiration | Short-lived data |
| **allkeys-random** | Random | Testing |

## Redis Cluster vs Sentinel

| Feature | Redis Sentinel | Redis Cluster |
|---------|---------------|---------------|
| **Scaling** | Read replicas (master + replica) | Data sharding (16384 hash slots) |
| **Auto-failover** | Yes (Sentinel) | Yes (gossip-based) |
| **Multi-key ops** | Yes (transactions on master) | Limited (same hash slot) |
| **Client communication** | Via Sentinel (deprecated) | Cluster nodes redirect (MOVED/ASK) |
| **Minimum nodes** | Master + Replica + 3 Sentinel | 3 masters (each with replica) |
| **Use case** | High availability, single shard | Multi-shard, horizontal scaling |

## Persistence

| Method | Description | RTO | RPO | Use case |
|--------|-------------|-----|-----|----------|
| **RDB** (Redis Database) | Periodic snapshot to dump.rdb | Minutes | Last snapshot | Cache, loss tolerated |
| **AOF** (Append-Only File) | Append-only log of all write operations | Seconds | 1 s (fsync every sec) | Data must not be lost |
| **RDB + AOF** | Combination | Seconds | 1 s | Recommended for production |

## Modules (RediSearch, RedisJSON, RedisGraph)

Redis is extensible via modules:
- **RediSearch** — full-text search, facets, prefix/suffix search
- **RedisJSON** — JSON path queries, document manipulation
- **RedisGraph** — graph DB (based on Cypher, deprecated since 2025)
- **RedisTimeSeries** — time-series with downsampling, retention policies
- **RedisBloom** — Bloom filters, Cuckoo filters, Top-K, Count-Min Sketch

## Memcached vs Redis

| Feature | Redis | Memcached |
|---------|-------|-----------|
| **Data structures** | String, Hash, List, Set, Sorted Set, Stream, JSON | String only |
| **Persistence** | RDB + AOF | None (purely in-memory) |
| **Replication** | Master-replica, Cluster | None (multi-threaded) |
| **Eviction** | 6 policies | LRU only |
| **Lua scripting** | Yes (EVAL) | No |
| **Transactions** | Yes (MULTI/EXEC) | No |
| **Pub/Sub** | Yes | No |
| **Streaming** | Yes (Stream) | No |

## Recommendations — where Redis is better

| Area | Redis | Competition | Why Redis |
|------|-------|-------------|-----------|
| **Cache (in-memory)** | < 1 ms latency, 6 eviction policies | Memcached (LRU string only) | Richer data types, persistence, cluster |
| **Session store** | Hash + TTL, Cluster for HA | DynamoDB (higher latency) | Simpler, faster, native expiration |
| **Rate limiting** | Sorted Set (sliding window counter) | Application in DB (complex) | Atomic operations, built-in logic |
| **Leaderboard / scoring** | Sorted Set (ZADD, ZRANK, ZREVRANGE) | SQL (ORDER BY + COUNT = expensive) | O(1) rank, O(log N) insert |
| **Message queue** | List/Stream (RPUSH+BLPOP) | Kafka (heavy, JVM) | Lightweight, embedded, no broker |
| **Real-time analytics** | HyperLogLog + Bitmap + Stream | ClickHouse (heavy analytics) | Real-time aggregation, small RAM |
| **Geolocation** | GEOADD, GEOSEARCH, GEODIST | PostGIS (heavier, disk-based) | In-memory, ideal for real-time |

### When to use Redis

- **Cache for API** — response cache, DB query cache, session cache
- **Session management** — distributed sessions across servers
- **Rate limiting** — API gateway, per-user/per-IP limits
- **Leaderboards / rankings** — real-time scoring
- **Message broker** — task queue (RQ, Celery with Redis), pub/sub notifications
- **Real-time analytics** — counting uniques, metrics, dashboards
- **Geo-proximity** — "find nearest branch" in < 1 ms

### When to use something else

- **Persistent data with SQL queries** → PostgreSQL or MySQL
- **Large volumes > RAM** → Memcached (multi-threaded), Dragonfly (more RAM utilization)
- **Long-term message queue** → Kafka, RabbitMQ (disk-based persistence)
- **Document DB** → MongoDB (persistent, complex queries)

## Redis licensing

Redis underwent a major license change in 2024:

| Period | License | Conditions |
|--------|---------|------------|
| **Until March 2024** | BSD 3-clause (open source) | Completely free use, including managed services |
| **Since March 2024** | RSALv2 + SSPL (dual license) | SSPL: if you offer Redis as a managed service, you must release the entire stack as open source. RSALv2: restrictions on cloud operators |
| **Valkey (fork, Linux Foundation)** | BSD 3-clause | Fully open source fork of Redis 7.2, supported by Linux Foundation, AWS, Google, Oracle |

**Impact**: Managed Redis services (AWS ElastiCache, Google Memorystore, Azure Cache for Redis) cannot use Redis 7.4+ without a commercial license → they are migrating to **Valkey**. For self-hosted Redis, no change — RSALv2/SSPL does not restrict internal use.

## Sources

References, books, and standards: [sources/databases/sources.md](sources/databases/sources.md)

*Last revision: 2026-06-03*