120 lines
6.2 KiB
Markdown
120 lines
6.2 KiB
Markdown
# 🔴 Redis
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## Overview
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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).
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## Data structures
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| Structure | Description | Use case |
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|-----------|-------------|----------|
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| **String** | Binary string (max 512 MB) | Cache values, session tokens, counters |
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| **Hash** | Map field-value | User profile, cached object |
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| **List** | Linked list (push/pop on both ends) | Queue (RPUSH/LPOP), log stream |
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| **Set** | Unique values (unordered) | Tags, deduplication, memberships |
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| **Sorted Set** | Unique + score (sorted) | Leaderboards, rate limiting, timeouts |
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| **Bitmap** | Bit field | Feature flags, daily active users |
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| **HyperLogLog** | Approximate cardinality (12 KB = 2^64) | Unique visitors (error < 1%) |
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| **Stream** | Append-only log (Kafka-like) | Event store, messaging |
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| **Geospatial** | Geo-indexing (GEOADD, GEOSEARCH) | Location queries, proximity search |
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| **JSON** | JSON document (RedisJSON module) | Document structures |
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## Eviction policies
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| Policy | Description | Use case |
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|--------|-------------|----------|
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| **noeviction** | Error on write when full | Transactional data, must not lose |
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| **allkeys-lru** | LRU on all keys | General cache, standard |
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| **allkeys-lfu** | LFU on all keys | Frequently accessed data |
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| **volatile-lru** | LRU on keys with TTL | Cache with expiration |
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| **volatile-ttl** | Closest to expiration | Short-lived data |
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| **allkeys-random** | Random | Testing |
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## Redis Cluster vs Sentinel
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| Feature | Redis Sentinel | Redis Cluster |
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|---------|---------------|---------------|
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| **Scaling** | Read replicas (master + replica) | Data sharding (16384 hash slots) |
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| **Auto-failover** | Yes (Sentinel) | Yes (gossip-based) |
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| **Multi-key ops** | Yes (transactions on master) | Limited (same hash slot) |
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| **Client communication** | Via Sentinel (deprecated) | Cluster nodes redirect (MOVED/ASK) |
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| **Minimum nodes** | Master + Replica + 3 Sentinel | 3 masters (each with replica) |
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| **Use case** | High availability, single shard | Multi-shard, horizontal scaling |
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## Persistence
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| Method | Description | RTO | RPO | Use case |
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|--------|-------------|-----|-----|----------|
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| **RDB** (Redis Database) | Periodic snapshot to dump.rdb | Minutes | Last snapshot | Cache, loss tolerated |
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| **AOF** (Append-Only File) | Append-only log of all write operations | Seconds | 1 s (fsync every sec) | Data must not be lost |
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| **RDB + AOF** | Combination | Seconds | 1 s | Recommended for production |
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## Modules (RediSearch, RedisJSON, RedisGraph)
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Redis is extensible via modules:
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- **RediSearch** — full-text search, facets, prefix/suffix search
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- **RedisJSON** — JSON path queries, document manipulation
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- **RedisGraph** — graph DB (based on Cypher, deprecated since 2025)
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- **RedisTimeSeries** — time-series with downsampling, retention policies
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- **RedisBloom** — Bloom filters, Cuckoo filters, Top-K, Count-Min Sketch
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## Memcached vs Redis
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| Feature | Redis | Memcached |
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|---------|-------|-----------|
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| **Data structures** | String, Hash, List, Set, Sorted Set, Stream, JSON | String only |
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| **Persistence** | RDB + AOF | None (purely in-memory) |
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| **Replication** | Master-replica, Cluster | None (multi-threaded) |
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| **Eviction** | 6 policies | LRU only |
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| **Lua scripting** | Yes (EVAL) | No |
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| **Transactions** | Yes (MULTI/EXEC) | No |
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| **Pub/Sub** | Yes | No |
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| **Streaming** | Yes (Stream) | No |
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## Recommendations — where Redis is better
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| Area | Redis | Competition | Why Redis |
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|------|-------|-------------|-----------|
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| **Cache (in-memory)** | < 1 ms latency, 6 eviction policies | Memcached (LRU string only) | Richer data types, persistence, cluster |
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| **Session store** | Hash + TTL, Cluster for HA | DynamoDB (higher latency) | Simpler, faster, native expiration |
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| **Rate limiting** | Sorted Set (sliding window counter) | Application in DB (complex) | Atomic operations, built-in logic |
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| **Leaderboard / scoring** | Sorted Set (ZADD, ZRANK, ZREVRANGE) | SQL (ORDER BY + COUNT = expensive) | O(1) rank, O(log N) insert |
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| **Message queue** | List/Stream (RPUSH+BLPOP) | Kafka (heavy, JVM) | Lightweight, embedded, no broker |
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| **Real-time analytics** | HyperLogLog + Bitmap + Stream | ClickHouse (heavy analytics) | Real-time aggregation, small RAM |
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| **Geolocation** | GEOADD, GEOSEARCH, GEODIST | PostGIS (heavier, disk-based) | In-memory, ideal for real-time |
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### When to use Redis
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- **Cache for API** — response cache, DB query cache, session cache
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- **Session management** — distributed sessions across servers
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- **Rate limiting** — API gateway, per-user/per-IP limits
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- **Leaderboards / rankings** — real-time scoring
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- **Message broker** — task queue (RQ, Celery with Redis), pub/sub notifications
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- **Real-time analytics** — counting uniques, metrics, dashboards
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- **Geo-proximity** — "find nearest branch" in < 1 ms
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### When to use something else
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- **Persistent data with SQL queries** → PostgreSQL or MySQL
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- **Large volumes > RAM** → Memcached (multi-threaded), Dragonfly (more RAM utilization)
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- **Long-term message queue** → Kafka, RabbitMQ (disk-based persistence)
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- **Document DB** → MongoDB (persistent, complex queries)
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## Redis licensing
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Redis underwent a major license change in 2024:
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| Period | License | Conditions |
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|--------|---------|------------|
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| **Until March 2024** | BSD 3-clause (open source) | Completely free use, including managed services |
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| **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 |
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| **Valkey (fork, Linux Foundation)** | BSD 3-clause | Fully open source fork of Redis 7.2, supported by Linux Foundation, AWS, Google, Oracle |
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**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.
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## Sources
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References, books, and standards: [sources/databases/sources.md](sources/databases/sources.md)
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*Last revision: 2026-06-03*
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