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Farhad Rahimi Klie
Farhad Rahimi Klie

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Redis Database: Complete Guide (Architecture, Internals, and Real-World Usage)

#redis #database #cache #programming

Redis (Remote Dictionary Server) is an in-memory data structure store used as a database, cache, message broker, and streaming engine.
It is designed for extreme performance, low latency, and high throughput, often operating at sub-millisecond response times.

This article is a full Redis reference, from internal architecture to every major command and real-world usage pattern.

1. What is Redis?

Redis is:

  • In-memory (RAM-based)
  • Key-value store
  • Single-threaded (core execution)
  • Non-relational (NoSQL)
  • Extremely fast
  • Persistent (optional)

Common Use Cases

  • Caching
  • Session storage
  • Real-time analytics
  • Rate limiting
  • Pub/Sub systems
  • Leaderboards
  • Queues
  • Distributed locks

2. Redis Architecture

High-Level Architecture 

Client
  |
TCP Connection
  |
Redis Server
  |
In-Memory Data Structures (RAM)
  |
Persistence (RDB / AOF)
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Key Architectural Properties

  • Single-threaded command execution
  • Event-driven I/O (epoll/kqueue)
  • In-memory first
  • Optional disk persistence
  • Replication and clustering support

3. Redis Internal Design

3.1 Event Loop (I/O Multiplexing)

Redis uses:

  • epoll (Linux)
  • kqueue (macOS/BSD)

This allows Redis to:

  • Handle thousands of concurrent connections
  • Remain single-threaded
  • Avoid context switching overhead

3.2 Memory Model

Redis stores data in:

  • RAM
  • Optimized internal encodings

Examples:

  • Small integers → integer encoding
  • Small strings → embedded in object
  • Large strings → heap allocation

3.3 Redis Objects

Every key maps to a Redis Object:

redisObject {
  type
  encoding
  refcount
  ptr
}
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Object types:

  • String
  • List
  • Set
  • Hash
  • ZSet
  • Stream

4. Redis Data Types (Complete)

4.1 Strings

Description

  • Binary safe
  • Max size: 512 MB

Commands 

SET key value
GET key
DEL key
EXISTS key
INCR counter
DECR counter
APPEND key "text"
STRLEN key
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With Expiration 

SET token abc123 EX 60
TTL token
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4.2 Lists (Linked Lists)

Description

  • Ordered
  • Allows duplicates
  • Stack & Queue behavior

Commands 

LPUSH mylist a b c
RPUSH mylist d
LPOP mylist
RPOP mylist
LRANGE mylist 0 -1
LLEN mylist
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Blocking Operations (Queues) 

BLPOP queue 0
BRPOP queue 0
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4.3 Sets (Unordered, Unique)

Commands 

SADD users alice bob charlie
SMEMBERS users
SISMEMBER users alice
SREM users bob
SCARD users
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Set Operations 

SUNION set1 set2
SINTER set1 set2
SDIFF set1 set2
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4.4 Hashes (Key-Value inside Key)

Commands 

HSET user:1 name "Ali" age 25
HGET user:1 name
HGETALL user:1
HDEL user:1 age
HEXISTS user:1 name
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4.5 Sorted Sets (ZSET)

Description

  • Unique members
  • Ordered by score (double)

Commands 

ZADD leaderboard 100 alice 200 bob
ZRANGE leaderboard 0 -1
ZREVRANGE leaderboard 0 -1
ZSCORE leaderboard alice
ZRANK leaderboard alice
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4.6 Streams (Log-Based Data)

Commands 

XADD logs * event login user ali
XRANGE logs - +
XREAD COUNT 2 STREAMS logs 0
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Consumer Groups 

XGROUP CREATE logs group1 0
XREADGROUP GROUP group1 consumer1 STREAMS logs >
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5. Keys Management 

KEYS *
SCAN 0
DEL key
EXPIRE key 60
PERSIST key
TTL key
TYPE key
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⚠️ Avoid KEYS * in production.

6. Redis Persistence

6.1 RDB (Snapshotting)

  • Periodic snapshots
  • Fast restart
  • Risk of data loss

Config:

save 900 1
save 300 10
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Manual:

SAVE
BGSAVE
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6.2 AOF (Append-Only File)

  • Logs every write operation
  • Safer than RDB
  • Slower

Config:

appendonly yes
appendfsync everysec
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Rewrite:

BGREWRITEAOF
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6.3 Hybrid Persistence

  • RDB + AOF combined

7. Redis Replication

Master → Replica 

Master
  |
Replica 1
Replica 2
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Commands:

REPLICAOF host port
INFO replication
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Properties:

  • Asynchronous
  • Read scaling
  • Failover ready

8. Redis Sentinel (High Availability)

Purpose

  • Monitor Redis
  • Automatic failover
  • Leader election

Components:

  • Sentinel processes
  • Master
  • Replicas

9. Redis Cluster (Horizontal Scaling)

Features

  • Sharding
  • 16384 hash slots
  • Automatic rebalancing

Hash slot calculation:

CRC16(key) % 16384
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Cluster commands:

CLUSTER INFO
CLUSTER NODES
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10. Transactions in Redis

MULTI / EXEC 

MULTI
SET a 10
INCR a
EXEC
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DISCARD 

DISCARD
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Optimistic Locking (WATCH) 

WATCH balance
MULTI
DECR balance
EXEC
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11. Lua Scripting

Purpose

  • Atomic operations
  • Complex logic

Example:

EVAL "return redis.call('GET', KEYS[1])" 1 mykey
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Lua guarantees:

  • Atomic execution
  • No race conditions

12. Pub/Sub Messaging

Publisher 

PUBLISH news "Hello World"
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Subscriber 

SUBSCRIBE news
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Properties:

  • Fire-and-forget
  • No persistence

13. Security in Redis

Authentication 

AUTH password
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Config:

requirepass strongpassword
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ACL (Redis 6+) 

ACL SETUSER ali on >password +get +set
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14. Performance Characteristics

  • O(1) average operations
  • Millions of ops/sec
  • Single-threaded = predictable latency

Optimization Tips

  • Avoid large values
  • Use pipelines
  • Use SCAN instead of KEYS
  • Set TTLs aggressively

15. Redis as Cache (Real Example)

Node.js Example 

import Redis from "ioredis";
const redis = new Redis();

await redis.set("user:1", JSON.stringify(user), "EX", 60);

const cached = await redis.get("user:1");
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16. Redis as Rate Limiter 

INCR ip:1
EXPIRE ip:1 60
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17. Redis as Queue 

LPUSH jobs task1
BRPOP jobs 0
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18. Redis vs Other Databases

Feature Redis MongoDB PostgreSQL
In-Memory Yes No No
Speed Ultra Fast Fast Moderate
Transactions Limited Yes Full ACID
Schema None Flexible Strict

19. When NOT to Use Redis

  • Large datasets exceeding RAM
  • Complex relational queries
  • Strong ACID guarantees required
  • Long-term data storage

20. Final Summary

Redis is:

  • A high-performance in-memory database
  • Ideal for real-time systems
  • Powerful when combined with SQL or NoSQL databases
  • Simple API, deep internals

Mastering Redis gives you:

  • Scalable systems
  • Faster applications
  • Production-grade architecture skills

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