# Cache: Redis & Memcached

Quick Reference: Write Policy | Replacement Policy | Step 2: In-Memory Databases

# Quick Reference

# Clear Definition

Redis and Memcached are in-memory caching systems that store data in RAM for extremely fast access. Redis is feature-rich with persistence options, while Memcached is simple and focused on pure caching performance.

💡 Key Insight: Use Redis for complex caching needs (data structures, persistence). Use Memcached for simple, high-performance caching.

# Core Concepts

# Redis Features

• Data Structures: Rich data types beyond key-value
• Persistence: Optional RDB snapshots and AOF logging
• Replication: Master-slave replication for high availability
• Pub/Sub: Publish-subscribe messaging
• Transactions: Multi-command transactions
• Lua Scripting: Server-side scripting

# Memcached Features

• Simplicity: Key-value store only
• Performance: Extremely fast, low overhead
• Volatility: No persistence, data lost on restart
• Distributed: Client-side sharding

# Use Cases

# Redis Use Cases

1. Complex Caching: Session storage, user profiles
2. Real-time Features: Leaderboards, counters
3. Pub/Sub: Real-time notifications
4. Distributed Locks: Coordination in distributed systems

# Memcached Use Cases

1. Simple Caching: HTML fragments, API responses
2. Session Storage: Simple session data
3. Database Query Caching: Cache expensive queries

# Best Practices

1. Choose Right Tool: Redis for complex needs, Memcached for simple
2. Set TTLs: Always set expiration times
3. Monitor Memory: Track memory usage and eviction
4. Handle Failures: Have fallback to database

# Interview Tips

🎯 Interview Focus: Know when to use Redis vs Memcached, cache patterns.

# Quick Reference Summary

Redis: Feature-rich caching with persistence, replication, and data structures. Use for complex caching needs.

Memcached: Simple, fast key-value cache. Use for high-performance, simple caching.

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