Redis

Redis (Remote Dictionary Server) is an in-memory data structure store, used as a database, cache, and message broker. It is incredibly fast.

When to Use

In-Memory Caching & Sub-Millisecond Reads: Caching hot database objects, session state, and computed API payloads in RAM.
Distributed Locking & Concurrency Control: Coordinating atomic multi-instance synchronization with Redlock or SET NX PX commands.
High-Velocity Queues & Pub/Sub Messaging: Powering message brokers, task queues (BullMQ, Sidekiq), and real-time event broadcasting.
Rate Limiting & Sliding Windows: Implementing sliding window rate limiters and leaderboards via Redis Sorted Sets (ZSET).

# Set a value with 10 second expiry
SET session:123 "active" EX 10

# Increment a counter atomically
INCR page:views