Install Redis with Docker: Persistence, Basic Commands and Caching

What is Redis #

Redis is an in-memory data store. Because it keeps data in RAM, reads and writes take well under a millisecond, which makes it the usual choice for caches, sessions, rate limiters and queues. In this tutorial you run it in a container, secure it, persist its data and use it as a cache.

Run Redis #

docker run -d --name redis -p 6379:6379 -v redis-data:/data redis:7 redis-server --appendonly yes --requirepass "change-me"

• -v redis-data:/data keeps the data in a named volume (see Docker volumes).
• --appendonly yes turns on the append-only file, so Redis replays every write after a restart. Without it only periodic snapshots (RDB) are saved.
• --requirepass sets a password. Never publish a Redis without one to a network you do not control.

Connect with the client inside the container:

docker exec -it redis redis-cli -a change-me

Basic commands #

SET greeting "hello"
GET greeting
SET session:42 "alice" EX 60      # expires in 60 seconds
TTL session:42
INCR visits                       # atomic counter
HSET user:1 name "Ana" plan "pro" # hash
HGETALL user:1
LPUSH jobs "send-email"           # list used as a queue
RPOP jobs
SADD tags "aws" "docker"          # set
KEYS *                            # only for debugging, use SCAN in production

Use a prefix with a colon (user:1, session:42) to organize keys.

Redis in Docker Compose #

services:
  redis:
    image: redis:7
    command: ["redis-server", "--appendonly", "yes", "--requirepass", "${REDIS_PASSWORD}"]
    ports:
      - "127.0.0.1:6379:6379"
    volumes:
      - redis-data:/data
    healthcheck:
      test: ["CMD", "redis-cli", "-a", "${REDIS_PASSWORD}", "ping"]
      interval: 10s
      timeout: 3s
      retries: 5
volumes:
  redis-data:

Binding to 127.0.0.1 exposes the port only on your machine. Other containers in the same Compose project reach Redis at redis:6379. See the Docker Compose tutorial.

Cache-aside pattern #

The application looks in Redis first and, on a miss, reads the database and stores the answer with an expiration.

import json, redis
r = redis.Redis(host="localhost", password="change-me", decode_responses=True)

def get_user(user_id, db):
    key = f"user:{user_id}"
    cached = r.get(key)
    if cached:
        return json.loads(cached)
    user = db.query_user(user_id)       # slow path
    r.set(key, json.dumps(user), ex=300) # cache for 5 minutes
    return user

The expiration keeps the cache from serving stale data forever.

Memory limits and eviction #

A cache must not grow without limit:

docker exec redis redis-cli -a change-me CONFIG SET maxmemory 256mb
docker exec redis redis-cli -a change-me CONFIG SET maxmemory-policy allkeys-lru

With allkeys-lru Redis evicts the least recently used keys when it reaches the limit. Check usage with INFO memory.

Redis on AWS #

For production on AWS use Amazon ElastiCache, which handles replication, failover and patching. Use the container for development and tests.