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Redis

Redis (Remote Dictionary Server) is an open-source, in-memory data structure store. It can be used as a database, cache, message broker, and streaming engine. Because it holds data in RAM, operations are exceptionally fast.

Key Features​

  • In-Memory Storage: Delivers microsecond latency for read and write operations.
  • Data Structures: Supports strings, hashes, lists, sets, sorted sets, bitmaps, hyperloglogs, and geospatial indexes.
  • Persistence: Can persist data to disk for durability, using RDB (snapshots) or AOF (Append Only File) mechanisms.
  • High Availability: Supports replication (primary-replica) and automatic failover via Redis Sentinel.
  • Clustering: Redis Cluster provides horizontal scaling and data sharding.

Installation​

Redis is easily installed from standard repositories.

  • Debian/Ubuntu: apt install redis-server
  • RHEL/CentOS: yum install redis (EPEL repository required)

Start and enable the service:

sudo systemctl start redis
sudo systemctl enable redis

Basic Operations​

Interact with Redis using the redis-cli command-line tool.

redis-cli

Common Commands​

  • Set a Key: SET mykey "Hello World"
  • Get a Key: GET mykey
  • Set with Expiration (Cache): SETEX mycachekey 60 "Temporary Data" (expires in 60 seconds)
  • List Operations:
    • LPUSH mylist "item1"
    • RPUSH mylist "item2"
    • LRANGE mylist 0 -1 (returns all items)
  • Hash Operations:
    • HSET user:1 name "John" age 30
    • HGET user:1 name
    • HGETALL user:1

Configuration and Best Practices​

The main configuration file is typically /etc/redis/redis.conf.

  • Security: By default, Redis binds to 127.0.0.1 and has no password.
    • If exposing Redis to the network, set a strong password using the requirepass directive.
    • Use the bind directive to restrict access to specific IP addresses.
  • Memory Management: Redis operates entirely in memory. It is crucial to set a memory limit to prevent it from exhausting system RAM and causing OOM (Out of Memory) kills.
    • Set maxmemory (e.g., maxmemory 2gb).
    • Configure the maxmemory-policy to dictate what happens when the limit is reached (e.g., allkeys-lru to evict least recently used keys, making it act as a cache).
  • Persistence: Choose the right persistence model based on the requirement.
    • RDB (Redis Database): Periodic snapshots. Faster restarts, but potential data loss between snapshots.
    • AOF (Append Only File): Logs every write operation. More durable but can be slower and result in larger files.