Data Eviction in Redis

Introduction to Data Eviction

Data eviction in Redis refers to the process of removing data from memory when certain conditions are met, such as when the available memory is fully utilized. Redis uses various eviction policies to determine which data should be removed in order to free up space.

Eviction Policies

Redis offers several eviction policies to manage memory usage:

• noeviction: Returns an error when the memory limit is reached and a write operation is attempted.
• allkeys-lru: Evicts the least recently used (LRU) keys in the entire keyspace.
• volatile-lru: Evicts the least recently used (LRU) keys with an expiry set.
• allkeys-random: Randomly evicts keys in the entire keyspace.
• volatile-random: Randomly evicts keys with an expiry set.
• volatile-ttl: Evicts keys with the shortest time-to-live (TTL).

Configuring Eviction Policies

To configure the eviction policy in Redis, you can set the maxmemory-policy directive in the Redis configuration file (redis.conf) or use the CONFIG SET command.

Setting Maxmemory

To enable eviction policies, you need to set a maximum memory limit using the maxmemory directive in the Redis configuration file or the CONFIG SET command.

Monitoring and Debugging

Redis provides several commands to monitor memory usage and eviction:

• INFO memory: Provides statistics about Redis memory usage.
• MEMORY STATS: Returns an array of memory-related metrics.
• MEMORY USAGE : Estimates the memory usage of a key.

Example Scenario

Consider a scenario where you have limited memory available and want to ensure that the most recently used data remains in memory while older data is evicted.

1. Set the maximum memory limit to 50MB:
CONFIG SET maxmemory 50mb
2. Set the eviction policy to allkeys-lru:
CONFIG SET maxmemory-policy allkeys-lru
3. Start adding data to Redis and observe the eviction process when the memory limit is reached.