Redis is an open source, key-value database built with an in-memory design that emphasizes speed. It has support for rich data types, atomic operations, and Lua scripting.
Redis stores its data, called keys, in memory only and uses eviction policies to free memory to write new data. Eviction policies fall into two main categories: general policies that apply to all keys and policies that use a Time to Live (TTL) expiration value. General policies consume less memory but require more CPU processing when Redis samples to choose which key to evict. TTL policies require you to set the TTL from your application. The extra TTL data consumes a bit more memory but TTL policies require less CPU processing when Redis is determining which keys to evict.
INFO command to reveal cache hits/misses to fine tune Redis.
General policies apply to any keys that do not have expiration set.
noeviction Don’t evict any data, returns error when memory limit is reached. Default
noeviction policy set, Redis may stop responding if it runs out of memory but no data is ever evicted. This policy is generally appropriate only when your application removes keys itself. This is the Redis default setting and poses the least chance of data loss.
allkeys-lru Evict any key, least recently used (LRU) first. Recommended.
allkeys-lru helps keep Redis from becoming unresponsive due to insufficient memory and operates on the assumption that you no longer need the least recently used keys. When Redis begins to run out of memory, it samples a small set of keys using an algorithm, then evicts the least recently key from that set. Because of the sampling algorithm, the key may not be the least recently used of all keys in memory.
allkeys-random Evict keys in a random order.
allkeys-random randomly evicts keys. It is appropriate for cases where your application continuously scans keys or no key is more important than any other key.
These policies require that some keys have an expiration set.
volatile-lru Evict keys with expiration only, least recently used (LRU) first.
This policy is similar to
allkeys-lru. Redis will begin evicting keys least recently used first, but will only sample keys that are expired. This policy operates on the assumption that expired keys that are also least recently used are no longer required by your application.
volatile-random Evict keys with expiration only in a random order.
Similar to the
allkeys-random policy, with this policy Redis evicts random keys but only those that have expired.
volatile-ttl Evict keys with expiration only, shortest time-to-live (TTL) first.
volatile-ttl policy frees memory by evicting expired keys, regardless of when the key was last used. This policy allows you to tell Redis which keys are most important by explicitly setting an expiration value.
Learn more about Redis eviction policies in the official Redis documentation, Using Redis as an LRU cache