Redis Eviction Policies Explained

Redis provides several eviction policies to decide which keys to remove when the memory limit (maxmemory) is reached. The policies are configured in the redis.conf file and can be examined in the source code.

Volatile policies (apply only to keys with an expiration time):

volatile-ttl : Removes keys that will expire soonest.

volatile-random : Randomly removes keys with an expiration time.

volatile-lru : Uses the Least Recently Used algorithm to evict the least recently accessed expiring keys.

volatile-lfu : Uses the Least Frequently Used algorithm, first discarding keys with few accesses and then, if needed, the oldest among those with equal access counts.

All‑keys policies (apply to all keys regardless of expiration):

allkeys-random : Randomly removes any key.

allkeys-lru : Evicts the least recently used key among all keys.

allkeys-lfu : Evicts the least frequently used key among all keys.

If none of these policies are selected, Redis defaults to noeviction , meaning that when memory exceeds maxmemory write operations will fail with an error.

The default policy is set in the server initialization code. In server.c , the initServer function contains the line:

void initServer(void) {
    ...
    server.maxmemory_policy = MAXMEMORY_NO_EVICTION;
    ...
}

This line establishes the noeviction behavior unless the configuration is changed.