Eviction policies (LRU, LFU, random) in Redis

Introduction

Eviction policies help Redis decide which data to remove when memory is full. This keeps Redis fast and avoids crashes.

When your Redis memory is limited and you want to keep it running smoothly.

When you want to keep the most useful data and remove less important data automatically.

When you want to control how Redis frees up space without manual cleanup.

When you want to test different ways Redis removes old data to find the best for your app.

Syntax

Redis

maxmemory-policy <policy_name>

Set this in your Redis configuration file or at runtime with the CONFIG SET command.

Common policies: allkeys-lru, allkeys-lfu, volatile-lru, volatile-lfu, volatile-random, allkeys-random, noeviction.

Examples

Evicts least recently used keys from all keys when memory is full.

Redis

CONFIG SET maxmemory-policy allkeys-lru

Evicts least frequently used keys but only those with an expiration set.

Redis

CONFIG SET maxmemory-policy volatile-lfu

Evicts random keys from all keys when memory is full.

Redis

CONFIG SET maxmemory-policy allkeys-random

Sample Program

This example sets Redis to use 100 MB max memory and the allkeys-lru eviction policy. When memory is full, Redis removes the least recently used keys from all keys.

Redis

CONFIG SET maxmemory 100mb
CONFIG SET maxmemory-policy allkeys-lru

SET key1 value1
SET key2 value2

# Simulate memory full by adding many keys
# Redis will evict least recently used keys automatically

GET key1
GET key2

Important Notes

LRU means Least Recently Used: Redis removes keys not used recently.

LFU means Least Frequently Used: Redis removes keys used less often.

Random eviction removes any key randomly, which is simple but less efficient.

Summary

Eviction policies control how Redis frees memory when full.

LRU and LFU keep the most useful data by usage patterns.

Random eviction removes keys without pattern, simpler but less smart.

Practice

1. Which Redis eviction policy removes the least recently used keys first when memory is full?

easy

A. LRU (Least Recently Used)

B. LFU (Least Frequently Used)

C. Random

D. No eviction

Solution

1. Step 1: Understand eviction policies in Redis

   Redis uses eviction policies to free memory when it is full by removing keys based on certain rules.

2. Step 2: Identify the policy that removes least recently used keys

   LRU stands for Least Recently Used, which means it removes keys that have not been accessed for the longest time.

3. Final Answer:

   LRU (Least Recently Used) -> Option A

4. Quick Check:

   Eviction policy removing least recently used keys = LRU [OK]

Hint: LRU = removes oldest unused keys first [OK]

Common Mistakes:

• Confusing LRU with LFU
• Thinking random eviction is based on usage
• Assuming no eviction means keys are removed

2. Which of the following is the correct Redis configuration directive to set the eviction policy to LFU?

easy

A. maxmemory-policy allkeys-lfu

B. maxmemory-policy random-eviction

C. maxmemory-policy volatile-lru

D. maxmemory-policy noeviction

Solution

1. Step 1: Recall Redis maxmemory-policy options

   Redis uses the 'maxmemory-policy' directive to set eviction behavior, with options like 'allkeys-lfu' for LFU eviction.

2. Step 2: Identify the correct syntax for LFU eviction

   The correct directive for LFU eviction on all keys is 'maxmemory-policy allkeys-lfu'.

3. Final Answer:

   maxmemory-policy allkeys-lfu -> Option A

4. Quick Check:

   LFU eviction config = maxmemory-policy allkeys-lfu [OK]

Hint: LFU config uses 'allkeys-lfu' in maxmemory-policy [OK]

Common Mistakes:

• Using 'random-eviction' instead of LFU
• Confusing 'volatile-lru' with LFU
• Missing 'maxmemory-policy' keyword

3. Given Redis is configured with the maxmemory-policy allkeys-lru and memory is full, what happens when a new key is added?

medium

A. A random key is evicted to make space

B. The least frequently used key is evicted to make space

C. The least recently used key is evicted to make space

D. The new key is rejected and not added

Solution

1. Step 1: Understand the 'allkeys-lru' eviction policy

   This policy evicts the least recently used key among all keys when memory is full.

2. Step 2: Predict behavior when adding a new key

   When memory is full, Redis removes the least recently used key to free space for the new key.

3. Final Answer:

   The least recently used key is evicted to make space -> Option C

4. Quick Check:

   allkeys-lru evicts least recently used key [OK]

Hint: allkeys-lru evicts oldest used key on full memory [OK]

Common Mistakes:

• Confusing LRU with LFU eviction
• Thinking random eviction applies here
• Assuming new keys are rejected automatically

4. You configured Redis with maxmemory-policy volatile-lfu but notice keys without expiration are never evicted. What is the likely cause?

medium

A. LFU policy does not work with volatile keys

B. Only keys with expiration are considered for eviction in volatile-lfu

C. Redis randomly evicts keys regardless of policy

D. Keys without expiration are evicted first

Solution

1. Step 1: Understand volatile-lfu eviction policy

   Volatile policies only evict keys that have an expiration set (TTL).

2. Step 2: Analyze why keys without expiration remain

   Since keys without expiration are not eligible, they are never evicted under volatile-lfu.

3. Final Answer:

   Only keys with expiration are considered for eviction in volatile-lfu -> Option B

4. Quick Check:

   Volatile policies evict only expiring keys [OK]

Hint: Volatile policies evict only keys with expiration [OK]

Common Mistakes:

• Assuming LFU ignores expiration
• Thinking random eviction applies here
• Believing keys without expiration are evicted first

5. You want Redis to evict keys randomly but only among keys with expiration set. Which eviction policy should you configure?

hard

A. volatile-lru

B. allkeys-random

C. allkeys-lfu

D. volatile-random

Solution

1. Step 1: Understand eviction policy prefixes

   'allkeys-' applies to all keys; 'volatile-' applies only to keys with expiration.

2. Step 2: Identify random eviction among volatile keys

   'volatile-random' evicts keys randomly but only among keys with expiration set.

3. Final Answer:

   volatile-random -> Option D

4. Quick Check:

   Random eviction on expiring keys = volatile-random [OK]

Hint: Volatile-random evicts random expiring keys only [OK]

Common Mistakes:

• Choosing allkeys-random which evicts all keys
• Confusing LFU or LRU with random eviction
• Selecting volatile-lru which is not random