Eviction policies (LRU, LFU, random) in Redis - Mini Project: Build & Apply

Configuring and Testing Redis Eviction Policies

📖 Scenario: You are managing a Redis cache server that stores user session data. To keep the cache efficient and prevent it from growing too large, you need to set an eviction policy. This policy decides which data Redis removes when it reaches its memory limit.Redis supports several eviction policies like Least Recently Used (LRU), Least Frequently Used (LFU), and random eviction. You will configure these policies using Redis CLI and test how Redis behaves when the cache is full.

🎯 Goal: Set up a Redis cache with a memory limit and configure different eviction policies (LRU, LFU, random) using CONFIG SET. Then, add keys to the cache to observe which keys get removed when the memory limit is reached.

📋 What You'll Learn

Set maxmemory limit to 102400 bytes (100 kilobytes) using CONFIG SET

Set the eviction policy to 'allkeys-lru' using CONFIG SET

Add multiple keys with values to the Redis cache using SET

Change the eviction policy to 'allkeys-lfu' and then to 'allkeys-random' using CONFIG SET and observe behavior

💡 Why This Matters

🌍 Real World

Caches like Redis often run on limited memory. Setting eviction policies helps keep the cache efficient by removing less important data automatically.

💼 Career

Understanding and configuring eviction policies is important for roles like backend developers, DevOps engineers, and system administrators who manage caching layers.

1

Set maxmemory limit

Using the Redis CLI, set the maxmemory to 102400 bytes (equivalent to 100kb) using the CONFIG SET command.

Redis

# Your code here

Hint

The CONFIG SET maxmemory 102400 sets the memory limit Redis can use (100KB in bytes).

2

Set eviction policy to allkeys-lru

Using the Redis CLI, set the maxmemory-policy to allkeys-lru using CONFIG SET.

Redis

CONFIG SET maxmemory 102400
# Your code here

Hint

Use CONFIG SET maxmemory-policy allkeys-lru for Least Recently Used eviction on all keys.

3

Add keys to Redis cache

Using the Redis CLI, add keys named key1 to key5 with values value1 to value5 respectively using the SET command. To test eviction later, you can add more keys.

Redis

CONFIG SET maxmemory 102400
CONFIG SET maxmemory-policy allkeys-lru
# Your code here

Hint

Use the SET key value command to add keys and values to Redis. Add more keys later to trigger eviction.

4

Change eviction policy to allkeys-lfu and then allkeys-random

Using CONFIG SET, change the eviction policy first to allkeys-lfu, add some more keys (e.g., key6 value6, etc.) to trigger eviction, observe with GET or KEYS *. Then change to allkeys-random and repeat.

Redis

CONFIG SET maxmemory 102400
CONFIG SET maxmemory-policy allkeys-lru
SET key1 value1
SET key2 value2
SET key3 value3
SET key4 value4
SET key5 value5
# Your code here

Hint

Use CONFIG SET maxmemory-policy allkeys-lfu then allkeys-random. Add extra SET commands to fill memory and observe evictions.

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