Eviction policies (LRU, LFU, random) in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens when Redis uses the LRU eviction policy?

Redis is configured with the allkeys-lru eviction policy and maxmemory is reached. Which keys are removed first?

A Keys with the largest size in memory are removed first.

B Keys are removed randomly without considering usage.

C Keys that were least recently accessed are removed first.

D Keys that were least frequently accessed are removed first.

💡 Hint

Think about what 'LRU' stands for in cache eviction.

LRU means Least Recently Used. Redis removes keys that have not been accessed for the longest time first.

❓ query_result

intermediate

How does the LFU eviction policy decide which keys to remove?

Redis is set to allkeys-lfu eviction policy. When memory is full, which keys get evicted?

A Keys with the oldest creation time are removed first.

B Keys that were least recently accessed are removed first.

C Keys are removed randomly without considering usage.

D Keys that were least frequently accessed are removed first.

💡 Hint

LFU stands for a frequency-based eviction.

LFU means Least Frequently Used. Redis removes keys that have been accessed the fewest times first.

📝 Syntax

advanced

Which Redis configuration line correctly sets the eviction policy to random?

Choose the correct Redis config line to set eviction policy to allkeys-random.

A maxmemory-policy random

B maxmemory-policy allkeys-random

C maxmemory-policy allkeys_random

D maxmemory-policy random-allkeys

💡 Hint

Check the exact spelling and format of Redis eviction policies.

The correct Redis config uses hyphens and the exact policy name: allkeys-random.

🧠 Conceptual

advanced

Why might LFU eviction be less efficient than LRU in some Redis workloads?

Consider Redis workloads with many keys accessed only once. Why could LFU eviction cause problems compared to LRU?

A LFU may keep rarely accessed keys longer because their frequency counters don't decay quickly.

B LFU removes keys randomly, causing unpredictable cache misses.

C LFU evicts keys based on size, which can remove important small keys.

D LFU requires more CPU, so Redis crashes under heavy load.

💡 Hint

Think about how frequency counters behave over time.

LFU counts how often keys are accessed but frequency counters may not decay fast, so some rarely used keys stay longer, reducing cache efficiency.

🔧 Debug

expert

What error occurs if you set an invalid eviction policy in Redis config?

You add maxmemory-policy allkeys-lrux to your Redis config and restart. What happens?

A Redis starts but ignores the invalid policy and uses no eviction.

B Redis fails to start and shows a configuration error.

C Redis starts and uses the default LRU eviction policy silently.

D Redis starts but crashes when memory limit is reached.

💡 Hint

Invalid config options usually cause startup errors.

Redis logs a warning for the unknown maxmemory-policy and overrides it with 'noeviction', allowing the server to start.

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