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.