Eviction policies (LRU, LFU, random) in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the eviction policy to Least Recently Used (LRU) in Redis configuration.

Redis

CONFIG SET maxmemory-policy [1]

Drag options to blanks, or click blank then click option'

Anoeviction

Bvolatile-lfu

Callkeys-lru

Drandom

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'volatile-lfu' which is for Least Frequently Used but only on volatile keys.

Using 'noeviction' which disables eviction.

Selecting 'random' which evicts keys randomly.

The allkeys-lru policy evicts the least recently used keys when memory is full.

2fill in blank

medium

Complete the code to set the eviction policy to Least Frequently Used (LFU) for volatile keys only.

Redis

CONFIG SET maxmemory-policy [1]

Drag options to blanks, or click blank then click option'

Avolatile-lru

Ballkeys-random

Callkeys-lfu

Dvolatile-lfu

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'allkeys-lfu' which applies LFU to all keys, not just volatile ones.

Selecting 'volatile-lru' which is LRU, not LFU.

Using 'allkeys-random' which evicts keys randomly.

The volatile-lfu policy evicts the least frequently used keys but only those with an expiration set.

3fill in blank

hard

Fix the error in the command to set a random eviction policy.

Redis

CONFIG SET maxmemory-policy [1]

Drag options to blanks, or click blank then click option'

Arandom

Ballkeys-random

Cvolatile-random

Drandom-eviction

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'random' alone which is not a valid policy name.

Choosing 'random-eviction' which is not recognized.

Selecting 'volatile-random' which only evicts volatile keys.

The correct policy name for random eviction of all keys is allkeys-random.

4fill in blank

hard

Fill both blanks to set the max memory to 100mb and eviction policy to allkeys-lfu.

Redis

CONFIG SET maxmemory [1]
CONFIG SET maxmemory-policy [2]

Drag options to blanks, or click blank then click option'

A100mb

Ballkeys-lfu

Cvolatile-lru

D50mb

Attempts:

3 left

💡 Hint

Common Mistakes

Using '50mb' instead of '100mb' for maxmemory.

Choosing 'volatile-lru' which is a different eviction policy.

Setting maxmemory to 100mb limits Redis memory usage, and allkeys-lfu evicts least frequently used keys from all keys.

5fill in blank

hard

Fill all three blanks to configure Redis to use 200mb max memory, volatile-lru eviction policy, and set a key expiration of 60 seconds.

Redis

CONFIG SET maxmemory [1]
CONFIG SET maxmemory-policy [2]
EXPIRE mykey [3]

Drag options to blanks, or click blank then click option'

Avolatile-lru

B60

C200mb

Dallkeys-random

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up eviction policies like choosing 'allkeys-random' instead of 'volatile-lru'.

Setting expiration time in wrong units.

We set maxmemory to 200mb, eviction policy to volatile-lru which evicts least recently used keys with expiration, and set expiration of 60 seconds on 'mykey'.

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