Eviction policies overview in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an eviction policy in Redis?

An eviction policy in Redis decides which keys to remove when the memory limit is reached to make space for new data.

beginner

Explain the noeviction policy in Redis.

The noeviction policy means Redis will return an error when memory is full and no keys are removed.

intermediate

What does the allkeys-lru eviction policy do?

The allkeys-lru policy removes the least recently used keys from all keys in Redis to free memory.

intermediate

Describe the difference between volatile-lru and allkeys-lru policies.

volatile-lru removes least recently used keys only among keys with an expiration set, while allkeys-lru considers all keys.

intermediate

What is the purpose of the volatile-ttl eviction policy?

The volatile-ttl policy removes keys with the shortest time to live (TTL) first, but only among keys with expiration set.

Which Redis eviction policy returns an error when memory is full?

A volatile-lru

B allkeys-lru

C noeviction

D allkeys-random

The noeviction policy stops Redis from removing keys and returns an error when memory is full.

What does the allkeys-random eviction policy do?

A Removes keys with shortest TTL

B Returns error on memory full

C Removes least recently used keys with expiration

D Removes random keys from all keys

allkeys-random removes random keys from all keys to free memory.

Which eviction policy only removes keys with an expiration set?

A volatile-lru

B allkeys-lfu

C noeviction

D allkeys-random

volatile-lru only removes keys that have an expiration set.

What does LRU stand for in eviction policies?

A Least Recently Used

B Last Random Update

C Longest Running User

D Latest Redis Update

LRU means Least Recently Used, removing keys that were used least recently.

Which eviction policy removes keys with the shortest time to live first?

A noeviction

B volatile-ttl

C allkeys-lru

D allkeys-lfu

volatile-ttl removes keys with the shortest TTL among keys with expiration.

Explain how Redis eviction policies help manage memory when it is full.

Think about what happens when Redis runs out of memory.

Describe the difference between allkeys-lru and volatile-lru eviction policies.

Focus on which keys each policy targets.

Practice

1. What is the main purpose of eviction policies in Redis?

easy

A. To encrypt Redis data

B. To configure the network settings of Redis

C. To backup data automatically

D. To decide how Redis frees memory when it is full

Solution

1. Step 1: Understand Redis memory management

   Redis uses eviction policies to manage memory when it reaches its limit.

2. Step 2: Identify the role of eviction policies

   Eviction policies decide which data to remove to free memory and keep Redis running smoothly.

3. Final Answer:

   To decide how Redis frees memory when it is full -> Option D

4. Quick Check:

   Eviction policies control memory freeing = A [OK]

Hint: Eviction policies manage memory freeing in Redis [OK]

Common Mistakes:

• Confusing eviction policies with backup or encryption
• Thinking eviction policies configure network
• Assuming eviction policies handle data encryption

2. Which command correctly sets the eviction policy to 'allkeys-lru' in Redis?

easy

A. CONFIG SET maxmemory-policy allkeys-lru

B. SET maxmemory-policy allkeys-lru

C. CONFIG GET maxmemory-policy allkeys-lru

D. EVICT POLICY allkeys-lru

Solution

1. Step 1: Recall the correct command format

   Redis uses CONFIG SET to change configuration settings.

2. Step 2: Match the correct syntax for eviction policy

   The correct syntax is CONFIG SET maxmemory-policy policy_name, so allkeys-lru fits here.

3. Final Answer:

   CONFIG SET maxmemory-policy allkeys-lru -> Option A

4. Quick Check:

   Use CONFIG SET for eviction policy = A [OK]

Hint: Use CONFIG SET maxmemory-policy to set eviction [OK]

Common Mistakes:

• Using SET instead of CONFIG SET
• Using CONFIG GET to set a value
• Using a non-existent EVICT POLICY command

3. Given Redis is set with eviction policy volatile-lru and maxmemory is reached, which keys are eligible for eviction?

medium

A. All keys regardless of expiration

B. Only keys with an expiration set

C. Only keys without expiration

D. No keys are evicted automatically

Solution

1. Step 1: Understand 'volatile-lru' policy meaning

   This policy evicts keys with expiration set using least recently used (LRU) order.

2. Step 2: Identify which keys can be evicted

   Only keys that have an expiration time are considered for eviction under 'volatile-lru'. Keys without expiration stay safe.

3. Final Answer:

   Only keys with an expiration set -> Option B

4. Quick Check:

   volatile-lru evicts expiring keys = B [OK]

Hint: volatile-lru evicts only keys with expiration [OK]

Common Mistakes:

• Thinking all keys are evicted
• Confusing volatile-lru with allkeys-lru
• Assuming no keys are evicted automatically

4. You set Redis eviction policy to noeviction but Redis still evicts keys when memory is full. What is the likely cause?

medium

A. The policy was not applied correctly or Redis was restarted without saving config

B. Redis always evicts keys regardless of policy

C. noeviction policy evicts keys randomly

D. Redis ignores eviction policies when maxmemory is set

Solution

1. Step 1: Understand 'noeviction' policy behavior

   This policy prevents Redis from evicting keys automatically when memory is full.

2. Step 2: Identify why eviction still happens

   If eviction occurs, likely the policy was not saved or applied properly, or Redis restarted without config persistence.

3. Final Answer:

   The policy was not applied correctly or Redis was restarted without saving config -> Option A

4. Quick Check:

   noeviction means no eviction unless misconfigured = C [OK]

Hint: Check config save and apply for noeviction policy [OK]

Common Mistakes:

• Believing Redis always evicts keys
• Thinking noeviction evicts randomly
• Assuming eviction policies are ignored when maxmemory is set

5. You want Redis to evict the least recently used keys regardless of expiration when memory is full. Which eviction policy should you choose?

hard

A. volatile-lru

B. noeviction

C. allkeys-lru

D. volatile-random

Solution

1. Step 1: Understand eviction policy options

   volatile-lru evicts only keys with expiration; allkeys-lru evicts any key using LRU.

2. Step 2: Match requirement to policy

   Since eviction should ignore expiration and evict least recently used keys, allkeys-lru fits best.

3. Final Answer:

   allkeys-lru -> Option C

4. Quick Check:

   Evict any key LRU = allkeys-lru = D [OK]

Hint: allkeys-lru evicts any key by least recent use [OK]

Common Mistakes:

• Choosing volatile-lru which only evicts expiring keys
• Selecting volatile-random which evicts randomly
• Picking noeviction which disables eviction