Eviction policies overview in Redis

Introduction

Eviction policies help Redis decide which data to remove when it runs out of memory. This keeps Redis fast and prevents crashes.

When your Redis server has limited memory and you want to keep it running smoothly.

When you want to control which data stays in Redis and which data gets removed automatically.

When you use Redis as a cache and want old or less important data to be removed first.

When you want to avoid errors caused by Redis running out of memory.

When you want to balance memory use and data freshness in your application.

Syntax

Redis

CONFIG SET maxmemory-policy <policy_name>

You set the eviction policy using the CONFIG SET command.

The <policy_name> is the name of the eviction policy you want to use.

Examples

This policy means Redis will not remove any data and will return errors when memory is full.

Redis

CONFIG SET maxmemory-policy noeviction

This policy removes the least recently used keys from all keys when memory is full.

Redis

CONFIG SET maxmemory-policy allkeys-lru

This policy removes the least recently used keys only among keys with an expiration time set.

Redis

CONFIG SET maxmemory-policy volatile-lru

This policy removes random keys from all keys when memory is full.

Redis

CONFIG SET maxmemory-policy allkeys-random

Sample Program

This sets Redis to use 100 megabytes of memory and to remove the least recently used keys from all keys when memory is full.

Redis

CONFIG SET maxmemory 100mb
CONFIG SET maxmemory-policy allkeys-lru

Important Notes

Eviction policies only work if you set a memory limit with maxmemory.

Choosing the right policy depends on your use case, like caching or persistent storage.

Common policies include noeviction, allkeys-lru, volatile-lru, allkeys-random, and volatile-random.

Summary

Eviction policies control how Redis frees memory when it is full.

You set eviction policies with CONFIG SET maxmemory-policy.

Pick a policy that fits your app's needs to keep Redis fast and stable.

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