Eviction policies overview in Redis - Step-by-Step Execution

Concept Flow - Eviction policies overview

Memory Limit Reached?

Yes↓

Select Eviction Policy

↓

Remove Keys Based on Policy

↓

Free Memory

↓

Continue Accepting Writes

When Redis memory is full, it checks the eviction policy to decide which keys to remove to free space or to reject writes.

Execution Sample

Redis

CONFIG SET maxmemory 100mb
CONFIG SET maxmemory-policy allkeys-lru
SET key1 value1
SET key2 value2
... (many keys)
SET keyN valueN

This example sets a memory limit and eviction policy, then adds keys until Redis evicts some keys based on the policy.

Execution Table

Step	Memory Usage	Action	Eviction Policy	Keys Removed	Result
1	10mb	Add key1	allkeys-lru	0	Success
2	20mb	Add key2	allkeys-lru	0	Success
3	95mb	Add keyN-1	allkeys-lru	0	Success
4	100mb	Add keyN	allkeys-lru	0	Memory limit reached, triggers eviction
5	90mb	Evict keys	allkeys-lru	1 (least recently used)	Space freed
6	100mb	Add keyN	allkeys-lru	0	Success
7	100mb	Add keyN+1	noeviction	0	Error - memory limit reached, no eviction

💡 Execution stops when memory limit is reached and no eviction policy allows freeing memory.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
Memory Usage	0mb	10mb	20mb	95mb	100mb	90mb	100mb	100mb
Keys Stored	0	1	2	N-1	N	N-1	N	N
Eviction Triggered	No	No	No	No	Yes	Yes	No	No
Write Success	N/A	Yes	Yes	Yes	Yes	Yes	Yes	No

Key Moments - 3 Insights

Why does Redis remove keys only after memory limit is reached?

Redis allows writes until memory usage hits the limit (see Step 4 in execution_table). Only then it triggers eviction to free space.

What happens if the eviction policy is set to 'noeviction'?

Redis refuses new writes when memory is full instead of removing keys (see Step 7 in execution_table).

How does the 'allkeys-lru' policy decide which key to remove?

It removes the least recently used key regardless of expiration (Step 5 shows one key removed based on usage).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Redis first trigger eviction?

A Step 4

B Step 3

C Step 5

D Step 7

💡 Hint

Check the 'Action' and 'Result' columns in execution_table rows for when eviction starts.

According to variable_tracker, what is the memory usage after eviction at Step 5?

A 100mb

B 90mb

C 95mb

D 92mb

💡 Hint

Look at the 'Memory Usage' row under 'After Step 5' in variable_tracker.

If the eviction policy was 'noeviction', what would happen at Step 7?

A Redis evicts keys to free memory

B Redis accepts the write successfully

C Redis returns an error due to memory limit

D Redis increases memory limit automatically

💡 Hint

Refer to Step 7 in execution_table under 'Result' for 'noeviction' policy.

Concept Snapshot

Redis Eviction Policies Overview:
- When memory limit is reached, Redis uses eviction policies to free space.
- Policies include allkeys-lru, volatile-lru, noeviction, etc.
- allkeys-lru removes least recently used keys regardless of expiration.
- noeviction rejects writes when memory is full.
- Proper policy choice helps maintain performance under memory pressure.

Full Transcript

This visual execution shows how Redis handles memory limits using eviction policies. When Redis reaches its configured max memory, it checks the eviction policy. If the policy allows, Redis removes keys based on that policy, such as least recently used keys in allkeys-lru. If the policy is noeviction, Redis refuses new writes once memory is full. The execution table traces memory usage and actions step-by-step, while the variable tracker shows how memory and keys change. Key moments clarify common confusions about when eviction triggers and how policies differ. The quiz tests understanding of these steps and outcomes.

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