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.