Key expiry for memory management in Redis - Step-by-Step Execution

Concept Flow - Key expiry for memory management

Set Key with Expiry

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Key Stored in Memory

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Time Passes

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Check Expiry Time

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Key Accessible

↩Back to Time Passes

This flow shows how Redis stores a key with an expiry time, checks if the key has expired over time, and deletes it to free memory if expired.

Execution Sample

Redis

SET session:user123 "data" EX 5
GET session:user123
(wait 6 seconds)
GET session:user123

Set a key with 5 seconds expiry, get it before and after expiry to see memory management in action.

Execution Table

Step	Command	Action	Result	Memory State
1	SET session:user123 "data" EX 5	Store key with 5s expiry	OK	session:user123 stored with expiry=5s
2	GET session:user123	Check key existence and expiry	"data"	Key exists, not expired
3	(wait 6 seconds)	Time passes beyond expiry	N/A	Expiry time reached
4	GET session:user123	Check key existence and expiry	(nil)	Key deleted from memory due to expiry

💡 Key expired after 5 seconds, so GET returns nil and key is removed from memory

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
session:user123	does not exist	exists with expiry=5s	exists with expiry=5s	expiry reached	deleted

Key Moments - 2 Insights

Why does the GET command return nil after waiting 6 seconds?

Because the key's expiry time of 5 seconds has passed (see execution_table step 3), Redis automatically deletes the key to free memory.

Does the key get deleted exactly at 5 seconds?

Redis checks expiry lazily on access or via periodic cleanup, so the key is deleted when accessed after expiry or during cleanup (step 4 shows deletion on GET).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value returned by GET?

A "data"

B nil

C Error

D "" (empty string)

💡 Hint

Check the 'Result' column at step 2 in execution_table

At which step does the key get deleted from memory?

A Step 1

B Step 2

C Step 4

D Step 3

💡 Hint

Look at the 'Memory State' column in execution_table for when the key is deleted

If the expiry was set to 10 seconds instead of 5, what would happen at step 4?

A GET returns nil because key is expired

B GET returns "data" because key is not expired

C GET returns error

D Key is deleted immediately at step 1

💡 Hint

Refer to variable_tracker and execution_table timing of expiry

Concept Snapshot

SET key with EX seconds sets expiry time
Key remains accessible until expiry
After expiry, key is deleted automatically
GET returns nil if key expired
Expiry helps free memory by removing unused keys

Full Transcript

This visual execution shows how Redis manages memory by expiring keys. First, a key is set with a 5-second expiry. The key is stored in memory and accessible immediately. After waiting 6 seconds, the expiry time is reached. When the key is accessed again, Redis detects the expiry and deletes the key from memory, returning nil. This automatic deletion frees memory by removing unused keys. Redis expiry is checked lazily on access or during periodic cleanup.