TTL-based expiry in Redis - Step-by-Step Execution

Concept Flow - TTL-based expiry

Set Key with TTL

↓

Start Countdown Timer

↓

Key Exists?

No→Key Expired and Deleted

Yes↓

Allow Access to Key

↓

Repeat Check Until TTL Ends

This flow shows how Redis sets a key with a time-to-live (TTL), counts down, and deletes the key when TTL expires.

Execution Sample

Redis

SET user:1 "Alice" EX 5
GET user:1
(wait 6 seconds)
GET user:1

Set a key 'user:1' with value 'Alice' that expires after 5 seconds, then get it before and after expiry.

Execution Table

Step	Command	Action	Key State	Output
1	SET user:1 "Alice" EX 5	Store key with TTL 5 seconds	user:1 = "Alice" (TTL=5s)	OK
2	GET user:1	Check key existence and TTL	user:1 = "Alice" (TTL=4s)	"Alice"
3	(wait 6 seconds)	TTL expires, key deleted	user:1 = <deleted>	N/A
4	GET user:1	Key not found after expiry	user:1 = <deleted>	(nil)

💡 After 5 seconds, TTL expires and Redis deletes the key automatically.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
user:1	<not set>	"Alice" (TTL=5s)	"Alice" (TTL=4s)	<deleted>	<deleted>

Key Moments - 2 Insights

Why does GET return "Alice" at step 2 but nil at step 4?

At step 2, the key still exists with TTL counting down (see execution_table row 2). By step 4, TTL expired and Redis deleted the key (row 3), so GET returns nil.

Does Redis delete the key exactly at TTL expiry or later?

Redis deletes the key as soon as TTL expires or shortly after during internal checks (row 3). So the key is not accessible after TTL ends.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of GET user:1 at step 2?

A "Alice"

B nil

C Error

D "Bob"

💡 Hint

Check execution_table row 2 under Output column.

At which step does the key user:1 get deleted due to TTL expiry?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at execution_table row 3 where key state changes to .

If the TTL was set to 10 seconds instead of 5, what would be the output at step 4?

A Error

B nil

C "Alice"

D "Bob"

💡 Hint

Refer to variable_tracker and execution_table timing of TTL expiry.

Concept Snapshot

TTL-based expiry in Redis:
- Use SET key value EX seconds to set TTL
- Redis counts down TTL internally
- Key is accessible until TTL expires
- After TTL, key is deleted automatically
- GET returns nil if key expired

Full Transcript

This visual execution shows how Redis handles TTL-based expiry. First, a key 'user:1' is set with a value 'Alice' and a TTL of 5 seconds. Redis stores the key and starts counting down the TTL. When we GET the key before TTL expires, Redis returns 'Alice'. After waiting 6 seconds, the TTL expires and Redis deletes the key automatically. A subsequent GET returns nil because the key no longer exists. This demonstrates how TTL controls automatic key expiry in Redis.