EXPIRE and TTL for time-to-live in Redis - Step-by-Step Execution

Concept Flow - EXPIRE and TTL for time-to-live

Set Key with Value

↓

Apply EXPIRE with seconds

↓

Key has TTL countdown

↓

Check TTL with TTL command

↓

If TTL > 0

→Key exists

No↓

TTL expired

→Key deleted

This flow shows how a key is set, given a time-to-live (TTL) with EXPIRE, and how TTL counts down until the key is deleted.

Execution Sample

Redis

SET session:user123 "active"
EXPIRE session:user123 5
TTL session:user123
(wait 6 seconds)
TTL session:user123

Set a key, apply a 5-second expiration, check TTL before and after expiration.

Execution Table

Step	Command	Action	TTL Value (seconds)	Key Exists?
1	SET session:user123 "active"	Key set with value	No TTL (persistent)	Yes
2	EXPIRE session:user123 5	TTL set to 5 seconds	5	Yes
3	TTL session:user123 (immediately)	Check TTL	5	Yes
4	(wait 6 seconds)	Time passes, TTL counts down	0 (expired)	No
5	TTL session:user123 (after wait)	Check TTL after expiration	-2 (key does not exist)	No

💡 Key expires after 5 seconds, TTL becomes -2 indicating key no longer exists.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
session:user123 (exists)	No	Yes	Yes	No	No
TTL (seconds)	N/A	No TTL	5	0 (expired)	-2 (no key)

Key Moments - 3 Insights

Why does TTL return -2 after the key expires?

TTL returns -2 when the key does not exist anymore, as shown in step 5 of the execution_table.

Does setting EXPIRE immediately delete the key?

No, EXPIRE sets a countdown. The key remains until TTL reaches zero, as seen between steps 2 and 4.

What happens if you check TTL before setting EXPIRE?

TTL returns -1 meaning the key exists but has no expiration, similar to step 1 where no TTL is set.

Visual Quiz - 3 Questions

Look at the execution_table, what is the TTL value immediately after setting EXPIRE?

A -1

B 0

C 5

D -2

💡 Hint

Check step 2 and 3 in the execution_table for TTL values after EXPIRE command.

At which step does the key stop existing?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look at the 'Key Exists?' column in the execution_table to find when it changes to 'No'.

If you do not set EXPIRE, what TTL value will TTL command return?

A -1

B 0

C -2

D 5

💡 Hint

Refer to the variable_tracker and key_moments about TTL values when no expiration is set.

Concept Snapshot

EXPIRE key seconds - sets time-to-live for a key
TTL key - shows remaining seconds before expiration
TTL returns -1 if no expiration set
TTL returns -2 if key does not exist
Key is deleted automatically when TTL reaches zero

Full Transcript

This visual execution shows how Redis EXPIRE sets a time-to-live on a key. First, a key is set with SET. Then EXPIRE sets a countdown in seconds. TTL command checks how many seconds remain. When TTL reaches zero, the key is deleted. TTL returns -1 if the key has no expiration and -2 if the key does not exist. This helps manage temporary data automatically.