Temporary data with TTL in Redis - Step-by-Step Execution

Concept Flow - Temporary data with TTL

Set key with value

↓

Assign TTL (time to live)

↓

Wait for TTL countdown

↓

TTL expires?

No→Key still exists

Yes↓

Key auto-deleted

↓

Key no longer accessible

This flow shows how a key is set with a value and a time limit (TTL). When the TTL ends, the key is automatically removed.

Execution Sample

Redis

SET session123 "user_data"
EXPIRE session123 5
GET session123
(wait 6 seconds)
GET session123

Set a key with a value and TTL 5 seconds, then get it before and after TTL expires.

Execution Table

Step	Command	Action	Key Exists?	Output
1	SET session123 "user_data"	Store key 'session123' with value 'user_data'	Yes	OK
2	EXPIRE session123 5	Set TTL 5 seconds on 'session123'	Yes	1
3	GET session123 (immediately)	Retrieve value of 'session123'	Yes	"user_data"
4	(Wait 6 seconds)	TTL countdown ends, key expires	No	nil
5	GET session123 (after wait)	Try to get expired key	No	nil

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

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
session123	does not exist	"user_data"	"user_data" (TTL 5s)	deleted	does not exist

Key Moments - 2 Insights

Why does GET return nil after waiting even though we set the key before?

Because the TTL expired after 5 seconds (see execution_table step 4), Redis automatically deleted the key.

What does the EXPIRE command return and why?

It returns 1 to confirm TTL was set successfully (see execution_table step 2). If key did not exist, it would return 0.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of GET session123 immediately after setting TTL?

A "user_data"

B nil

C Error

D 1

💡 Hint

Check execution_table row 3 under Output column.

At which step does the key 'session123' get deleted automatically?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at execution_table step 4 Action and Key Exists? columns.

If we set TTL to 10 seconds instead of 5, what changes in the execution_table?

A EXPIRE returns 0

B GET returns nil immediately

C Key expires at step 4 after 10 seconds instead of 5

D Key never expires

💡 Hint

TTL controls when the key expires, see execution_table step 4.

Concept Snapshot

SET key value - stores data
EXPIRE key seconds - sets TTL
GET key - retrieves data
After TTL ends, key auto-deletes
GET returns nil if key expired

Full Transcript

This visual trace shows how Redis handles temporary data using TTL (time to live). First, a key 'session123' is set with a value 'user_data'. Then, a TTL of 5 seconds is assigned. Immediately after, getting the key returns the stored value. After waiting 6 seconds, the TTL expires and Redis deletes the key automatically. Trying to get the key now returns nil because it no longer exists. This demonstrates how TTL helps manage temporary data that expires automatically.

Practice

1. What does the EX option do when used with the Redis SET command?

easy

A. Deletes the key immediately after setting it

B. Sets a time-to-live (TTL) in seconds for the key, after which it expires

C. Sets the key to be persistent and never expire

D. Renames the key to a new name

Solution

1. Step 1: Understand the SET command with EX option

   The SET command stores a key-value pair. The EX option specifies the expiration time in seconds.

2. Step 2: Effect of EX on the key

   When EX is used, the key will automatically be deleted after the given time passes.

3. Final Answer:

   Sets a time-to-live (TTL) in seconds for the key, after which it expires -> Option B

4. Quick Check:

   EX = TTL in seconds [OK]

Hint: EX sets expiration time in seconds for keys [OK]

Common Mistakes:

• Thinking EX makes the key permanent
• Confusing EX with immediate deletion
• Assuming EX renames the key

2. Which of the following is the correct syntax to set a key session with value abc123 that expires in 120 seconds in Redis?

easy

A. SET session abc123 PX 120

B. SET session EX 120 abc123

C. SET EX 120 session abc123

D. SET session abc123 EX 120

Solution

1. Step 1: Recall Redis SET command syntax with EX

   The correct syntax is SET key value EX seconds.

2. Step 2: Match the syntax with options

   SET session abc123 EX 120 matches the correct order: key, value, EX, seconds.

3. Final Answer:

   SET session abc123 EX 120 -> Option D

4. Quick Check:

   SET key value EX seconds [OK]

Hint: Remember: SET key value EX seconds order [OK]

Common Mistakes:

• Placing EX before the key or value
• Using PX instead of EX for seconds
• Mixing order of arguments

3. Given the commands below, what will be the output of TTL user:123 immediately after execution?

SET user:123 "John" EX 60
TTL user:123

medium

A. 60

B. -1

C. -2

D. 0

Solution

1. Step 1: Understand the SET with EX 60

   The key user:123 is set with a TTL of 60 seconds.

2. Step 2: Check TTL command output

   The TTL command returns the remaining time to live in seconds. Immediately after setting, it should be 60.

3. Final Answer:

   60 -> Option A

4. Quick Check:

   TTL after SET with EX 60 = 60 [OK]

Hint: TTL shows seconds left, right after SET with EX it's full time [OK]

Common Mistakes:

• Expecting TTL to return -1 (no expiration)
• Confusing -2 (key does not exist) with TTL
• Thinking TTL returns 0 immediately

4. You run the following commands:

SET temp_key "data" EX 30
EXPIRE temp_key 60
TTL temp_key

What will TTL temp_key return?

medium

A. 60

B. 30

C. -1

D. -2

Solution

1. Step 1: SET with EX 30 sets TTL to 30 seconds

   The key temp_key initially has 30 seconds TTL.

2. Step 2: EXPIRE command updates TTL to 60 seconds

   The EXPIRE temp_key 60 resets the TTL to 60 seconds.

3. Step 3: TTL command returns updated TTL

   So, TTL temp_key returns 60 seconds remaining.

4. Final Answer:

   60 -> Option A

5. Quick Check:

   EXPIRE updates TTL, TTL shows new value [OK]

Hint: EXPIRE resets TTL, TTL shows latest expiration [OK]

Common Mistakes:

• Assuming TTL stays at original 30 seconds
• Confusing TTL -1 (no expiration) with updated TTL
• Thinking EXPIRE does not change TTL

5. You want to store a temporary login token in Redis that expires after 5 minutes. Which sequence of commands correctly sets the token and checks its remaining TTL after 3 minutes?

hard

A. SET login_token abc EX 300
SLEEP 180
EXISTS login_token

B. SET login_token abc PX 300
WAIT 180
EXISTS login_token

C. SET login_token abc EX 300
SLEEP 180
TTL login_token

D. SET login_token abc EX 300
WAIT 180
EXISTS login_token

Solution

1. Step 1: Use SET with EX 300 for 5 minutes expiration

   The command SET login_token abc EX 300 sets the token with 300 seconds TTL.

2. Step 2: Wait 180 seconds (3 minutes) and check TTL

   Using SLEEP 180 simulates waiting. Then TTL login_token shows remaining time, confirming token still exists.

3. Step 3: EXISTS only returns 1 or 0, TTL shows time left

   Checking TTL is better to confirm token is still valid and how long it lasts.

4. Final Answer:

   SET login_token abc EX 300 SLEEP 180 TTL login_token -> Option C

5. Quick Check:

   SET with EX + wait + TTL check = correct flow [OK]

Hint: Use SET with EX, wait, then TTL to check remaining time [OK]

Common Mistakes:

• Using PX instead of EX for seconds
• Using WAIT instead of SLEEP (WAIT waits for replicas, not sleep)
• Using EXISTS which only returns existence, not TTL