Temporary data with TTL in Redis

Introduction

Sometimes you want to store data that only lasts for a short time and then disappears automatically. This helps keep your database clean and saves space.

Storing user session information that expires after a few minutes.

Caching web page data to speed up loading but refresh it regularly.

Saving one-time verification codes that should expire quickly.

Keeping temporary flags or counters that reset after some time.

Syntax

Redis

SET key value EX seconds

The EX option sets the expiration time in seconds.

After the time runs out, the key is deleted automatically.

Examples

Stores 'user_data' in 'session123' key that expires in 300 seconds (5 minutes).

Redis

SET session123 user_data EX 300

Stores a temporary code that expires in 60 seconds.

Redis

SET temp_code 123456 EX 60

Caches a web page for 1 hour.

Redis

SET cache_page "<html>...</html>" EX 3600

Sample Program

This sets a temporary login token that expires in 120 seconds, then retrieves its value and remaining time to live.

Redis

SET login_token abcdef EX 120
GET login_token
TTL login_token

Important Notes

If you try to GET a key after it expires, Redis returns nil (no value).

You can check how many seconds remain before expiration using TTL command.

Setting a key without EX means it will stay forever unless deleted manually.

Summary

Use SET with EX to store temporary data that expires automatically.

TTL helps you see how long the data will last.

Temporary data keeps your database clean and efficient.

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