EXPIRE and TTL for time-to-live in Redis - Mini Project: Build & Apply

Using EXPIRE and TTL commands in Redis

📖 Scenario: You are managing a Redis database for a web application that stores temporary session data for users. To keep the database clean and efficient, you want to set expiration times on session keys so they automatically get deleted after a certain period.

🎯 Goal: Learn how to set expiration times on Redis keys using the EXPIRE command and check the remaining time-to-live with the TTL command.

📋 What You'll Learn

Create a Redis key with a session ID and a value

Set an expiration time of 120 seconds on the session key using EXPIRE

Check the remaining time-to-live of the session key using TTL

Set a new expiration time of 300 seconds on the session key

💡 Why This Matters

🌍 Real World

Setting expiration times on session keys helps automatically clean up old sessions, saving memory and improving performance in web applications.

💼 Career

Understanding EXPIRE and TTL commands is essential for backend developers and DevOps engineers managing Redis databases to ensure efficient data lifecycle management.

1

Create a session key in Redis

Create a Redis key called session:12345 with the value user_data using the SET command.

Redis

SET session:12345 user_data
# Your code here

Hint

Use the SET command followed by the key and value.

2

Set expiration time on the session key

Use the EXPIRE command to set the expiration time of 120 seconds on the Redis key session:12345.

Redis

SET session:12345 user_data
EXPIRE session:12345 120
# Your code here

Hint

The EXPIRE command takes the key name and the number of seconds before expiration.

3

Check the time-to-live of the session key

Use the TTL command to check the remaining time-to-live of the Redis key session:12345.

Redis

SET session:12345 user_data
EXPIRE session:12345 120
TTL session:12345
# Your code here

Hint

The TTL command returns the remaining seconds before the key expires.

4

Update the expiration time on the session key

Use the EXPIRE command again to update the expiration time to 300 seconds on the Redis key session:12345.

Redis

SET session:12345 user_data
EXPIRE session:12345 120
TTL session:12345
EXPIRE session:12345 300
# Your code here

Hint

You can reset the expiration time by calling EXPIRE again with a new time.

Practice

1. What does the Redis command EXPIRE key 60 do?

easy

A. Returns the remaining time to live for the key.

B. Immediately deletes the key named 'key'.

C. Sets the key to expire and be deleted after 60 seconds.

D. Renames the key to '60'.

Solution

1. Step 1: Understand the EXPIRE command

   The EXPIRE command sets a countdown timer on a key in seconds.

2. Step 2: Apply the command to the example

   Here, EXPIRE key 60 sets the key to expire after 60 seconds.

3. Final Answer:

   Sets the key to expire and be deleted after 60 seconds. -> Option C

4. Quick Check:

   EXPIRE sets countdown = A [OK]

Hint: EXPIRE sets time in seconds before deletion [OK]

Common Mistakes:

• Thinking EXPIRE deletes immediately
• Confusing EXPIRE with TTL command
• Assuming EXPIRE renames keys

2. Which of the following is the correct syntax to check the remaining time to live of a key named session in Redis?

easy

A. TTL session

B. EXPIRE session

C. GETTTL session

D. TIMELEFT session

Solution

1. Step 1: Identify the command to check TTL

   The Redis command to check remaining time is TTL key.

2. Step 2: Match the syntax with the key name

   For key session, the correct command is TTL session.

3. Final Answer:

   TTL session -> Option A

4. Quick Check:

   TTL command syntax = D [OK]

Hint: TTL shows remaining seconds before expiry [OK]

Common Mistakes:

• Using EXPIRE to check TTL
• Assuming GETTTL is a Redis command
• Using TIMELEFT which does not exist

3. Consider the following Redis commands executed in order:

SET temp "data"
EXPIRE temp 10
TTL temp

What will the TTL temp command return immediately after setting the expire?

medium

A. Error

B. -1

C. 0

D. 10

Solution

1. Step 1: Set key and expire time

   The key temp is set with value "data" and an expire time of 10 seconds.

2. Step 2: Check TTL immediately after EXPIRE

   Immediately after setting expire, TTL returns the remaining seconds, which is 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   TTL after EXPIRE = 10 [OK]

Hint: TTL returns seconds left right after EXPIRE [OK]

Common Mistakes:

• Expecting TTL to return -1 (no expire)
• Thinking TTL returns 0 immediately
• Assuming TTL causes an error if key exists

4. You run the following commands:

SET user "Alice"
EXPIRE user -5
TTL user

What is the result of TTL user and why?

medium

A. -2 because key is deleted immediately

B. Error due to invalid expire time

C. -1 because negative expire removes expiration

D. 5 because negative is treated as positive

Solution

1. Step 1: Understand EXPIRE with negative time

   Setting EXPIRE with a negative value deletes the key immediately.

2. Step 2: Check TTL after immediate deletion

   TTL returns -2 when the key does not exist.

3. Final Answer:

   -2 because key is deleted immediately -> Option A

4. Quick Check:

   Negative EXPIRE deletes key, TTL = -2 [OK]

Hint: Negative EXPIRE deletes key, TTL returns -2 [OK]

Common Mistakes:

• Thinking negative expire removes expiration but keeps key
• Expecting TTL to return -1 for no expire
• Assuming negative expire causes error

5. You want to set a key cache with a value and ensure it expires in 30 seconds only if it does not already have an expiration. Which Redis command sequence achieves this safely?

hard

A. SET cache "value" NX EX 30

B. SET cache "value"
TTL cache
EXPIRE cache 30 if TTL is -1

C. SET cache "value" EX 30
EXPIRE cache 30

D. SET cache "value"
EXPIRE cache 30

Solution

1. Step 1: Understand the requirement

   We want to set the key and only add expiration if none exists (TTL = -1).

2. Step 2: Analyze options

   SET cache "value" TTL cache EXPIRE cache 30 if TTL is -1 checks TTL first, then sets EXPIRE only if TTL is -1, ensuring no overwrite of existing expiration.

3. Final Answer:

   SET cache "value" TTL cache EXPIRE cache 30 if TTL is -1 -> Option B

4. Quick Check:

   Check TTL before EXPIRE to avoid overwriting [OK]

Hint: Check TTL before EXPIRE to avoid overwriting [OK]

Common Mistakes:

• Using NX option which only affects key existence
• Setting EXPIRE twice unnecessarily
• Not checking TTL before setting EXPIRE