EXPIRE and TTL for time-to-live in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the TTL after setting EXPIRE?

You run the following Redis commands:

SET session:user123 "active"
EXPIRE session:user123 120

What will TTL session:user123 return immediately after?

Redis

SET session:user123 "active"
EXPIRE session:user123 120
TTL session:user123

A 120

B -1

C -2

D 0

💡 Hint

TTL returns the remaining time in seconds before the key expires.

After setting EXPIRE with 120 seconds, TTL returns the remaining seconds, which is 120 immediately after.

❓ query_result

intermediate

What does TTL return for a key without expiration?

If you run:

SET config:theme "dark"

and then:

TTL config:theme

What is the expected output?

Redis

SET config:theme "dark"
TTL config:theme

A -1

B 0

C -2

D null

💡 Hint

TTL returns -1 if the key exists but has no expiration.

TTL returns -1 when the key exists but does not have an expiration time set.

📝 Syntax

advanced

Which EXPIRE command syntax is correct?

Which of the following Redis commands correctly sets a key to expire in 60 seconds?

A EXPIRE 60 mykey

B EXPIRE mykey TO 60

C EXPIRE mykey 60

D EXPIRE mykey EX 60

💡 Hint

The EXPIRE command syntax is: EXPIRE key seconds

The correct syntax is EXPIRE followed by the key and then the number of seconds.

❓ query_result

advanced

What happens to TTL after key deletion?

You run:

SET temp:data "123"
EXPIRE temp:data 30
DEL temp:data

What will TTL temp:data return after deletion?

Redis

SET temp:data "123"
EXPIRE temp:data 30
DEL temp:data
TTL temp:data

A -1

B -2

C 0

D 30

💡 Hint

TTL returns -2 if the key does not exist.

After deleting the key, it no longer exists, so TTL returns -2.

🧠 Conceptual

expert

Why use EXPIRE instead of manual deletion?

Which is the main advantage of using EXPIRE to set a time-to-live on keys instead of manually deleting keys later?

A EXPIRE makes keys immutable until expiration.

B EXPIRE duplicates keys to backup storage before deletion.

C EXPIRE encrypts keys for security until they expire.

D EXPIRE automatically removes keys after a set time, reducing memory usage without manual intervention.

💡 Hint

Think about automation and resource management.

EXPIRE automates key removal after a set time, helping manage memory efficiently without manual cleanup.

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