EXPIRE and TTL for time-to-live in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXPIRE command do in Redis?

The EXPIRE command sets a time-to-live (TTL) in seconds on a key. After this time, the key will be automatically deleted.

beginner

How can you check the remaining time before a Redis key expires?

Use the TTL command to see how many seconds remain before the key expires. It returns -1 if the key has no expiration and -2 if the key does not exist.

intermediate

What happens if you set EXPIRE on a key that does not exist?

The EXPIRE command will return 0, meaning it did not set the expiration because the key does not exist.

intermediate

Can you remove the expiration from a Redis key? If yes, how?

Yes, use the PERSIST command to remove the expiration and make the key permanent.

advanced

What is the difference between EXPIRE and PEXPIRE in Redis?

EXPIRE sets the TTL in seconds, while PEXPIRE sets the TTL in milliseconds, allowing more precise expiration times.

What does the TTL command return if a key does not have an expiration?

A -2

B 0

C 1

D -1

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

Which command sets a key to expire after a certain number of seconds?

A PERSIST

B TTL

C EXPIRE

D GET

EXPIRE sets the time-to-live in seconds for a key.

If you want to remove the expiration from a key, which command should you use?

A PERSIST

B EXPIRE

C TTL

D DEL

PERSIST removes the expiration, making the key permanent.

What does the EXPIRE command return if the key does not exist?

A 1

B 0

C -1

D -2

EXPIRE returns 0 if the key does not exist.

Which command sets the expiration time in milliseconds?

A PEXPIRE

B PTTL

C TTL

D EXPIRE

PEXPIRE sets the expiration time in milliseconds.

Explain how to set and check the expiration time of a key in Redis.

Think about how you tell Redis to delete a key after some time and how to see how much time is left.

Describe how to remove the expiration from a Redis key and what effect it has.

Consider what happens if you want a key to stay forever after setting an expiration.

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