TTL-based expiry in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after setting a TTL?

You run these Redis commands:

SET session:123 user1
EXPIRE session:123 10
TTL session:123

What will the TTL command return immediately after setting the expiry?

Redis

SET session:123 user1
EXPIRE session:123 10
TTL session:123

A 10

B -1

C 0

D -2

💡 Hint

TTL returns the remaining time to live in seconds.

After setting EXPIRE with 10 seconds, TTL returns 10 immediately.

🧠 Conceptual

intermediate

What does a TTL of -2 mean?

In Redis, what does a TTL command returning -2 indicate about the key?

A The key does not exist

B The key exists but has no expiry

C The key has expired but still exists

D The key has a persistent expiry

💡 Hint

Think about what happens when a key is missing.

TTL returns -2 when the key does not exist in Redis.

📝 Syntax

advanced

Which command correctly sets a key with a 5-second expiry?

Choose the Redis command that sets key 'temp' with value 'data' and expires it after 5 seconds in one atomic operation.

A SET temp data EX 5

B SETEX temp 5 data

C EXPIRE temp 5 SET data

D SET temp data PX 5000

💡 Hint

SETEX sets value and expiry in one command.

SETEX key seconds value sets the key with expiry atomically.

❓ optimization

advanced

How to efficiently check if a key will expire soon?

You want to check if a Redis key 'cache:item' will expire within the next 3 seconds. Which approach is best?

A Use TTL cache:item and check if result is between 0 and 3

B Use GET cache:item and check if value is null

C Use EXISTS cache:item and check if it returns 1

D Use PTTL cache:item and check if result is between 0 and 3000

💡 Hint

PTTL returns milliseconds remaining.

PTTL returns time to live in milliseconds, so checking if it's between 0 and 3000 is precise.

🔧 Debug

expert

Why does a key persist despite EXPIRE command?

You run these commands:

SET mykey value
EXPIRE mykey 10
DEL mykey
TTL mykey

The TTL command returns -2. But after 5 seconds, you check and the key still exists. Why?

A DEL command failed silently and key was never deleted

B TTL returns -2 only if key exists without expiry

C The key was recreated by another client after deletion

D Redis does not support expiry on deleted keys

💡 Hint

Consider concurrent clients and key recreation.

If another client recreated the key after deletion, TTL returns -2 because the original key was deleted, but the new key exists without expiry.

Practice

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

easy

A. Renames the key to '60'

B. Deletes the key immediately

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

D. Returns the current TTL of the key

Solution

1. Step 1: Understand the EXPIRE command

   The EXPIRE command sets a time-to-live (TTL) in seconds for a key in Redis.

2. Step 2: Apply the command to the example

   Using EXPIRE key 60 means the key will be deleted automatically after 60 seconds.

3. Final Answer:

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

4. Quick Check:

   EXPIRE sets TTL = expire after time [OK]

Hint: EXPIRE sets countdown in seconds for key deletion [OK]

Common Mistakes:

• Thinking EXPIRE deletes key immediately
• Confusing EXPIRE with TTL command
• Assuming EXPIRE renames the key

2. Which of the following is the correct syntax to check the remaining TTL of a key named session?

easy

A. TTL session

B. EXPIRE session

C. GETTTL session

D. PERSIST session

Solution

1. Step 1: Identify the command to check TTL

   The Redis command to check remaining time-to-live is TTL followed by the key name.

2. Step 2: Match the syntax

   TTL session correctly checks the remaining TTL of the key 'session'. Other options are either invalid or perform different actions.

3. Final Answer:

   TTL session -> Option A

4. Quick Check:

   TTL command returns remaining expiry time [OK]

Hint: Use TTL key to see remaining expiry time in seconds [OK]

Common Mistakes:

• Using EXPIRE to check TTL instead of setting it
• Using PERSIST which removes expiry, not checks it
• Assuming GETTTL is a valid Redis command

3. Consider the following Redis commands executed in order:
SET temp 100
EXPIRE temp 10
TTL temp
What is the expected output of the last command?

medium

A. -1

B. 0

C. Error

D. 10

Solution

1. Step 1: Set key and expiry

   The key 'temp' is set with value '100' and then given a TTL of 10 seconds.

2. Step 2: Check TTL immediately

   Immediately after setting expiry, TTL temp returns the remaining time, which should be close to 10 seconds.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   TTL after EXPIRE shows seconds left [OK]

Hint: TTL right after EXPIRE shows the set seconds [OK]

Common Mistakes:

• Expecting TTL to return -1 (means no expiry)
• Thinking TTL returns 0 immediately after EXPIRE
• Assuming TTL command causes error

4. You run the following commands:
SET user123 "data"
EXPIRE user123 30
PERSIST user123
TTL user123
But the TTL command returns -1. What is the issue?

medium

A. Key user123 does not exist

B. PERSIST removed the expiry, so TTL returns -1

C. TTL command syntax is wrong

D. EXPIRE was not set correctly

Solution

1. Step 1: Understand PERSIST effect

   PERSIST removes the expiry from a key, making it permanent.

2. Step 2: Check TTL after PERSIST

   After PERSIST, TTL returns -1 indicating no expiry is set on the key.

3. Final Answer:

   PERSIST removed the expiry, so TTL returns -1 -> Option B

4. Quick Check:

   PERSIST removes TTL, TTL returns -1 [OK]

Hint: PERSIST removes expiry; TTL then returns -1 [OK]

Common Mistakes:

• Thinking TTL returns error if expiry removed
• Assuming EXPIRE failed without checking
• Confusing TTL -1 with key missing

5. You want to set a key cache_item with a value and ensure it expires exactly after 5 minutes. Which sequence of commands correctly achieves this and verifies the expiry?

hard

A. SET cache_item "value"\nEXPIRE cache_item 300\nTTL cache_item

B. SET cache_item "value"\nTTL cache_item 300\nEXPIRE cache_item

C. EXPIRE cache_item 300\nSET cache_item "value"\nTTL cache_item

D. SET cache_item "value"\nPERSIST cache_item\nTTL cache_item

Solution

1. Step 1: Set the key with value

   Use SET to create the key 'cache_item' with the desired value.

2. Step 2: Set expiry to 300 seconds (5 minutes)

   Use EXPIRE cache_item 300 to set TTL to 5 minutes.

3. Step 3: Verify TTL

   Use TTL cache_item to check remaining time, which should be close to 300 seconds.

4. Final Answer:

   SET cache_item "value"\nEXPIRE cache_item 300\nTTL cache_item -> Option A

5. Quick Check:

   SET then EXPIRE then TTL to verify expiry [OK]

Hint: SET key, then EXPIRE seconds, then TTL to check [OK]

Common Mistakes:

• Using TTL to set expiry (wrong command)
• Setting EXPIRE before SET (key must exist first)
• Using PERSIST which removes expiry