Key expiry for memory management in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is key expiry in Redis?

Key expiry is a feature in Redis that allows you to set a time limit on a key. After this time, the key is automatically deleted to free up memory.

beginner

How do you set a key to expire in Redis?

You use the EXPIRE command followed by the key name and the time in seconds. For example, EXPIRE mykey 60 sets the key to expire in 60 seconds.

beginner

What happens when a key expires in Redis?

When a key expires, Redis automatically removes it from the database, freeing up the memory it used.

beginner

Can you check the remaining time to live (TTL) of a key in Redis?

Yes, you use the TTL command with the key name. It returns the remaining time in seconds before the key expires.

beginner

Why is key expiry important for memory management in Redis?

Key expiry helps automatically clean up unused or temporary data, preventing memory from filling up and keeping Redis fast and efficient.

Which Redis command sets a key to expire after a certain time?

A DELETE

B SETEXPIRE

C TIMEOUT

D EXPIRE

The EXPIRE command sets a time limit on a key after which it is deleted.

What does the TTL command return in Redis?

A The value of the key

B The remaining time to live of a key in seconds

C The total number of keys

D The memory used by the key

TTL returns how many seconds remain before the key expires.

What happens when a key expires in Redis?

A It is automatically deleted

B It is archived

C It is moved to a backup database

D It becomes read-only

Expired keys are automatically removed to free memory.

Why is setting key expiry useful in Redis?

A To automatically remove unused data and save memory

B To speed up network connections

C To keep data forever

D To encrypt data

Key expiry helps manage memory by removing data that is no longer needed.

Which of these is NOT a valid way to set expiry on a key in Redis?

A EXPIRE key seconds

B SET key value EX seconds

C TTL key seconds

D PEXPIRE key milliseconds

TTL is used to check expiry time, not to set it.

Explain how key expiry helps with memory management in Redis.

Think about what happens to keys after their expiry time.

Describe how to set and check expiry time for a key in Redis.

Commands EXPIRE and TTL are key here.

Practice

1. What does the EXPIRE command do in Redis?

easy

A. Sets a time limit after which a key will be automatically deleted

B. Immediately deletes a key from the database

C. Shows the remaining time before a key expires

D. Removes the expiration time from a key

Solution

1. Step 1: Understand the purpose of EXPIRE

   The EXPIRE command sets a countdown timer on a key, after which Redis deletes it automatically.

2. Step 2: Differentiate from other commands

   Unlike DEL which deletes immediately, EXPIRE schedules deletion after a set time.

3. Final Answer:

   Sets a time limit after which a key will be automatically deleted -> Option A

4. Quick Check:

   EXPIRE = set auto-delete timer [OK]

Hint: EXPIRE sets timer; key deletes after time [OK]

Common Mistakes:

• Confusing EXPIRE with DEL command
• Thinking EXPIRE shows time left instead of setting it
• Assuming EXPIRE removes expiration instead of adding it

2. Which of the following is the correct syntax to set a key session to expire in 120 seconds?

easy

A. SETEX session 120

B. EXPIRE 120 session

C. TTL session 120

D. EXPIRE session 120

Solution

1. Step 1: Recall EXPIRE command syntax

   The correct syntax is EXPIRE key seconds, so EXPIRE session 120 sets expiry to 120 seconds.

2. Step 2: Check other options

   EXPIRE 120 session reverses arguments, A is incomplete syntax for SETEX, D is invalid usage of TTL.

3. Final Answer:

   EXPIRE session 120 -> Option D

4. Quick Check:

   EXPIRE key seconds = EXPIRE session 120 [OK]

Hint: EXPIRE syntax: EXPIRE key seconds [OK]

Common Mistakes:

• Swapping key and seconds in EXPIRE
• Using TTL to set expiry instead of check
• Confusing SETEX syntax

3. Given the commands:

SET user:1 "Alice"
EXPIRE user:1 10
TTL user:1

What will the TTL user:1 command return immediately after setting expiry?

medium

A. -1

B. Error

C. 10

D. 0

Solution

1. Step 1: Understand TTL after EXPIRE

   After setting EXPIRE to 10 seconds, TTL returns the remaining time in seconds, which is 10 immediately.

2. Step 2: Interpret other options

   -1 means no expiry, 0 means expired, Error is invalid here.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   TTL after EXPIRE = expiry seconds [OK]

Hint: TTL shows seconds left after EXPIRE set [OK]

Common Mistakes:

• Expecting TTL to return -1 immediately
• Thinking TTL returns 0 right after EXPIRE
• Confusing TTL with DEL command

4. You run the command EXPIRE mykey 30 but TTL mykey returns -2. What is the likely problem?

medium

A. The key mykey has no expiry set

B. The key mykey does not exist

C. The EXPIRE command syntax is incorrect

D. The Redis server is down

Solution

1. Step 1: Understand TTL return values

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

2. Step 2: Analyze the scenario

   Since TTL returns -2, it means mykey is missing, so EXPIRE had no effect.

3. Final Answer:

   The key mykey does not exist -> Option B

4. Quick Check:

   TTL = -2 means key missing [OK]

Hint: TTL -2 means key missing, not syntax error [OK]

Common Mistakes:

• Assuming -2 means no expiry set
• Thinking EXPIRE syntax is wrong
• Ignoring that key might be deleted

5. You want to keep a Redis key cache:data but remove its expiry to prevent automatic deletion. Which command should you use?

hard

A. PERSIST cache:data

B. EXPIRE cache:data 0

C. DEL cache:data

D. TTL cache:data

Solution

1. Step 1: Identify command to remove expiry

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

2. Step 2: Check other options

   EXPIRE with 0 deletes immediately, DEL deletes key, TTL only checks expiry time.

3. Final Answer:

   PERSIST cache:data -> Option A

4. Quick Check:

   PERSIST removes expiry timer [OK]

Hint: Use PERSIST to cancel expiry, keep key [OK]

Common Mistakes:

• Using EXPIRE 0 which deletes key immediately
• Confusing DEL with removing expiry
• Using TTL expecting it to remove expiry