DEL and UNLINK for deletion in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the DEL command do in Redis?

DEL removes the specified keys from the database immediately and synchronously.

intermediate

How does the UNLINK command differ from DEL in Redis?

UNLINK removes keys asynchronously by unlinking them from the keyspace and freeing memory in the background.

intermediate

When should you prefer using UNLINK over DEL?

Use UNLINK when you want to avoid blocking Redis during large key deletions, improving performance.

beginner

What happens if you try to DEL or UNLINK a key that does not exist?

Both DEL and UNLINK return 0 and do nothing if the key does not exist.

beginner

Can DEL and UNLINK delete multiple keys at once?

Yes, both commands accept multiple keys and delete all specified keys in one call.

Which Redis command deletes keys asynchronously?

A DEL

B UNLINK

C EXPIRE

D FLUSHDB

UNLINK deletes keys asynchronously, freeing memory in the background.

What does DEL do when deleting a large key?

A Returns an error

B Deletes it asynchronously

C Renames the key

D Deletes it immediately and blocks Redis

DEL deletes keys immediately and can block Redis if the key is large.

If a key does not exist, what is the return value of DEL or UNLINK?

A 0

B Error

C 1

D Null

Both DEL and UNLINK return 0 when the key does not exist.

Which command would you use to avoid blocking Redis during key deletion?

A UNLINK

B SET

C GET

D DEL

UNLINK frees memory asynchronously, avoiding blocking Redis.

Can DEL and UNLINK delete multiple keys in one command?

A Only DEL can

B No

C Yes

D Only UNLINK can

Both DEL and UNLINK accept multiple keys to delete at once.

Explain the difference between DEL and UNLINK commands in Redis.

Think about how each command affects Redis performance during deletion.

When would you choose UNLINK over DEL in a Redis database?

Consider the impact on Redis responsiveness.

Practice

1. What is the main difference between DEL and UNLINK commands in Redis?

easy

A. DEL deletes keys immediately and blocks Redis, while UNLINK deletes keys asynchronously in the background.

B. DEL deletes keys asynchronously, while UNLINK deletes keys immediately and blocks Redis.

C. DEL only deletes one key at a time, UNLINK deletes multiple keys.

D. DEL moves keys to a recycle bin, UNLINK permanently deletes keys.

Solution

1. Step 1: Understand DEL behavior

   DEL removes keys immediately and blocks Redis until deletion finishes.

2. Step 2: Understand UNLINK behavior

   UNLINK removes keys asynchronously in the background without blocking Redis.

3. Final Answer:

   DEL deletes keys immediately and blocks Redis, while UNLINK deletes keys asynchronously in the background. -> Option A

4. Quick Check:

   Deletion blocking = DEL [OK]

Hint: Remember: DEL blocks, UNLINK is non-blocking [OK]

Common Mistakes:

• Thinking UNLINK blocks Redis like DEL
• Believing DEL deletes keys asynchronously
• Confusing number of keys each command can delete

2. Which of the following is the correct syntax to delete keys key1 and key2 using the UNLINK command in Redis?

easy

A. UNLINK (key1, key2)

B. UNLINK key1 key2

C. UNLINK [key1, key2]

D. UNLINK 'key1' 'key2'

Solution

1. Step 1: Recall Redis command syntax

   Redis commands take keys as space-separated arguments without brackets or quotes.

2. Step 2: Match syntax for multiple keys

   UNLINK key1 key2 correctly deletes both keys at once.

3. Final Answer:

   UNLINK key1 key2 -> Option B

4. Quick Check:

   Space-separated keys = correct syntax [OK]

Hint: Use space-separated keys without brackets or quotes [OK]

Common Mistakes:

• Adding parentheses or brackets around keys
• Using quotes around keys unnecessarily
• Separating keys with commas

3. Given the following Redis commands executed in order:
SET key1 "value1"
SET key2 "value2"
DEL key1
UNLINK key2
What keys remain in the database after these commands?

medium

A. No keys remain

B. Only key1 remains

C. Only key2 remains

D. Both key1 and key2 remain

Solution

1. Step 1: Analyze DEL key1

   DEL key1 deletes key1 immediately.

2. Step 2: Analyze UNLINK key2

   UNLINK key2 deletes key2 asynchronously, but it will be removed eventually.

3. Final Answer:

   No keys remain -> Option A

4. Quick Check:

   Both keys deleted = no keys left [OK]

Hint: Both DEL and UNLINK remove keys, so none remain [OK]

Common Mistakes:

• Assuming UNLINK does not delete keys
• Thinking UNLINK delays deletion indefinitely
• Believing DEL only marks keys for deletion

4. You run the command DEL key1 key2 but get an error. Which of the following is the most likely cause?

medium

A. One or both keys do not exist

B. You need to use UNLINK instead of DEL

C. DEL cannot delete multiple keys at once

D. You used commas between keys like DEL key1, key2

Solution

1. Step 1: Check syntax for multiple keys

   Redis commands require keys separated by spaces, not commas.

2. Step 2: Identify error cause

   Using commas like DEL key1, key2 causes syntax error.

3. Final Answer:

   You used commas between keys like DEL key1, key2 -> Option D

4. Quick Check:

   Keys must be space-separated [OK]

Hint: Avoid commas; separate keys with spaces only [OK]

Common Mistakes:

• Using commas between keys
• Thinking DEL can't delete multiple keys
• Confusing missing keys with syntax errors

5. You want to delete 1000 large keys in Redis without blocking the server. Which command and approach is best?

hard

A. Delete keys one by one with DEL in a loop

B. Use UNLINK with all 1000 keys at once

C. Use UNLINK on keys in small batches asynchronously

D. Use DEL with all 1000 keys at once

Solution

1. Step 1: Understand blocking behavior of DEL

   DEL blocks Redis and deleting 1000 large keys at once can cause latency spikes.

2. Step 2: Use UNLINK in batches

   UNLINK deletes keys asynchronously; batching avoids large memory spikes and keeps Redis responsive.

3. Final Answer:

   Use UNLINK on keys in small batches asynchronously -> Option C

4. Quick Check:

   Batch UNLINK = safe large deletion [OK]

Hint: Batch UNLINK to delete large keys without blocking [OK]

Common Mistakes:

• Using DEL for large bulk deletes causing blocking
• Deleting keys one by one causing slow performance
• Using UNLINK without batching causing memory spikes