DEL and UNLINK for deletion in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Difference in behavior between DEL and UNLINK

Given a Redis key named mykey with a large value, what is the main difference in the output or effect when running DEL mykey versus UNLINK mykey?

A DEL removes the key immediately and blocks the server; UNLINK schedules the key for asynchronous deletion and returns immediately.

B Both commands immediately remove the key and block the server until deletion finishes.

C UNLINK removes the key immediately and blocks the server; DEL schedules the key for asynchronous deletion and returns immediately.

D Both commands schedule the key for asynchronous deletion and return immediately without blocking.

💡 Hint

Think about how Redis handles large keys and server blocking.

DEL deletes the key synchronously, blocking the server until the deletion completes, which can cause latency spikes for large keys. UNLINK, introduced later, schedules the key for asynchronous deletion, allowing the server to continue processing other commands immediately.

❓ query_result

intermediate

Output of DEL and UNLINK commands

Assuming the key tempkey exists in Redis, what will be the output of the commands DEL tempkey and UNLINK tempkey respectively?

A Both DEL and UNLINK return 1 if the key existed and was deleted or scheduled for deletion.

B DEL returns 0 if the key existed; UNLINK returns 1 if the key was scheduled for deletion.

C DEL returns 1 if the key existed and was deleted; UNLINK returns 0 regardless.

D DEL returns the deleted key's value; UNLINK returns the key's TTL.

💡 Hint

Check the Redis documentation for return values of DEL and UNLINK.

Both DEL and UNLINK return the number of keys that were removed or scheduled for removal. If the key existed, both return 1.

🧠 Conceptual

advanced

Why use UNLINK over DEL for large keys?

Which of the following best explains why you might prefer to use UNLINK instead of DEL when deleting large keys in Redis?

A UNLINK automatically backs up the key before deleting it.

B UNLINK compresses the key before deletion to save memory.

C UNLINK deletes keys faster by using multiple CPU cores simultaneously.

D UNLINK avoids blocking the Redis server by performing deletion asynchronously, preventing latency spikes.

💡 Hint

Consider server responsiveness during large key deletions.

UNLINK schedules the key for asynchronous deletion, allowing Redis to remain responsive and avoid latency spikes caused by blocking synchronous deletion.

📝 Syntax

advanced

Identify the invalid Redis command usage

Which of the following Redis commands will cause a syntax error?

A DEL mykey

B DEL

C UNLINK mykey anotherkey

D UNLINK mykey

💡 Hint

Check if the command requires at least one key argument.

DEL requires at least one key argument. Using DEL without any key causes a syntax error. UNLINK supports multiple keys, so option B is valid.

❓ optimization

expert

Optimizing deletion of multiple large keys

You need to delete 100 large keys in Redis without blocking the server. Which approach is the best to minimize latency impact?

A Run <code>DEL</code> on all 100 keys in a single command.

B Run <code>DEL</code> on each key one by one with a delay between each.

C Run <code>UNLINK</code> on all 100 keys in a single command.

D Run <code>UNLINK</code> on each key one by one with a delay between each.

💡 Hint

Think about asynchronous deletion and command batching.

UNLINK supports multiple keys and deletes them asynchronously in a single command, minimizing latency and server blocking. Running DEL on many keys blocks the server. Running commands one by one with delays is less efficient.

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