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.