DEL and UNLINK for deletion in Redis - Step-by-Step Execution

Concept Flow - DEL and UNLINK for deletion

Start

↓

Receive DEL or UNLINK command

↓

Check if key exists

Yes No↓

DEL: Remove key synchronously

↓

UNLINK: Remove key asynchronously

↓

Return number of keys deleted

↓

End

The flow shows how Redis processes DEL and UNLINK commands: it checks if the key exists, then deletes it either synchronously (DEL) or asynchronously (UNLINK), and returns the count of deleted keys.

Execution Sample

Redis

DEL mykey
UNLINK mykey

Illustrates deletion of key 'mykey' using DEL (synchronous) and UNLINK (asynchronous).

Execution Table

Step	Command	Key Exists?	Action	Result	Notes
1	DEL mykey	Yes	Remove key synchronously	1	Key 'mykey' deleted immediately
2	DEL mykey	No	No action	0	Key 'mykey' does not exist
3	UNLINK mykey	Yes	Schedule key removal asynchronously	1	Key 'mykey' marked for async deletion
4	UNLINK mykey	No	No action	0	Key 'mykey' does not exist

💡 Execution stops after command returns number of keys deleted or 0 if key not found

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
mykey existence	Varies	Does not exist	Does not exist	Deleted (async removal scheduled)	Does not exist

Key Moments - 2 Insights

Why does DEL delete the key immediately but UNLINK does not?

DEL removes the key synchronously during the command execution (see Step 1), while UNLINK schedules the key for asynchronous deletion to avoid blocking Redis (see Step 3).

What happens if the key does not exist when DEL or UNLINK is called?

Both commands return 0 and do nothing if the key does not exist (see Steps 2 and 4 in the execution table).

Visual Quiz - 3 Questions

Look at the execution table, what is the result of DEL when the key exists?

A 1

B 0

C Error

D Key remains

💡 Hint

Check Step 1 in the execution_table under the Result column

At which step does UNLINK schedule the key for asynchronous deletion?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Action column for UNLINK commands in the execution_table

If the key does not exist, what do both DEL and UNLINK return?

A 1

B 0

C Error

D Null

💡 Hint

See Steps 2 and 4 in the execution_table Result column

Concept Snapshot

DEL and UNLINK remove keys in Redis.
DEL deletes keys immediately and blocks until done.
UNLINK schedules keys for asynchronous deletion to avoid blocking.
Both return the number of keys deleted (0 if none).
Use DEL for immediate removal, UNLINK for non-blocking deletion.

Full Transcript

This visual execution shows how Redis handles the DEL and UNLINK commands for deleting keys. When a command is received, Redis checks if the key exists. If it does, DEL removes the key immediately and returns 1, indicating one key deleted. UNLINK, on the other hand, schedules the key for asynchronous deletion to avoid blocking Redis, also returning 1. If the key does not exist, both commands return 0 and do nothing. The variable tracker shows the key's existence changing from present to deleted after DEL, and marked for async removal after UNLINK. Key moments clarify why DEL blocks and UNLINK does not, and what happens when keys are missing. The quiz tests understanding of these steps and results.

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