SETNX for set-if-not-exists in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command SETNX do?

SETNX sets a key to a value only if the key does not already exist in the database.

beginner

What will SETNX mykey "hello" return if mykey already exists?

It will return 0, meaning the key was not set because it already exists.

intermediate

How can SETNX be useful in real-life applications?

It can be used to create locks or ensure a value is set only once, like setting a unique user ID or a one-time token.

beginner

What is the difference between SET and SETNX in Redis?

SET always sets the key to the value, overwriting if it exists. SETNX sets the key only if it does not exist.

beginner

If SETNX returns 1, what does that mean?

It means the key was successfully set because it did not exist before.

What does the Redis command SETNX key value do?

A Deletes the key if it exists

B Always sets the key to the value

C Returns the current value of the key

D Sets the key only if it does not exist

SETNX means 'set if not exists'. It sets the key only if it does not already exist.

What does SETNX return if the key already exists?

A null

B 1

C 0

D error

SETNX returns 0 if the key already exists and the value was not set.

Which Redis command would you use to set a key only if it does not exist?

A GET

B SETNX

C SET

D DEL

SETNX is designed to set a key only if it does not exist.

Why might you use SETNX in a distributed system?

A To create a lock preventing race conditions

B To get the value of a key

C To always overwrite values

D To delete keys safely

SETNX can be used to create locks by setting a key only if it does not exist, preventing multiple clients from doing the same action simultaneously.

If you want to set a key regardless of its existence, which command should you use?

A SET

B SETNX

C GET

D EXISTS

SET sets the key value no matter if the key exists or not.

Explain how the Redis command SETNX works and give an example of when you might use it.

Think about setting a value only once.

What is the difference between SET and SETNX in Redis?

Consider what happens if the key already exists.

Practice

1. What does the Redis command SETNX do?

easy

A. Sets a key only if it does not already exist

B. Always sets a key, overwriting any existing value

C. Deletes a key if it exists

D. Returns the value of a key without changing it

Solution

1. Step 1: Understand the purpose of SETNX

   SETNX stands for 'SET if Not eXists'. It sets a key only if the key does not already exist in Redis.

2. Step 2: Compare with other commands

   Unlike SET, which always sets the key, SETNX avoids overwriting existing keys.

3. Final Answer:

   Sets a key only if it does not already exist -> Option A

4. Quick Check:

   SETNX = set-if-not-exists [OK]

Hint: Remember NX means 'Not eXists' for setting keys [OK]

Common Mistakes:

• Thinking SETNX always overwrites keys
• Confusing SETNX with GET command
• Assuming SETNX deletes keys

2. Which of the following is the correct syntax to set key 'user:1' to 'Alice' only if it does not exist using SETNX?

easy

A. SETNX user:1 Alice

B. SET user:1 Alice NX

C. SETNX 'user:1' 'Alice'

D. SET user:1 Alice XX

Solution

1. Step 1: Recall SETNX syntax

   The correct syntax is SETNX key value without quotes unless the key or value contains spaces.

2. Step 2: Analyze options

   SETNX user:1 Alice uses correct syntax without quotes, which is valid. SETNX 'user:1' 'Alice' uses quotes which are not required in Redis CLI and may cause errors.

3. Final Answer:

   SETNX user:1 Alice -> Option A

4. Quick Check:

   SETNX key value is correct syntax [OK]

Hint: Use SETNX key value without quotes for simple keys [OK]

Common Mistakes:

• Using SET with NX option incorrectly
• Adding quotes unnecessarily in Redis CLI
• Using XX option which means 'only set if exists'

3. Given the following Redis commands executed in order:
SETNX lock 1
SETNX lock 2
What will be the result of the second SETNX command?

medium

A. 1

B. 2

C. 0

D. Error

Solution

1. Step 1: Execute first SETNX

   The first command SETNX lock 1 sets the key 'lock' to '1' because it does not exist yet. It returns 1 indicating success.

2. Step 2: Execute second SETNX

   The second command SETNX lock 2 tries to set 'lock' again, but since 'lock' already exists, it does not overwrite and returns 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   SETNX returns 0 if key exists [OK]

Hint: SETNX returns 0 if key already exists [OK]

Common Mistakes:

• Assuming SETNX overwrites existing keys
• Expecting the second SETNX to return 1
• Confusing return values with stored values

4. You run the command SETNX mykey without a value. What is the expected result?

medium

A. Returns 1 and sets mykey to NULL

B. Syntax error due to missing value

C. Sets mykey to an empty string

D. Returns 0 because key exists

Solution

1. Step 1: Check command syntax requirements

   SETNX requires exactly two arguments: a key and a value. Omitting the value causes a syntax error.

2. Step 2: Analyze the command given

   The command SETNX mykey has only one argument after the command name, so Redis will return a syntax error.

3. Final Answer:

   Syntax error due to missing value -> Option B

4. Quick Check:

   SETNX needs key and value arguments [OK]

Hint: Always provide both key and value for SETNX [OK]

Common Mistakes:

• Trying to set key without value
• Assuming missing value defaults to empty string
• Expecting 0 return instead of error

5. You want to implement a simple lock using Redis SETNX. Which sequence correctly acquires the lock and handles the case when the lock is already held?

hard

A. Use DEL lock before SETNX to ensure lock is free

B. Use SET lock 1 NX; if returns OK, lock acquired; else delete lock

C. Use SETNX lock 1; if returns 0, overwrite lock with SET

D. Use SETNX lock 1; if returns 1, lock acquired; else wait and retry

Solution

1. Step 1: Understand lock acquisition with SETNX

   SETNX sets the lock key only if it does not exist, returning 1 if successful, which means lock acquired.

2. Step 2: Handle lock already held

   If SETNX returns 0, the lock exists, so the client should wait and retry later instead of overwriting or deleting the lock.

3. Final Answer:

   Use SETNX lock 1; if returns 1, lock acquired; else wait and retry -> Option D

4. Quick Check:

   SETNX + retry loop is standard lock pattern [OK]

Hint: SETNX returns 1 means lock acquired; else retry [OK]

Common Mistakes:

• Overwriting lock without checking SETNX result
• Deleting lock blindly before setting
• Assuming SETNX returns OK instead of 1 or 0