SETNX for set-if-not-exists in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Consider the following Redis commands executed in order:

SETNX mykey 10
SETNX mykey 20
GET mykey

What will be the output of the GET command?

Redis

SETNX mykey 10
SETNX mykey 20
GET mykey

A nil

B "20"

C "10"

D Error: key does not exist

💡 Hint

SETNX sets the key only if it does not exist.

The first SETNX sets 'mykey' to '10' because it does not exist yet. The second SETNX does nothing because 'mykey' already exists. So GET returns '10'.

🧠 Conceptual

intermediate

What does the SETNX command return when the key already exists?

If you run SETNX on a key that already exists in Redis, what is the return value?

A 0

B 1

C nil

D Error

💡 Hint

SETNX returns 1 if it sets the key, 0 otherwise.

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

📝 Syntax

advanced

Which of these is the correct syntax to set a key only if it does not exist in Redis?

Choose the correct Redis command syntax to set key 'user:1' to 'active' only if it does not exist.

A SET user:1 active XX

B SET user:1 active NX

C SETNX user:1 active

D SET user:1 active IFNOTEXISTS

💡 Hint

SETNX is a dedicated command for set-if-not-exists.

SETNX key value sets the key only if it does not exist. The SET command with NX option is valid in newer Redis versions and is the preferred syntax here.

❓ optimization

advanced

How can you atomically set a key with expiration only if it does not exist?

You want to set key 'lock' with value '1' only if it does not exist, and have it expire after 10 seconds atomically. Which Redis command achieves this?

A

SETNX lock 1
EXPIRE lock 10

B SET lock 1 EX 10

C SETNX lock 1 EX 10

D SET lock 1 NX EX 10

💡 Hint

Use the SET command with NX and EX options together.

The SET command supports NX (set if not exists) and EX (expire seconds) options atomically. SETNX does not support expiration directly.

🔧 Debug

expert

Why does this Lua script using SETNX fail to set expiration correctly?

Given this Redis Lua script:

local set = redis.call('SETNX', KEYS[1], ARGV[1])
if set == 1 then
redis.call('EXPIRE', KEYS[1], ARGV[2])
end
return set

The script sometimes leaves the key without expiration. Why?

A Because SETNX and EXPIRE are not atomic together, a crash between calls can leave key without expiration

B Because EXPIRE command is invalid inside Lua scripts

C Because KEYS and ARGV are not accessible in Lua scripts

D Because SETNX returns a string, not a number, causing the if condition to fail

💡 Hint

Consider atomicity of multiple commands in Lua scripts.

Although Lua scripts run atomically, if the script crashes or is interrupted between SETNX and EXPIRE calls, the key may be set without expiration. The two commands are separate calls.

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