SETNX for set-if-not-exists in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a key only if it does not exist.

Redis

SETNX mykey [1]

Drag options to blanks, or click blank then click option'

Avalue

BEX

CNX

DPX

Attempts:

3 left

💡 Hint

Common Mistakes

Using expiration options like EX or PX instead of the value.

Confusing SETNX with SET command options.

The SETNX command sets the key to the given value only if the key does not already exist.

2fill in blank

medium

Complete the command to set key 'user' with value 'Alice' only if it does not exist.

Redis

SETNX [1] Alice

Drag options to blanks, or click blank then click option'

Aname

Buser

Cusername

Duser_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different key name than 'user'.

Confusing the key with the value.

The key to set is user, so the command sets 'user' to 'Alice' if 'user' does not exist.

3fill in blank

hard

Fix the error in the command to set key 'session' with value 'abc123' only if it does not exist.

Redis

SETNX session [1]

Drag options to blanks, or click blank then click option'

ASET

B'abc123'

Csession

Dabc123

Attempts:

3 left

💡 Hint

Common Mistakes

Adding quotes around the value.

Using the key name as the value.

The value should be abc123 without quotes in Redis CLI commands.

4fill in blank

hard

Fill both blanks to set key 'config' with value 'enabled' only if it does not exist, and check the result.

Redis

SETNX [1] [2]

Drag options to blanks, or click blank then click option'

Aconfig

Benabled

Csettings

Ddisabled

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping key and value.

Using wrong key or value names.

The key is config and the value is enabled. The command sets the key only if it does not exist.

5fill in blank

hard

Fill all three blanks to set key 'flag' with value 'true' only if it does not exist, then get the key.

Redis

SETNX [1] [2]
GET [3]

Drag options to blanks, or click blank then click option'

Aflag

Btrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keys in SETNX and GET.

Using wrong values like 'false'.

The key is flag, the value is true, and the GET command retrieves the value of flag.

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