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.