Lua script syntax in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this Redis Lua script return?

Consider this Lua script run inside Redis:

return redis.call('SET', KEYS[1], ARGV[1])

What is the output if the key does not exist before?

Redis

return redis.call('SET', KEYS[1], ARGV[1])

A Error: wrong number of arguments

B nil

C 0

D "OK"

💡 Hint

The SET command returns a simple string reply when successful.

The redis.call('SET', key, value) command returns the string "OK" when it successfully sets the key.

📝 Syntax

intermediate

Which Lua script syntax is correct for incrementing a key's value?

You want to increment the integer value stored at a key using Redis Lua script. Which option is syntactically correct?

A redis.call('INCR', KEYS[1])

B redis.call('INCR', ARGV[1])

C redis.call('INCR')

D redis.call('INCR', KEYS)

💡 Hint

The INCR command requires exactly one key argument.

The INCR command increments the value stored at the key specified by KEYS[1]. ARGV is for arguments, not keys.

🔧 Debug

advanced

Why does this Lua script cause an error in Redis?

Given this script:

local val = redis.call('GET', KEYS[1])
return val + 1

What error will it cause and why?

Redis

local val = redis.call('GET', KEYS[1])
return val + 1

A Attempt to perform arithmetic on a nil value error

B Syntax error: unexpected symbol near '+'

C Attempt to perform arithmetic on a string value error

D No error, returns incremented value

💡 Hint

redis.call('GET', key) returns a string or nil, not a number.

redis.call('GET', ...) returns a string or nil. Lua cannot add a number to a string without conversion, so it raises an error.

🧠 Conceptual

advanced

What is the purpose of KEYS and ARGV in Redis Lua scripts?

In Redis Lua scripting, what do KEYS and ARGV represent and why are they separated?

A KEYS holds keys to access; ARGV holds additional arguments; separation helps Redis optimize command execution.

B KEYS holds values; ARGV holds keys.

C KEYS and ARGV both hold keys but in different formats.

D KEYS holds all arguments; ARGV is unused in scripts.

💡 Hint

Think about how Redis needs to know which keys a script will access.

KEYS contains the keys the script will access, ARGV contains other arguments. This separation helps Redis know which keys are involved for replication and sharding.

❓ optimization

expert

How to optimize a Redis Lua script that increments multiple keys atomically?

You want to increment 100 keys atomically in Redis using Lua. Which approach is best for performance?

A Call redis.call('INCR', key) 100 times in separate scripts.

B Use a single Lua script with a loop calling redis.call('INCR', key) for each key.

C Use redis.pcall inside the script to increment keys concurrently.

D Use a Lua script that builds a multi-command string and calls redis.call once.

💡 Hint

Lua scripts run atomically and reduce network round-trips.

A single Lua script looping over keys and calling redis.call('INCR', key) is efficient and atomic, minimizing network overhead.