EVAL command for Lua execution in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a simple Lua script with EVAL

What is the output of this Redis EVAL command?

EVAL "return ARGV[1] .. ARGV[2]" 0 Hello World

Redis

EVAL "return ARGV[1] .. ARGV[2]" 0 Hello World

A "Hello World"

B "ARGV[1]ARGV[2]"

C Error: wrong number of arguments

D "HelloWorld"

💡 Hint

Remember that ARGV is a list of arguments passed after the key count, and concatenation in Lua uses .. operator.

The Lua script concatenates ARGV[1] and ARGV[2] without spaces, so the output is "HelloWorld".

📝 Syntax

intermediate

Identify the syntax error in Lua script for EVAL

Which option contains a syntax error when used as the Lua script in Redis EVAL?

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

B return redis.call('get', KEYS[1])

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

D return redis.call('del', KEYS[1])

💡 Hint

Check for missing or mismatched parentheses in the Lua code.

Option A is missing a closing parenthesis after KEYS[1], causing a syntax error.

❓ optimization

advanced

Optimizing Lua script to increment multiple keys

You want to increment the values of multiple keys atomically using EVAL. Which Lua script is the most efficient?

A for i=1,#KEYS do redis.call('incr', KEYS[i]) end return 'OK'

B redis.call('incr', KEYS[1]) redis.call('incr', KEYS[2]) return 'OK'

C return redis.call('incr', KEYS[1]) + redis.call('incr', KEYS[2])

D for i=1,#ARGV do redis.call('incr', ARGV[i]) end return 'OK'

💡 Hint

Use KEYS array to loop over keys, not ARGV.

Option A loops over all keys passed in KEYS and increments each one, which is efficient and scalable.

🧠 Conceptual

advanced

Understanding KEYS and ARGV in EVAL

In Redis EVAL command, what is the main difference between KEYS and ARGV?

A KEYS are only used for read operations; ARGV only for write operations.

B KEYS are the keys the script will access; ARGV are additional arguments passed to the script.

C KEYS are arguments passed after ARGV; ARGV are keys to access.

D KEYS and ARGV are interchangeable and mean the same.

💡 Hint

Think about how Redis separates keys from other parameters for scripting.

KEYS contains the list of keys the script will access, while ARGV contains other arguments passed to the script.

🔧 Debug

expert

Debugging a Lua script that returns nil unexpectedly

Given this EVAL command:

EVAL "return redis.call('get', KEYS[1])" 1 mykey

If the key 'mykey' does not exist, what will the script return?

Redis

EVAL "return redis.call('get', KEYS[1])" 1 mykey

A "" (empty string)

B Error: key does not exist

C nil

D 0

💡 Hint

Consider what Redis returns when getting a non-existent key.

Redis returns nil when a key does not exist, so the Lua script returns nil.