Accessing keys and arguments in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Accessing KEYS and ARGV in a Lua script

Given the following Redis Lua script, what will be the output if KEYS = ['user:1', 'user:2'] and ARGV = ['100', '200']?

return {KEYS[1], ARGV[2]}

Redis

return {KEYS[1], ARGV[2]}

A ["user:1", "200"]

B ["user:2", "100"]

C ["user:1", "100"]

D ["user:2", "200"]

💡 Hint

Remember that KEYS and ARGV are 1-based indexed in Redis Lua scripts.

In Redis Lua scripts, KEYS and ARGV arrays start at index 1. KEYS[1] is 'user:1' and ARGV[2] is '200'. So the returned list is ['user:1', '200'].

🧠 Conceptual

intermediate

Understanding KEYS and ARGV usage in Redis scripts

Why does Redis separate input parameters into KEYS and ARGV in Lua scripts?

A To allow passing only keys without any arguments.

B To optimize memory usage by separating keys and arguments.

C To distinguish keys that will be accessed for locking and replication from other arguments.

D To enforce that keys must be strings and arguments must be numbers.

💡 Hint

Think about Redis cluster and command atomicity.

Redis requires keys to be explicitly declared in KEYS so it can manage locking and replication correctly. ARGV holds other arguments that are not keys.

📝 Syntax

advanced

Identify the syntax error in accessing KEYS and ARGV

Which option contains a syntax error when trying to access the first key and first argument in a Redis Lua script?

Redis

return {KEYS[0], ARGV[1]}

A return {KEYS[1], ARGV[1]}

B return {KEYS[0], ARGV[1]}

C return {KEYS[1], ARGV[0]}

D return {KEYS[1], ARGV[2]}

💡 Hint

Lua arrays start at 1, not 0.

Lua arrays are 1-based. Accessing KEYS[0] or ARGV[0] is invalid and causes a nil value or error. ARGV[1] and KEYS[1] are valid.

🔧 Debug

advanced

Debugging argument access in Redis Lua script

A Redis Lua script returns nil when trying to access ARGV[3], but ARGV has 2 elements. What is the cause?

A ARGV is empty because KEYS were passed instead.

B ARGV indexing starts at 0, so ARGV[3] is invalid.

C ARGV must be accessed with string keys, not numeric indices.

D ARGV[3] is out of range because only 2 arguments were passed.

💡 Hint

Check how many arguments are passed in ARGV.

ARGV is an array with length equal to the number of arguments passed. Accessing ARGV[3] when only 2 arguments exist returns nil.

❓ optimization

expert

Optimizing Redis Lua script argument usage

You have a Redis Lua script that receives 10 keys and 10 arguments. You only need to access the first 3 keys and first 5 arguments. Which approach is best for performance?

A Pass only the needed keys and arguments to the script to reduce input size.

B Loop over all 10 KEYS and 10 ARGV and ignore unused ones.

C Copy KEYS and ARGV into new tables with only needed elements before use.

D Access only KEYS[1] to KEYS[3] and ARGV[1] to ARGV[5] directly without looping over all.

💡 Hint

Minimize data passed to the script for best performance.

Passing only the needed keys and arguments reduces memory and processing overhead inside the script, improving performance.