Why Lua scripts enable atomicity in Redis - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why are Lua scripts atomic in Redis?

In Redis, Lua scripts run atomically. What does this mean for commands inside a Lua script?

A Commands inside the script run one by one but can be interrupted by other clients.

B All commands inside the script run as a single, uninterrupted operation.

C Each command inside the script runs in a separate transaction.

D Lua scripts run asynchronously and may overlap with other commands.

💡 Hint

Think about what atomic means in the context of operations.

Atomic means the entire script runs without interruption, so no other commands can run in the middle.

❓ query_result

intermediate

Output of a Lua script modifying a key atomically

Given this Lua script run in Redis:

redis.call('SET', 'counter', 0)
for i = 1, 3 do
  redis.call('INCR', 'counter')
end
return redis.call('GET', 'counter')

What is the output after running this script?

A "1"

B "0"

C "3"

D nil

💡 Hint

The script increments the counter three times starting from zero.

The script sets 'counter' to 0, then increments it 3 times, so the final value is 3.

📝 Syntax

advanced

Identify the syntax error in this Lua script for Redis

Which option contains a syntax error that prevents the Lua script from running atomically in Redis?

for i = 1, 5 do
  redis.call('INCR', 'score')
end
return redis.call('GET', 'score')

A for i = 1, 5 do redis.call('INCR', 'score') end return redis.call('GET', 'score')

B )'erocs' ,'TEG'(llac.sider nruter dne )'erocs' ,'RCNI'(llac.sider od 5 ,1 = i rof

C or i = 1, 5 do redis.call('INCR', 'score') end return redis.call('GET', 'score')

D for i = 1, 5 redis.call('INCR', 'score') end return redis.call('GET', 'score')

💡 Hint

Check the for loop syntax carefully.

Option D is missing the 'do' keyword after the for loop declaration, causing a syntax error.

❓ optimization

advanced

Optimizing Lua script atomic operations in Redis

Which Lua script is the most efficient way to atomically increment two keys 'a' and 'b' by 1 in Redis?

A

local a = redis.call('INCR', 'a')
local b = redis.call('INCR', 'b')
return {a, b}

B

redis.call('INCR', 'a')
return redis.call('INCR', 'b')

C

redis.call('INCR', 'a')
redis.call('INCR', 'b')
return {redis.call('GET', 'a'), redis.call('GET', 'b')}

D

redis.call('INCRBY', 'a', 1)
redis.call('INCRBY', 'b', 1)
return {redis.call('GET', 'a'), redis.call('GET', 'b')}

💡 Hint

Consider minimizing calls and returning values directly.

Option A increments both keys and stores the results locally, then returns them, reducing extra calls.

🔧 Debug

expert

Why does this Lua script cause a runtime error in Redis?

Consider this Lua script:

local val = redis.call('GET', 'missing_key')
return val + 1

What error will Redis raise when running this script and why?

A Attempt to perform arithmetic on a nil value because 'missing_key' does not exist.

B Syntax error due to missing parentheses in redis.call.

C Redis returns nil without error because missing keys are allowed.

D Type error because redis.call returns a table, not a string.

💡 Hint

Think about what happens when you try to add a number to nil in Lua.

When 'missing_key' does not exist, redis.call returns nil. Adding 1 to nil causes a runtime error.