Atomic operations with Lua in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Lua script in Redis?

Consider the following Lua script executed in Redis:

redis.call('SET', 'counter', 10)
local val = redis.call('INCRBY', 'counter', 5)
return val

What will be the output after running this script?

Redis

redis.call('SET', 'counter', 10)
local val = redis.call('INCRBY', 'counter', 5)
return val

A nil

B 10

C 5

D 15

💡 Hint

Remember that INCRBY increases the value stored at the key by the given amount and returns the new value.

The script sets 'counter' to 10, then increments it by 5 using INCRBY, which returns the new value 15.

📝 Syntax

intermediate

Which Lua script will cause a syntax error in Redis?

Identify the Lua script that will cause a syntax error when executed in Redis.

A local val = redis.call('INCR', 'counter') return val

B local val = redis.call('GET', 'key' return val

C local val = redis.call('SET', 'key', 'value') return val

D return redis.call('GET', 'key')

💡 Hint

Look for missing or misplaced parentheses.

Option B is missing a closing parenthesis in the redis.call function, causing a syntax error.

❓ optimization

advanced

Which Lua script is the most efficient for incrementing a counter and returning its new value?

You want to increment a Redis key 'visits' atomically and get the updated value. Which script is the most efficient?

A return redis.call('INCR', 'visits')

B

local current = redis.call('GET', 'visits')
local new = tonumber(current) + 1
redis.call('SET', 'visits', new)
return new

C

redis.call('SET', 'visits', redis.call('GET', 'visits') + 1)
return redis.call('GET', 'visits')

D

local new = redis.call('INCRBY', 'visits', 1)
return new

💡 Hint

Consider atomicity and number of Redis calls.

Option A uses the atomic INCR command which increments and returns the new value in one call, making it most efficient.

🧠 Conceptual

advanced

Why are Lua scripts in Redis considered atomic?

Choose the best explanation for why Lua scripts executed in Redis are atomic.

A Because Redis queues Lua scripts and executes them one at a time without interruption.

B Because Redis locks the entire database during script execution, preventing other commands.

C Because Lua scripts run in a separate thread from Redis commands.

D Because Lua scripts automatically rollback if an error occurs.

💡 Hint

Think about how Redis handles commands and scripts internally.

Redis executes Lua scripts sequentially, blocking other commands until the script finishes, ensuring atomicity.

🔧 Debug

expert

What error does this Lua script raise in Redis?

Given the Lua script:

local val = redis.call('INCRBY', 'counter', 'five')
return val

What error will Redis return when executing this script?

Redis

local val = redis.call('INCRBY', 'counter', 'five')
return val

A nil

B SyntaxError: unexpected string

C ERR value is not an integer or out of range

D TypeError: argument must be a number

💡 Hint

INCRBY expects a numeric increment value.

Passing a non-numeric string 'five' to INCRBY causes Redis to return an error about invalid integer value.