Low-level caching with Rails.cache - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Rails.cache.fetch block return?

Consider this code snippet in a Rails controller action:

value = Rails.cache.fetch('user_count', expires_in: 5.minutes) do
  User.count
end

What will value contain if the cache is empty?

Ruby on Rails

value = Rails.cache.fetch('user_count', expires_in: 5.minutes) do
  User.count
end

A The current count of User records from the database, and it caches this value for 5 minutes.

B Always nil because the cache key 'user_count' does not exist yet.

C An error because the block is missing a return statement.

D The string 'User.count' is cached and returned as a string.

💡 Hint

Think about what Rails.cache.fetch does when the key is missing.

Rails.cache.fetch tries to read the key 'user_count'. If missing, it runs the block, caches the result (User.count), and returns it.

📝 Syntax

intermediate

Identify the syntax error in this Rails.cache.write usage

Which option shows the correct syntax to write a value to cache with a 10-minute expiration?

Ruby on Rails

Rails.cache.write('key', 'value', expires_in: 10.minutes)

A Rails.cache.write('key', value: 'value', expires_in: 10.minutes)

B Rails.cache.write('key', 'value', expire_in: 10.minutes)

C Rails.cache.write('key', 'value', expires_in: 10.minutes)

D Rails.cache.write('key', 'value', expires: 10.minutes)

💡 Hint

Check the exact option name for expiration time.

The correct option is 'expires_in'. Other options like 'expire_in' or 'expires' are invalid and cause errors.

🔧 Debug

advanced

Why does this Rails.cache.fetch block never update the cached value?

Given this code:

Rails.cache.fetch('random_number', expires_in: 1.minute) do
  rand(100)
end

Why does the cached value never change even after 1 minute?

Ruby on Rails

Rails.cache.fetch('random_number', expires_in: 1.minute) do
  rand(100)
end

A The block is never executed because the key always exists in cache until manually deleted.

B The expires_in option is ignored because it must be passed as a string, not a symbol.

C The cache key is misspelled, so it fetches a different key each time.

D The cache store does not support expiration, so the value stays forever.

💡 Hint

Think about the cache store configuration and expiration support.

Some cache stores like MemoryStore do not support expiration properly, so expires_in is ignored and cached value stays.

❓ state_output

advanced

What is the output of this Rails.cache.fetch with race_condition_ttl?

Consider this code snippet:

value = Rails.cache.fetch('expensive_calc', expires_in: 10.seconds, race_condition_ttl: 5.seconds) do
  sleep 2
  Time.now.to_i
end
puts value

If two requests run this code at the same time after the cache expired, what will happen?

Ruby on Rails

value = Rails.cache.fetch('expensive_calc', expires_in: 10.seconds, race_condition_ttl: 5.seconds) do
  sleep 2
  Time.now.to_i
end
puts value

A Both requests recompute the value simultaneously causing duplicate work.

B The first request recomputes and caches the new time; the second request returns the old cached value during recomputation.

C Both requests return nil because the cache expired and block is not executed.

D The code raises an error because race_condition_ttl is not a valid option.

💡 Hint

Race condition TTL helps avoid multiple recomputations at once.

race_condition_ttl allows the old cached value to be served while one request recomputes the new value, preventing duplicate work.

🧠 Conceptual

expert

Which cache store supports distributed caching in Rails by default?

You want to share cached data across multiple Rails app servers. Which cache store should you choose?

A MemCacheStore

B FileStore

C MemoryStore

D NullStore

💡 Hint

Distributed caching means multiple servers share the same cache data.

MemCacheStore uses Memcached, a distributed cache system, allowing multiple app servers to share cached data.