Low-level caching with Rails.cache - Step-by-Step Execution

Concept Flow - Low-level caching with Rails.cache

Call Rails.cache.fetch(key)

↓

Check if key exists in cache

↓

Return cached value

↓

Store value in cache

↓

Return new cached value

Rails.cache.fetch checks if a key exists. If yes, it returns cached data. If no, it runs the block, stores the result, then returns it.

Execution Sample

Ruby on Rails

value = Rails.cache.fetch('greeting') do
  'Hello, world!'
end
puts value

This code tries to get 'greeting' from cache; if missing, it stores and returns 'Hello, world!'.

Execution Table

Step	Action	Cache Key Exists?	Block Executed?	Cache Store Updated?	Returned Value
1	Call Rails.cache.fetch('greeting')	No	Yes	Yes	'Hello, world!'
2	Call Rails.cache.fetch('greeting') again	Yes	No	No	'Hello, world!'

💡 Execution stops after returning cached value on second fetch call.

Variable Tracker

Variable	Start	After Step 1	After Step 2
cache['greeting']	nil	'Hello, world!'	'Hello, world!'
value	nil	'Hello, world!'	'Hello, world!'

Key Moments - 2 Insights

Why does the block run only the first time?

Because in step 1, the cache key 'greeting' does not exist, so the block runs and stores the value. In step 2, the key exists, so the block is skipped (see execution_table rows 1 and 2).

What happens if the cache key expires or is cleared?

Rails.cache.fetch will treat it like a missing key, run the block again, store the new value, and return it, similar to step 1 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the returned value at step 2?

A Block content not returned

B 'Hello, world!'

C nil

D Error

💡 Hint

Check the 'Returned Value' column in execution_table row 2.

At which step does the cache store get updated?

A Step 2

B Both steps

C Step 1

D Never

💡 Hint

Look at 'Cache Store Updated?' column in execution_table.

If the cache key 'greeting' is deleted before step 2, what happens?

A Block runs again and cache updates

B Block is skipped and nil returned

C Error is raised

D Old cached value returned

💡 Hint

Refer to key_moments about cache expiration and step 1 behavior.

Concept Snapshot

Rails.cache.fetch(key) checks cache for key.
If found, returns cached value immediately.
If missing, runs block, stores result, returns it.
Blocks run only when cache miss occurs.
Useful for fast repeated data access.

Full Transcript

Low-level caching with Rails.cache uses the fetch method to check if a cache key exists. When you call Rails.cache.fetch with a key, it first looks in the cache. If the key is found, it returns the cached value without running the block. If the key is not found, it runs the block, stores the block's result in the cache under that key, and returns the new value. This means the block runs only once per cache miss. Subsequent calls with the same key return the cached value quickly. If the cache is cleared or the key expires, the block runs again to refresh the cache. This pattern helps speed up applications by avoiding repeated expensive operations.