Fragment caching in Ruby on Rails - Step-by-Step Execution

Concept Flow - Fragment caching

Render View

↓

Check Cache for Fragment

↓

Use Cached

↓

Insert Cached

↓

Continue Rendering

↓

Send Response

Rails checks if a cached fragment exists. If yes, it uses it. If no, it renders and caches the fragment before continuing.

Execution Sample

Ruby on Rails

<% cache('menu') do %>
  <ul>
    <li>Home</li>
    <li>About</li>
  </ul>
<% end %>

This code caches the HTML list inside the cache block under the key 'menu'.

Execution Table

Step	Action	Cache Check Result	Fragment Rendered	Cache Updated	Output
1	Start rendering view	N/A	No	No	No output yet
2	Check cache for key 'menu'	Miss (not found)	No	No	No output yet
3	Render fragment inside cache block	N/A	Yes	No	<ul><li>Home</li><li>About</li></ul>
4	Store rendered fragment in cache with key 'menu'	N/A	Yes	Yes	<ul><li>Home</li><li>About</li></ul>
5	Continue rendering rest of view	N/A	Yes	Yes	<ul><li>Home</li><li>About</li></ul> + rest of view
6	Next request: Check cache for key 'menu'	Hit (found)	No	No	<ul><li>Home</li><li>About</li></ul> from cache
7	Insert cached fragment, skip rendering	Hit (found)	No	No	<ul><li>Home</li><li>About</li></ul> from cache
8	Continue rendering rest of view	N/A	No	No	<ul><li>Home</li><li>About</li></ul> from cache + rest of view

💡 Execution stops after full view is rendered and response sent.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
cache['menu']	nil	nil	<ul><li>Home</li><li>About</li></ul>	<ul><li>Home</li><li>About</li></ul>	<ul><li>Home</li><li>About</li></ul>

Key Moments - 2 Insights

Why does Rails skip rendering the fragment on the second request?

Because the cache check at step 6 finds the fragment stored at step 4, so it uses the cached HTML instead of rendering again.

What happens if the cache key changes?

Rails treats it as a cache miss, so it renders the fragment again and stores it under the new key, as shown by the cache miss at step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the cache check result at step 6?

A Hit (found)

B Miss (not found)

C Error

D Not checked

💡 Hint

Check the 'Cache Check Result' column at step 6 in the execution_table.

At which step is the fragment first stored in the cache?

A Step 6

B Step 2

C Step 4

D Step 8

💡 Hint

Look at the 'Cache Updated' column to find when the cache is updated.

If the cache key changes, what will happen at the cache check?

A Cache hit and fragment reused

B Cache miss and fragment rendered again

C Cache cleared automatically

D Error thrown

💡 Hint

Refer to the key_moments explanation about cache key changes causing misses.

Concept Snapshot

Fragment caching in Rails:
Use <% cache(key) do %> ... <% end %> to cache parts of views.
Rails checks cache for the key before rendering.
If found, cached HTML is used directly.
If not, fragment renders and stores in cache.
Speeds up repeated view rendering by reusing HTML.

Full Transcript

Fragment caching in Rails helps speed up rendering by saving parts of the view as HTML in cache. When Rails renders a view with a cache block, it first checks if the cached fragment exists for the given key. If it does, Rails inserts the cached HTML directly, skipping rendering. If not, Rails renders the fragment, stores it in cache, then continues rendering the rest of the view. On subsequent requests, the cached fragment is reused until the cache expires or is cleared. This reduces repeated work and improves performance.