Fragment caching in Ruby on Rails - Performance & Optimization

Performance: Fragment caching

HIGH IMPACT

Fragment caching improves page load speed by storing parts of the page output to avoid repeated rendering.

Rendering a complex partial multiple times on a page

Ruby on Rails

<% cache('comments_fragment') do %>
  <%= render 'comments' %>
<% end %>

Stores rendered output once, reuses it on subsequent requests without reprocessing.

📈 Performance GainReduces server render time by 70-90%, improving LCP and reducing server load

Rendering a complex partial multiple times on a page

Ruby on Rails

<%= render 'comments' %> repeated multiple times without caching

Each render triggers full template processing and database queries, slowing response.

📉 Performance CostBlocks rendering for 100+ ms per render, increasing LCP significantly

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No caching, repeated partial renders	High (many nodes rendered repeatedly)	Multiple reflows per partial	High paint cost due to delayed content	[X] Bad
Fragment caching with stored HTML	Low (cached HTML reused)	Single reflow on initial load	Low paint cost, faster content display	[OK] Good

Rendering Pipeline

Fragment caching bypasses template rendering and database calls by serving stored HTML, reducing server processing before sending to browser.

→Server Rendering

→Network Transfer

→Browser Paint

⚠️ BottleneckServer Rendering (template processing and DB queries)

Core Web Vital Affected

LCP

Fragment caching improves page load speed by storing parts of the page output to avoid repeated rendering.

Optimization Tips

1Cache only parts of the page that are expensive to render and do not change often.

2Set proper expiration or cache keys to avoid showing stale content.

3Use fragment caching to reduce server rendering time and improve LCP.

Performance Quiz - 3 Questions

What is the main performance benefit of fragment caching in Rails?

A Reduces server rendering time by reusing cached HTML fragments

B Improves client-side JavaScript execution speed

C Decreases CSS file size

D Prevents browser reflows

DevTools: Performance

How to check: Record page load, look for long server-side scripting times and repeated template rendering calls.

What to look for: Reduced server scripting time and faster Time to First Byte indicate effective fragment caching.

Practice

1. What is the main purpose of fragment caching in Rails?

easy

A. To manage database connections efficiently

B. To store user login information securely

C. To save parts of a page so they load faster next time

D. To compress images for faster download

Solution

1. Step 1: Understand fragment caching concept

   Fragment caching stores parts of a webpage to avoid regenerating them every time.

2. Step 2: Identify the main benefit

   This speeds up page rendering by reusing cached parts instead of rebuilding them.

3. Final Answer:

   To save parts of a page so they load faster next time -> Option C

4. Quick Check:

   Fragment caching = save parts for speed [OK]

Hint: Fragment caching speeds up page parts loading [OK]

Common Mistakes:

• Confusing caching with user authentication
• Thinking it compresses images
• Assuming it manages database connections

2. Which of the following is the correct syntax to use fragment caching in a Rails view?

easy

A. <% cache 'unique_key' do %> ... <% end %>

B. <% cache do 'unique_key' %> ... <% end %>

C. <% cache 'unique_key' %> ... <% end %>

D. <% cache 'unique_key' { ... } %>

Solution

1. Step 1: Recall correct fragment caching syntax

   The correct syntax uses cache 'key' do ... end block in ERB.

2. Step 2: Check each option

   <% cache 'unique_key' do %> ... <% end %> matches the correct block form. Others have wrong block usage or missing do.

3. Final Answer:

   <% cache 'unique_key' do %> ... <% end %> -> Option A

4. Quick Check:

   Use cache 'key' do block [OK]

Hint: Remember: cache needs 'do' block syntax [OK]

Common Mistakes:

• Omitting the 'do' keyword
• Using braces {} instead of do...end in ERB
• Placing the key inside the block instead of as argument

3. Given this Rails view code:

<% cache 'post_1' do %>
  <h1>Post Title</h1>
  <p>Post content here.</p>
<% end %>

What happens if the post content changes but the cache is not expired?

medium

A. The page shows an error about stale cache

B. The page shows the old cached content, not the updated post

C. Rails automatically updates the cache with new content

D. The cache is ignored and content always updates

Solution

1. Step 1: Understand cache behavior on data change

   Fragment cache stores rendered content and does not update unless expired or cleared.

2. Step 2: Identify effect of not expiring cache

   If cache is not expired, Rails serves the old cached content even if data changed.

3. Final Answer:

   The page shows the old cached content, not the updated post -> Option B

4. Quick Check:

   Cache stale without expiry = old content shown [OK]

Hint: Cache must be expired to show updated content [OK]

Common Mistakes:

• Assuming cache auto-updates on data change
• Expecting an error when cache is stale
• Thinking cache is ignored automatically

4. You have this fragment cache in your view:

<% cache 'user_42_profile' do %>
  <p>User profile info</p>
<% end %>

But after updating user data, the cache still shows old info. What is the likely problem?

medium

A. The cache key 'user_42_profile' was not changed or expired after update

B. The cache block syntax is incorrect and causes failure

C. Rails does not support fragment caching for user profiles

D. The view template is missing the cache helper method

Solution

1. Step 1: Check cache key usage

   Cache keys must change or be expired to reflect updated data.

2. Step 2: Identify why old data shows

   If key stays same and cache not expired, old cached content remains visible.

3. Final Answer:

   The cache key 'user_42_profile' was not changed or expired after update -> Option A

4. Quick Check:

   Cache key unchanged = stale cache shown [OK]

Hint: Expire or change cache key after data updates [OK]

Common Mistakes:

• Assuming syntax error causes stale cache
• Believing Rails disables caching for certain views
• Forgetting to use cache helper in views

5. You want to cache a list of comments for a post, but each comment can be edited independently. Which approach best ensures cache updates correctly?

hard

A. Cache only the post content and ignore comments

B. Cache the entire comments list with a single static key

C. Do not use fragment caching for comments to avoid stale data

D. Cache each comment fragment separately using a unique key per comment

Solution

1. Step 1: Understand caching granularity

   When parts update independently, caching each part separately allows selective expiration.

2. Step 2: Apply to comments list

   Caching each comment with a unique key lets you expire only changed comments, keeping others cached.

3. Step 3: Evaluate other options

   Single static key caches whole list, causing stale data if one comment changes. Avoiding caching loses speed benefits.

4. Final Answer:

   Cache each comment fragment separately using a unique key per comment -> Option D

5. Quick Check:

   Separate keys for independent parts = correct caching [OK]

Hint: Cache parts with unique keys for independent updates [OK]

Common Mistakes:

• Caching entire list causing stale comments
• Skipping caching due to fear of stale data
• Ignoring comment updates in cache strategy