Russian doll caching in Ruby on Rails - Performance & Optimization

Performance: Russian doll caching

HIGH IMPACT

This affects page load speed by reducing server rendering time and network payload size through nested cache fragments.

Rendering a complex page with nested components that change independently

Ruby on Rails

def show
  @post = Post.find(params[:id])
  render :show
end

# In view:
<% cache @post do %>
  <%= render @post %>
  <% cache @post.comments do %>
    <%= render @post.comments %>
  <% end %>
<% end %>

Caches nested parts separately, so only changed fragments re-render, reducing server load and response time.

📈 Performance GainReduces rendering time by 70-90%; avoids unnecessary reflows on client by sending smaller HTML

Rendering a complex page with nested components that change independently

Ruby on Rails

def show
  @post = Post.find(params[:id])
  render :show
end

# In view:
<%= render @post %>
<%= render @post.comments %>

Renders entire page and all nested parts every time, causing slow response and high CPU load.

📉 Performance CostBlocks rendering for 200-500ms on complex pages; triggers full template re-render

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No caching, full render	High (full DOM tree)	Multiple (full page reflow)	High (large HTML)	[X] Bad
Russian doll caching	Low (only changed fragments)	Minimal (partial updates)	Low (smaller HTML)	[OK] Good

Rendering Pipeline

Russian doll caching reduces server-side rendering work by serving cached HTML fragments. This lowers the time spent in template rendering and reduces the HTML size sent to the browser, speeding up the critical rendering path.

→Server Rendering

→Network Transfer

→Browser Parsing

⚠️ BottleneckServer Rendering (template processing)

Core Web Vital Affected

LCP

This affects page load speed by reducing server rendering time and network payload size through nested cache fragments.

Optimization Tips

1Cache nested parts of views separately to maximize reuse.

2Expire only the cache fragments that changed to avoid full re-renders.

3Use Russian doll caching to reduce server rendering time and HTML size.

Performance Quiz - 3 Questions

What is the main performance benefit of Russian doll caching in Rails?

A It avoids re-rendering unchanged nested parts of a page.

B It reduces CSS file size.

C It speeds up JavaScript execution.

D It compresses images automatically.

DevTools: Network

How to check: Open DevTools Network tab, reload page, and inspect HTML response size and load time. Compare with and without caching.

What to look for: Look for smaller HTML payload and faster Time to First Byte (TTFB) indicating effective caching.

Practice

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

easy

A. To cache nested parts of a page separately for faster updates

B. To cache the entire page as a single block

C. To disable caching for nested components

D. To cache only the database queries

Solution

1. Step 1: Understand caching scope

   Russian doll caching caches nested parts of a page separately, not the whole page at once.

2. Step 2: Identify the benefit

   This allows only changed parts to be updated, speeding up page rendering.

3. Final Answer:

   To cache nested parts of a page separately for faster updates -> Option A

4. Quick Check:

   Russian doll caching = nested caching [OK]

Hint: Think nested caching updates only changed parts [OK]

Common Mistakes:

• Confusing with full page caching
• Thinking it disables caching
• Assuming it caches only queries

2. Which syntax correctly uses Russian doll caching in a Rails view?

easy

A. <% cache @post do %> <% cache @comment do %> ... <% end %> <% end %>

B. <% cache do @post, @comment %> ... <% end %>

C. <% cache @post + @comment do %> ... <% end %>

D. <% cache @post && @comment do %> ... <% end %>

Solution

1. Step 1: Check nested cache blocks

   Russian doll caching uses nested cache blocks for each object separately.

2. Step 2: Validate syntax correctness

   <% cache @post do %> <% cache @comment do %> ... <% end %> <% end %> correctly nests cache @post and inside it cache @comment blocks.

3. Final Answer:

   <% cache @post do %> <% cache @comment do %> ... <% end %> <% end %> -> Option A

4. Quick Check:

   Nested cache blocks = correct syntax [OK]

Hint: Look for nested cache blocks with separate objects [OK]

Common Mistakes:

• Trying to cache multiple objects in one block incorrectly
• Using operators like + or && inside cache call
• Not nesting cache blocks properly

3. Given this Rails view code snippet, what will happen when @comment is updated but @post is unchanged?

<% cache @post do %>
  <p>Post content</p>
  <% cache @comment do %>
    <p>Comment content</p>
  <% end %>
<% end %>

medium

A. Both post and comment caches are expired and re-rendered

B. Only the comment cache is expired and re-rendered

C. Only the post cache is expired and re-rendered

D. Neither cache is expired; page stays the same

Solution

1. Step 1: Understand cache dependencies

   Russian doll caching nests caches so inner caches expire independently from outer caches.

2. Step 2: Apply update effect

   Updating @comment expires only the comment cache, not the post cache.

3. Final Answer:

   Only the comment cache is expired and re-rendered -> Option B

4. Quick Check:

   Inner cache update = inner cache expired [OK]

Hint: Only changed nested cache re-renders, outer stays [OK]

Common Mistakes:

• Assuming outer cache always expires with inner
• Thinking no cache expires on update
• Confusing which cache block is affected

4. Identify the error in this Rails view code using Russian doll caching:

<% cache @post do %>
  <p>Post content</p>
<% cache @comment %>
  <p>Comment content</p>
<% end %>
<% end %>

medium

A. Extra end keyword causing syntax error

B. Nested cache blocks cannot be used

C. Incorrect variable names for caching

D. Missing block for inner cache call

Solution

1. Step 1: Check inner cache syntax

   The inner cache @comment call lacks a block (no do ... end).

2. Step 2: Understand block requirement

   Cache calls must have a block to define cached content; missing block causes error.

3. Final Answer:

   Missing block for inner cache call -> Option D

4. Quick Check:

   Cache calls need blocks [OK]

Hint: Always use do ... end with cache blocks [OK]

Common Mistakes:

• Omitting block for cache call
• Misplacing end keywords
• Assuming cache works without block

5. You want to implement Russian doll caching for a blog page with posts and comments. Which approach best ensures only changed comments or posts update their cache without re-rendering the whole page?

hard

A. Use cache @post.comments once for all comments without nesting

B. Cache the entire blog page in one cache block with a combined key

C. Nest cache @post do blocks around each post and inside nest cache comment do blocks for each comment

D. Disable caching for comments and cache only posts

Solution

1. Step 1: Understand nested caching strategy

   Russian doll caching works best by nesting cache blocks for each object separately.

2. Step 2: Apply to posts and comments

   Wrapping each post in a cache block and nesting comment cache blocks inside allows independent expiration.

3. Step 3: Evaluate other options

   Other options either cache too broadly or disable caching, losing efficiency.

4. Final Answer:

   Nest cache @post do blocks around each post and inside nest cache comment do blocks for each comment -> Option C

5. Quick Check:

   Nested cache blocks for each object = best practice [OK]

Hint: Nest cache blocks per object for fine-grained updates [OK]

Common Mistakes:

• Caching entire page at once
• Caching all comments together without nesting
• Disabling caching for parts