Page and action caching in Ruby on Rails - Performance & Optimization

Performance: Page and action caching

HIGH IMPACT

Page and action caching mainly affect how fast the server can respond and how quickly the browser receives fully rendered content.

Serving frequently requested pages quickly

Ruby on Rails

caches_page :show

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

Serves pre-rendered HTML directly from cache without hitting database or rendering views.

📈 Performance GainReduces response time to under 10 ms; saves server CPU cycles

Serving frequently requested pages quickly

Ruby on Rails

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

Every request hits the database and renders the view, causing slower response and more server load.

📉 Performance CostBlocks rendering for 100+ ms per request; increases server CPU usage

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
No caching, full render each request	High (new DOM each time)	Multiple reflows per request	High paint cost	[X] Bad
Page caching serving static HTML	Low (cached DOM reused)	Single reflow on load	Low paint cost	[OK] Good
Action caching with dynamic cache keys	Medium (cached per param)	Few reflows	Medium paint cost	[!] OK

Rendering Pipeline

Page and action caching bypass the usual server rendering steps by serving stored HTML directly, reducing server processing and speeding up content delivery.

→Server Processing

→Network Transfer

→Browser Rendering

⚠️ BottleneckServer Processing (rendering views and querying database)

Core Web Vital Affected

LCP

Page and action caching mainly affect how fast the server can respond and how quickly the browser receives fully rendered content.

Optimization Tips

1Cache full pages to serve content faster and reduce server load.

2Use dynamic cache keys for actions with variable content to avoid stale data.

3Monitor cache invalidation to maintain content freshness and avoid CLS.

Performance Quiz - 3 Questions

How does page caching improve Largest Contentful Paint (LCP)?

A By reducing CSS file size

B By delaying loading of images until after page load

C By serving pre-rendered HTML directly, reducing server processing time

D By using JavaScript to render content on the client

DevTools: Performance

How to check: Record a page load and look at the 'Main' thread for scripting and rendering time; check if server response time is low.

What to look for: Short server response time and quick first contentful paint indicate effective caching.

Practice

1. What is the main difference between page caching and action caching in Rails?

easy

A. Page caching requires database queries, action caching does not.

B. Page caching saves only JSON data, action caching saves HTML pages.

C. Action caching is faster because it skips filters, page caching runs filters every time.

D. Page caching saves the full HTML page, while action caching saves the controller action output with filters.

Solution

1. Step 1: Understand what page caching does

   Page caching saves the entire HTML page as a static file, so the web server can serve it directly without hitting Rails.

2. Step 2: Understand what action caching does

   Action caching saves the output of a controller action but still runs filters like authentication before serving cached content.

3. Final Answer:

   Page caching saves the full HTML page, while action caching saves the controller action output with filters. -> Option D

4. Quick Check:

   Page caching = full HTML, action caching = action output + filters [OK]

Hint: Page caching skips filters; action caching runs filters first [OK]

Common Mistakes:

• Confusing page caching with action caching filters
• Thinking action caching saves static files
• Assuming page caching runs controller filters

2. Which of the following is the correct syntax to enable page caching for the index action in a Rails controller?

easy

A. caches_page :index

B. cache_page :index

C. page_cache :index

D. enable_page_cache :index

Solution

1. Step 1: Recall Rails page caching syntax

   Rails uses the method caches_page followed by the action name symbol to enable page caching.

2. Step 2: Verify the correct method name

   The correct method is caches_page :index. Other options are invalid method names.

3. Final Answer:

   caches_page :index -> Option A

4. Quick Check:

   Page caching method = caches_page [OK]

Hint: Page caching uses 'caches_page' method with action name [OK]

Common Mistakes:

• Using 'cache_page' instead of 'caches_page'
• Confusing page caching syntax with action caching
• Adding parentheses incorrectly

3. Given this Rails controller snippet with action caching:

class ProductsController < ApplicationController
  caches_action :show

  before_action :authenticate_user!

  def show
    @product = Product.find(params[:id])
  end
end

What happens when a cached show page is requested by a user who is not authenticated?

medium

A. The user is redirected to login because filters run before serving cached content.

B. The cached page is served directly without authentication check.

C. An error occurs because cache skips filters.

D. The cache is ignored and the action runs fresh.

Solution

1. Step 1: Understand action caching behavior with filters

   Action caching runs controller filters like before_action before serving cached content.

2. Step 2: Apply to authentication filter

   The authenticate_user! filter runs before serving the cached show page, so unauthenticated users are redirected.

3. Final Answer:

   The user is redirected to login because filters run before serving cached content. -> Option A

4. Quick Check:

   Action caching runs filters before cache [OK]

Hint: Action caching runs filters before serving cache [OK]

Common Mistakes:

• Thinking cached page skips authentication
• Assuming cache serves static files directly
• Believing cache is ignored if filters exist

4. You added caches_page :index to your controller but the page does not seem cached. Which of the following is the most likely cause?

medium

A. You forgot to run rake tmp:cache:clear to clear old cache.

B. You need to add skip_before_action for caching to work.

C. Your web server is not configured to serve cached files from public/ directory.

D. Page caching only works with JSON responses.

Solution

1. Step 1: Understand how page caching serves cached pages

   Page caching saves static HTML files in the public/ folder, served directly by the web server.

2. Step 2: Check web server configuration

   If the web server (like Nginx or Apache) is not set to serve files from public/, cached pages won't be served.

3. Final Answer:

   Your web server is not configured to serve cached files from public/ directory. -> Option C

4. Quick Check:

   Web server must serve public/ for page caching [OK]

Hint: Page cache files served by web server from public/ folder [OK]

Common Mistakes:

• Assuming Rails serves cached pages directly
• Clearing cache without checking server config
• Thinking skip_before_action is needed for caching

5. You want to cache the show action of a controller but only for logged-in users, and you want filters like authentication to run before serving cached content. Which caching method should you use and how?

hard

A. Use cache_fragment :show inside the view to cache only parts of the page.

B. Use caches_action :show to cache with filters running before serving.

C. Use caches_page :show because it caches faster and skips filters.

D. Disable caching because caching with authentication is not possible.

Solution

1. Step 1: Identify caching method that runs filters

   Action caching runs filters like authentication before serving cached content, suitable for logged-in user pages.

2. Step 2: Apply correct syntax for action caching

   Use caches_action :show in the controller to cache the action output with filters.

3. Final Answer:

   Use caches_action :show to cache with filters running before serving. -> Option B

4. Quick Check:

   Action caching = cache + filters [OK]

Hint: Action caching runs filters; page caching skips them [OK]

Common Mistakes:

• Choosing page caching which skips filters
• Confusing fragment caching with action caching
• Thinking caching can't work with authentication