Page and action caching in Ruby on Rails

Introduction

Page and action caching help your website load faster by saving copies of pages or actions. This means the server can quickly show saved content instead of making it again every time.

When you want to speed up pages that don't change often, like a homepage or about page.

When many users visit the same page and you want to reduce server work.

When you want to save time by not running the same code repeatedly for each visitor.

When you want to improve user experience by showing pages faster.

When your site has actions that produce the same result for many users.

Syntax

Ruby on Rails

caches_page :index

caches_action :show

caches_page saves the entire HTML output of a page.

caches_action saves the result of a controller action, which can be more flexible.

Examples

This saves the full HTML of the index page so it loads instantly next time.

Ruby on Rails

class ProductsController < ApplicationController
  caches_page :index
end

This saves the output of the show action, speeding up loading for each article.

Ruby on Rails

class ArticlesController < ApplicationController
  caches_action :show
end

You can use both caching types in one controller for different actions.

Ruby on Rails

class HomeController < ApplicationController
  caches_page :welcome
  caches_action :dashboard
end

Sample Program

This example caches the full HTML of the posts index page. When users visit again, Rails serves the saved page quickly without running the index code again.

Ruby on Rails

class PostsController < ApplicationController
  caches_page :index

  def index
    @posts = Post.all
  end
end

Important Notes

Page caching works best for static content but is less flexible if you need to show different content to different users.

Action caching allows filters and authentication to run before serving cached content.

Remember to expire caches when content changes, so users see fresh data.

Summary

Page caching saves the full HTML page for fast loading.

Action caching saves the controller action output and can run filters before serving.

Use caching to improve speed but manage cache expiration carefully.

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