Page and action caching in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is page caching in Rails?

Page caching saves the entire HTML output of a page as a static file. When a user requests that page, the server serves the static file directly without running Rails code, making it very fast.

intermediate

How does action caching differ from page caching?

Action caching runs Rails filters and code before serving a cached version of the action's output. It allows authentication and other logic to run, unlike page caching which skips Rails entirely.

beginner

Which caching method is better for pages that require user authentication: page caching or action caching?

Action caching is better because it runs Rails filters like authentication before serving cached content. Page caching skips Rails and cannot check user permissions.

beginner

How do you enable page caching for a controller action in Rails?

You use the caches_page :action_name method inside the controller. This tells Rails to save the full HTML output of that action as a static file.

intermediate

What is a common downside of page caching?

Page caching can serve outdated content if the cached file is not expired or deleted after data changes. It also cannot handle dynamic content per user.

What does page caching in Rails do?

A Saves the full HTML output as a static file

B Caches only database queries

C Runs filters before caching

D Caches only JSON responses

Page caching saves the entire HTML output as a static file to serve quickly without running Rails.

Which caching method allows running authentication filters before serving cached content?

A Page caching

B Low-level caching

C Fragment caching

D Action caching

Action caching runs Rails filters like authentication before serving cached content.

How do you enable page caching for an action named 'show'?

A caches_action :show

B caches_page :show

C cache_page :show

D cache_action :show

The correct method is caches_page :show to enable page caching for the 'show' action.

What is a limitation of page caching?

A It is slower than action caching

B It cannot cache static files

C It skips Rails filters and cannot handle dynamic user content

D It requires database queries on every request

Page caching skips Rails filters and cannot handle dynamic content per user.

When is action caching preferred over page caching?

A When you need to run filters like authentication before caching

B When you want to cache static assets

C When you want to cache only partial views

D When you want to cache database queries

Action caching is preferred when filters like authentication must run before serving cached content.

Explain the difference between page caching and action caching in Rails.

Think about when Rails code runs in each caching method.

Describe a scenario where action caching is better than page caching.

Consider pages that show different content to logged-in users.

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