Page and action caching in Ruby on Rails - Step-by-Step Execution

Concept Flow - Page and action caching

User Request Received

↓

Check Page Cache

Yes / No?↓

Serve Cached

↓

Response

↓

Serve Cached

↓

Response

↓

Store Action Cache

↓

Response

When a user requests a page, Rails first checks if a full page cache exists. If yes, it serves it immediately. If not, it checks for an action cache. If that exists, it serves it. Otherwise, it runs the controller action, generates the response, stores it in action cache, then serves it.

Execution Sample

Ruby on Rails

class ProductsController < ApplicationController
  caches_page :index
  caches_action :show

  def index
    @products = Product.all
  end

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

This Rails controller caches the entire index page and caches the show action output separately.

Execution Table

Step	Cache Checked	Cache Hit?	Action Taken	Response Served
1	Page Cache for /products	No	Check Action Cache	No response yet
2	No action cache configured for index	N/A	Run index action	No response yet
3	N/A	N/A	Generate index page HTML	No response yet
4	N/A	N/A	Store page cache for /products	No response yet
5	N/A	N/A	Serve generated index page	Index page HTML served
6	Page Cache for /products	Yes	Serve cached page	Index page HTML served
7	Page Cache for /products/1	No	Check Action Cache	No response yet
8	Action Cache for show id=1	No	Run show action	No response yet
9	N/A	N/A	Generate show page HTML	No response yet
10	N/A	N/A	Store action cache for show id=1	No response yet
11	N/A	N/A	Serve generated show page	Show page HTML served
12	Page Cache for /products/1	No	Check Action Cache	No response yet
13	Action Cache for show id=1	Yes	Serve cached action	Show page HTML served
14	Page Cache for /products/1	No	Check Action Cache	No response yet

💡 Execution stops after serving cached or generated response.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 8	After Step 10	Final
page_cache_index	empty	empty	stored HTML	stored HTML	stored HTML	stored HTML
action_cache_show_1	empty	empty	empty	empty	stored HTML	stored HTML

Key Moments - 3 Insights

Why does Rails check page cache before action cache?

Page cache stores the full HTML page and is faster to serve. If it exists, Rails can skip running any controller code. This is shown in execution_table rows 1 and 2 where page cache is checked first.

What happens if page cache is missing but action cache exists?

Rails serves the cached action output without running the controller action. This is shown in execution_table rows 12-13 where page cache misses but action cache hits to serve the show page.

When is the cache stored during the request?

Cache is stored after generating the response, as seen in rows 4 and 10 where page and action caches are saved after running actions.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the page cache stored for the index page?

A Step 2

B Step 4

C Step 10

D Step 8

💡 Hint

Check the 'Action Taken' column for storing page cache related to index page.

At which step does Rails serve the cached action for show id=1?

A Step 11

B Step 8

C Step 13

D Step 14

💡 Hint

Look for 'Serve cached action' in the 'Action Taken' column for show id=1.

If the page cache for /products existed at the start, what would happen at step 1?

A Rails would serve the cached page immediately

B Rails would check action cache next

C Rails would run the index action

D Rails would generate a new page

💡 Hint

Refer to the 'Cache Hit?' and 'Action Taken' columns at step 1.

Concept Snapshot

Page and action caching in Rails:
- Page caching stores full HTML pages for instant serving.
- Action caching stores controller action output with filters.
- Rails checks page cache first, then action cache.
- If no cache, runs action and stores cache.
- Speeds up response by skipping controller processing.

Full Transcript

Page and action caching in Rails help speed up web responses. When a user requests a page, Rails first looks for a full page cache. If it finds one, it sends that cached page immediately without running any code. If not, it checks for an action cache, which stores the output of controller actions. If action cache exists, Rails serves it. Otherwise, Rails runs the controller action, generates the page, stores the cache, and then serves the response. This process reduces server work and makes pages load faster. The example controller caches the index page fully and caches the show action separately. The execution table shows step-by-step how Rails checks caches, runs actions, stores caches, and serves responses. Variables track when caches are empty or stored. Key moments clarify why page cache is checked before action cache, when caches are stored, and what happens if caches exist or not. The quiz tests understanding of cache storage and serving steps. This caching pattern is important for improving Rails app performance.

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