Fragment caching in Ruby on Rails

Introduction

Fragment caching helps speed up your web pages by saving parts of a page so they don't have to be rebuilt every time.

When a page has parts that change rarely but other parts change often.

When rendering a list of items where each item is expensive to build.

When you want to reduce server load by reusing HTML snippets.

When you want faster page loads for users by avoiding repeated work.

Syntax

Ruby on Rails

<% cache('cache_key') do %>
  <!-- content to cache -->
<% end %>

Use a unique cache_key to identify the cached fragment.

Rails stores the cached HTML and reuses it until the cache expires or is cleared.

Examples

This caches the navigation menu so it doesn't get rebuilt on every page load.

Ruby on Rails

<% cache('menu') do %>
  <nav>
    <!-- menu items -->
  </nav>
<% end %>

This caches a product's HTML using a key that includes the product's id, so each product has its own cache.

Ruby on Rails

<% cache(['product', product.id]) do %>
  <div><%= product.name %></div>
<% end %>

You can use arrays as keys to create namespaced cache keys that change when the user changes.

Ruby on Rails

<% cache(['user', user, 'profile']) do %>
  <p><%= user.name %></p>
<% end %>

Sample Program

This example caches the product details section so it doesn't get rebuilt every time the product page is viewed. The cache key includes the product id, so each product has its own cache. Expire the cache manually when the product changes.

Ruby on Rails

<!-- app/views/products/show.html.erb -->
<h1>Product Details</h1>
<% cache(['product', @product.id]) do %>
  <div>
    <h2><%= @product.name %></h2>
    <p><%= @product.description %></p>
  </div>
<% end %>

Important Notes

Cache keys should be unique and reflect the content they represent.

When the underlying data changes, you should expire or update the cache to show fresh content.

Fragment caching improves performance but adds complexity in cache management.

Summary

Fragment caching saves parts of a page to speed up rendering.

Use unique keys to cache different parts separately.

Remember to expire caches when data changes to keep content fresh.

Practice

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

easy

A. To manage database connections efficiently

B. To store user login information securely

C. To save parts of a page so they load faster next time

D. To compress images for faster download

Solution

1. Step 1: Understand fragment caching concept

   Fragment caching stores parts of a webpage to avoid regenerating them every time.

2. Step 2: Identify the main benefit

   This speeds up page rendering by reusing cached parts instead of rebuilding them.

3. Final Answer:

   To save parts of a page so they load faster next time -> Option C

4. Quick Check:

   Fragment caching = save parts for speed [OK]

Hint: Fragment caching speeds up page parts loading [OK]

Common Mistakes:

• Confusing caching with user authentication
• Thinking it compresses images
• Assuming it manages database connections

2. Which of the following is the correct syntax to use fragment caching in a Rails view?

easy

A. <% cache 'unique_key' do %> ... <% end %>

B. <% cache do 'unique_key' %> ... <% end %>

C. <% cache 'unique_key' %> ... <% end %>

D. <% cache 'unique_key' { ... } %>

Solution

1. Step 1: Recall correct fragment caching syntax

   The correct syntax uses cache 'key' do ... end block in ERB.

2. Step 2: Check each option

   <% cache 'unique_key' do %> ... <% end %> matches the correct block form. Others have wrong block usage or missing do.

3. Final Answer:

   <% cache 'unique_key' do %> ... <% end %> -> Option A

4. Quick Check:

   Use cache 'key' do block [OK]

Hint: Remember: cache needs 'do' block syntax [OK]

Common Mistakes:

• Omitting the 'do' keyword
• Using braces {} instead of do...end in ERB
• Placing the key inside the block instead of as argument

3. Given this Rails view code:

<% cache 'post_1' do %>
  <h1>Post Title</h1>
  <p>Post content here.</p>
<% end %>

What happens if the post content changes but the cache is not expired?

medium

A. The page shows an error about stale cache

B. The page shows the old cached content, not the updated post

C. Rails automatically updates the cache with new content

D. The cache is ignored and content always updates

Solution

1. Step 1: Understand cache behavior on data change

   Fragment cache stores rendered content and does not update unless expired or cleared.

2. Step 2: Identify effect of not expiring cache

   If cache is not expired, Rails serves the old cached content even if data changed.

3. Final Answer:

   The page shows the old cached content, not the updated post -> Option B

4. Quick Check:

   Cache stale without expiry = old content shown [OK]

Hint: Cache must be expired to show updated content [OK]

Common Mistakes:

• Assuming cache auto-updates on data change
• Expecting an error when cache is stale
• Thinking cache is ignored automatically

4. You have this fragment cache in your view:

<% cache 'user_42_profile' do %>
  <p>User profile info</p>
<% end %>

But after updating user data, the cache still shows old info. What is the likely problem?

medium

A. The cache key 'user_42_profile' was not changed or expired after update

B. The cache block syntax is incorrect and causes failure

C. Rails does not support fragment caching for user profiles

D. The view template is missing the cache helper method

Solution

1. Step 1: Check cache key usage

   Cache keys must change or be expired to reflect updated data.

2. Step 2: Identify why old data shows

   If key stays same and cache not expired, old cached content remains visible.

3. Final Answer:

   The cache key 'user_42_profile' was not changed or expired after update -> Option A

4. Quick Check:

   Cache key unchanged = stale cache shown [OK]

Hint: Expire or change cache key after data updates [OK]

Common Mistakes:

• Assuming syntax error causes stale cache
• Believing Rails disables caching for certain views
• Forgetting to use cache helper in views

5. You want to cache a list of comments for a post, but each comment can be edited independently. Which approach best ensures cache updates correctly?

hard

A. Cache only the post content and ignore comments

B. Cache the entire comments list with a single static key

C. Do not use fragment caching for comments to avoid stale data

D. Cache each comment fragment separately using a unique key per comment

Solution

1. Step 1: Understand caching granularity

   When parts update independently, caching each part separately allows selective expiration.

2. Step 2: Apply to comments list

   Caching each comment with a unique key lets you expire only changed comments, keeping others cached.

3. Step 3: Evaluate other options

   Single static key caches whole list, causing stale data if one comment changes. Avoiding caching loses speed benefits.

4. Final Answer:

   Cache each comment fragment separately using a unique key per comment -> Option D

5. Quick Check:

   Separate keys for independent parts = correct caching [OK]

Hint: Cache parts with unique keys for independent updates [OK]

Common Mistakes:

• Caching entire list causing stale comments
• Skipping caching due to fear of stale data
• Ignoring comment updates in cache strategy