Russian Doll Caching

Introduction

Russian doll caching nests cached fragments inside each other, so when an inner fragment changes, only that fragment and its direct parents are invalidated — siblings remain cached. This dramatically reduces cache misses.

Key Concepts

• Nested Caching: Cache fragments contain other cache fragments, forming a hierarchy.
• Touch Propagation: When a child record updates, touch: true propagates the timestamp change up the chain.
• Cache Digest: Rails includes a template digest in cache keys, so template changes automatically invalidate caches.

Real World Context

An e-commerce category page with 100 products, each showing reviews: when one review is added, only that product's cache invalidates. The other 99 products serve from cache instantly.

Deep Dive

The Pattern

erb
<% cache ['posts', @posts.maximum(:updated_at)] do %>
  <% @posts.each do |post| %>
    <% cache post do %>
      <article>
        <h2><%= post.title %></h2>
        <% cache [post, 'comments'] do %>
          <div class="comments">
            <% post.comments.each do |comment| %>
              <% cache comment do %>
                <p><%= comment.body %></p>
              <% end %>
            <% end %>
          </div>
        <% end %>
      </article>
    <% end %>
  <% end %>
<% end %>

Setting Up Touch Propagation

ruby
class Comment < ApplicationRecord
  belongs_to :post, touch: true
end

class Post < ApplicationRecord
  belongs_to :author, touch: true
  has_many :comments, dependent: :destroy
end

When a comment changes:

1. Comment's cache invalidates
2. Post's updated_at updates via touch: true
3. Post's cache invalidates
4. Sibling posts remain cached

Template Digest Keys

Rails automatically includes a digest of the template in cache keys:

erb
<% cache post do %>
  <%= post.title %>  <!-- Change this template, cache auto-refreshes -->
<% end %>

Common Pitfalls

1. Missing touch declarations — If you forget touch: true on the child association, parent caches won't invalidate when children change.
2. Too many nesting levels — More than 3-4 levels of nesting adds complexity without much benefit. Keep it practical.

Best Practices

1. Always add touch: true on belongs_to — Any association whose parent is cached should propagate timestamp changes.
2. Use maximum(:updated_at) for collection keys — This ensures the outer cache invalidates when any item in the collection changes.

Summary

• Russian doll caching nests fragments so siblings stay cached when one changes.
• touch: true propagates timestamp changes up the association chain.
• Rails auto-includes template digests in cache keys.
• Keep nesting to 3-4 levels maximum for maintainability.

# Touch propagation setup
class Comment < ApplicationRecord
  belongs_to :post, touch: true
end

class Post < ApplicationRecord
  belongs_to :category, touch: true
  has_many :comments
end

# When a comment is saved:
# 1. comment.updated_at changes
# 2. post.updated_at changes (touch)
# 3. category.updated_at changes (touch)
# All related caches invalidate automatically