Russian doll caching in Ruby on Rails - Step-by-Step Execution

Concept Flow - Russian doll caching

Render outer cache block

↓

Check outer cache hit?

Yes→Use cached outer content

No↓

Render inner cache block

↓

Check inner cache hit?

Yes→Use cached inner content

No↓

Render inner content

↓

Store inner content in cache

↓

Render outer content with inner cached

↓

Store outer content in cache

↓

Display full cached content

The outer cache block checks if it has cached content. If not, it renders the inner cache block, which also checks its cache. This nesting allows efficient reuse of cached parts.

Execution Sample

Ruby on Rails

<% cache @post do %>
  <h1><%= @post.title %></h1>
  <% cache @post.comments do %>
    <%= render @post.comments %>
  <% end %>
<% end %>

This code caches a post and its comments separately, nesting the comment cache inside the post cache.

Execution Table

Step	Cache Block	Cache Hit?	Action	Cache Stored	Output
1	Outer (@post)	No	Render outer block start	No	Start rendering post
2	Inner (@post.comments)	No	Render inner block start	No	Start rendering comments
3	Inner (@post.comments)	No	Render comments content	Yes	Comments HTML rendered and cached
4	Outer (@post)	No	Render post content with cached comments	Yes	Post HTML with comments included cached
5	Display	N/A	Show cached post with comments	N/A	Full post and comments displayed
6	Outer (@post)	Yes	Use cached outer content	N/A	Full post HTML with comments served from cache
7	Display	N/A	Show cached post with comments	N/A	Full post and comments displayed from cache

💡 Caching stops rendering when cache hits occur, serving stored HTML instead.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
outer_cache	empty	empty	cached post with comments	cached post with comments
inner_cache	empty	cached comments	cached comments	cached comments
output	empty	post title + comments HTML	post HTML with comments	post HTML with comments

Key Moments - 3 Insights

Why does the inner cache block render even if the outer cache is missing?

Because the outer cache is missing, Rails renders the outer block, which includes rendering the inner cache block. The inner cache block checks its own cache independently (see steps 2 and 3 in execution_table).

What happens when both outer and inner caches hit?

Rails skips rendering the outer block (and thus the inner) and serves the fully cached HTML directly (see steps 6 and 7). This saves time by reusing all cached content.

How does nesting caches improve performance?

If only comments change, only the inner cache updates, while the outer cache remains valid. This avoids re-rendering the whole post (see variable_tracker showing inner_cache updated separately).

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the inner cache first stored?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Check the 'Cache Stored' column for the inner cache block in execution_table.

According to variable_tracker, what is the state of outer_cache after step 4?

A empty

B cached post with comments

C cached comments

D not defined

💡 Hint

Look at the 'outer_cache' row and 'After Step 4' column in variable_tracker.

If the outer cache hits but the inner cache misses, what would happen?

A Outer cache is invalidated and re-rendered

B Both caches are ignored and content is re-rendered

C This situation cannot happen because inner cache is inside outer cache

D Outer cache is used, inner cache is ignored

💡 Hint

Consider the nesting logic shown in concept_flow and execution_table.

Concept Snapshot

Russian doll caching nests cache blocks inside each other.
Outer cache wraps inner caches.
If outer cache misses, inner caches check their own caches.
Caches store rendered HTML to reuse.
Improves performance by caching parts separately.
Used in Rails views with <% cache %> blocks.

Full Transcript

Russian doll caching in Rails means putting cache blocks inside other cache blocks. When rendering, Rails first checks if the outer cache exists. If it does, it uses that cached content directly. If not, it renders the outer block, which includes rendering the inner cache block. The inner cache block also checks its own cache independently. This nesting allows Rails to reuse cached parts efficiently. For example, if only comments change, only the inner cache updates, saving time by not re-rendering the whole post. The execution table shows steps where caches are checked, rendered, and stored. Variable tracking shows how caches update separately. This method improves performance by caching smaller parts inside bigger parts.