caching-strategies

name	caching-strategies
description	Implements Rails caching patterns for performance optimization. Use when adding fragment caching, Russian doll caching, low-level caching, cache invalidation, or when user mentions caching, performance, cache keys, or memoization. WHEN NOT: General query optimization (use performance-optimization), background job processing, or problems caused by N+1 queries rather than missing caches.
paths	app/views/**/*.erb, app/controllers/**/*.rb

Caching Strategies for Rails 8

Overview

Rails provides multiple caching layers:

• Fragment caching: Cache view partials
• Russian doll caching: Nested cache fragments
• Low-level caching: Cache arbitrary data
• HTTP caching: Browser and CDN caching
• Query caching: Automatic within requests

Quick Start

# config/environments/development.rb
config.action_controller.perform_caching = true
config.cache_store = :memory_store

# config/environments/production.rb
config.cache_store = :solid_cache_store  # Rails 8 default
# OR
config.cache_store = :redis_cache_store, { url: ENV["REDIS_URL"] }

Enable caching in development:

bin/rails dev:cache

Cache Store Options

Store	Use Case	Pros	Cons
:memory_store	Development	Fast, no setup	Not shared, limited size
:solid_cache_store	Production (Rails 8)	Database-backed, no Redis	Slightly slower
:redis_cache_store	Production	Fast, shared	Requires Redis
:file_store	Simple production	Persistent, no Redis	Slow, not shared
:null_store	Testing	No caching	N/A

Fragment Caching

Basic Fragment Cache

<%# app/views/events/_event.html.erb %>
<% cache event do %>
  <article class="event-card">
    <h3><%= event.name %></h3>
    <p><%= event.description %></p>
    <time><%= l(event.event_date, format: :long) %></time>
    <%= render event.venue %>
  </article>
<% end %>

Cache Key Components

Rails generates cache keys from:

• Model name
• Model ID
• updated_at timestamp
• Template digest (automatic)

# Generated key example:
# views/events/123-20240115120000000000/abc123digest

Custom Cache Keys

<%# With version %>
<% cache [event, "v2"] do %>
  ...
<% end %>

<%# With user-specific content %>
<% cache [event, current_user] do %>
  ...
<% end %>

<%# With explicit key %>
<% cache "featured-events-#{Date.current}" do %>
  <%= render @featured_events %>
<% end %>

Russian Doll Caching

Nested caches where inner caches are reused when outer cache is invalidated:

<%# app/views/events/show.html.erb %>
<% cache @event do %>
  <h1><%= @event.name %></h1>

  <section class="vendors">
    <% @event.vendors.each do |vendor| %>
      <% cache vendor do %>
        <%= render partial: "vendors/card", locals: { vendor: vendor } %>
      <% end %>
    <% end %>
  </section>

  <section class="comments">
    <% @event.comments.each do |comment| %>
      <% cache comment do %>
        <%= render comment %>
      <% end %>
    <% end %>
  </section>
<% end %>

Use touch: true on belongs_to associations to cascade invalidation up the chain. See cache-invalidation.md for examples.

Collection Caching

Efficient Collection Rendering

<%# Caches each item individually %>
<%= render partial: "events/event", collection: @events, cached: true %>

<%# With custom cache key %>
<%= render partial: "events/event",
           collection: @events,
           cached: ->(event) { [event, current_user.admin?] } %>

Low-Level Caching

Use Rails.cache.fetch with a block for the most common pattern. See low-level-caching.md for:

• Basic read/write/fetch examples
• Caching in service objects
• Caching in query objects
• Instance variable memoization
• Request-scoped memoization with CurrentAttributes

Cache Invalidation

Three strategies: time-based expiration, key-based expiration (using updated_at), and manual deletion. See cache-invalidation.md for:

• Time-based and key-based expiration
• Manual invalidation in model callbacks and services
• Pattern-based deletion (delete_matched)
• touch: true for Russian doll cascade
• Built-in and custom counter caches

HTTP Caching

Use stale? for conditional GET (ETags/Last-Modified) and expires_in for Cache-Control headers. See http-caching-and-testing.md for full examples.

Testing Caching

Use a :caching metadata tag to enable caching in specs. See http-caching-and-testing.md for:

• rails_helper.rb configuration
• Testing cached view invalidation
• Testing cache invalidation in services
• Performance monitoring and instrumentation

Checklist

• Cache store configured for environment
• Fragment caching on expensive partials
• touch: true on belongs_to for Russian doll
• Collection caching with cached: true
• Low-level caching for expensive queries
• Cache invalidation strategy defined
• Counter caches for counts
• HTTP caching headers for API
• Cache warming for cold starts (if needed)
• Monitoring for hit/miss rates

References

• low-level-caching.md - fetch/read/write, service objects, query objects, memoization
• cache-invalidation.md - expiration strategies, manual invalidation, touch, counter caches
• http-caching-and-testing.md - ETags, Cache-Control, spec configuration, monitoring