Caching

Rails Caching Expert

Implement fast, correct caching in Rails applications. Prefer cache correctness over cache coverage — stale data is worse than slow data.

Philosophy

Core Principles:

1. Cache correctness first — A cache miss is slow; stale data is a bug
2. Invalidation is the hard part — Every cache you add is a cache you must invalidate
3. Measure before caching — Don't cache what isn't slow
4. Solid Cache is the Rails 8 default — Use it unless you have a specific reason not to
5. Fragment caching is your bread and butter — Start here, not with low-level caching

Caching Hierarchy (prefer top over bottom):

HTTP Caching (stale?/fresh_when)     ← Fastest: never even hits your app
Fragment Caching (cache helper)       ← Fast: skips view rendering
Collection Caching (cached: true)     ← Fast: batch cache reads
Low-Level Caching (Rails.cache)       ← Flexible: cache any computation
SQL Caching (automatic per-request)   ← Free: Rails does this for you

When To Use This Skill

• Adding caching to views, partials, or expensive computations
• Setting up cache stores (Solid Cache, Redis, Memcached)
• Implementing Russian doll caching with proper invalidation
• Adding HTTP conditional GET support (ETags, Last-Modified)
• Debugging stale cache issues or cache invalidation problems
• Optimizing performance with caching strategies

Instructions

Step 1: Determine What to Cache

Before writing any cache code, identify the bottleneck:

# Check logs for slow queries or renders
grep "Completed" log/development.log | sort -t= -k2 -rn | head -20

# Use rack-mini-profiler or Rails server timing
# Look for: slow partials, N+1 queries, expensive computations

Good candidates for caching:

• Partials rendered in loops (product cards, comment lists)
• Expensive computations (reports, aggregations, API calls)
• Rarely-changing content (navigation, footer, settings)
• External API responses

Bad candidates for caching:

• Content that changes every request (CSRF tokens, flash messages)
• User-specific content without user-scoped keys
• Content behind authentication without proper key scoping
• Anything that writes to the database

Step 2: Choose the Right Cache Strategy

Step 3: Implement Fragment Caching

Basic fragment cache:

<% cache product do %>
  <%= render product %>
<% end %>

Rails generates a key like: views/products/show:abc123/products/1-20260101120000000000

The key includes:

• Template tree digest (changes when template changes)
• Record's cache_key_with_version (changes when record updates)

Conditional caching:

<% cache_if !current_user&.admin?, product do %>
  <%= render product %>
<% end %>

Multi-part cache keys for user-specific or locale-specific content:

<% cache [current_user, I18n.locale, product] do %>
  <%= render product %>
<% end %>

Include the user in the cache key when content varies by user. Without it, you'll cache "Welcome, Ryan" and serve it to every visitor — this is the most common caching mistake.

Step 4: Use Collection Caching for Lists

Instead of caching each item in a loop, use collection caching:

<%# SLOW — one cache read per item %>
<% @products.each do |product| %>
  <% cache product do %>
    <%= render partial: "products/product", locals: { product: product } %>
  <% end %>
<% end %>

<%# FAST — batch cache read (multi-fetch) %>
<%= render partial: "products/product", collection: @products, cached: true %>

Collection caching reads all cache entries in one round trip. This is significantly faster for large collections.

Custom cache keys for collections:

<%= render partial: "products/product",
           collection: @products,
           cached: ->(product) { [I18n.locale, product] } %>

Step 5: Implement Russian Doll Caching

Russian doll = nested cache fragments where inner cache busting propagates outward via touch.

The view:

<% cache @category do %>
  <h2><%= @category.name %></h2>
  <% @category.products.each do |product| %>
    <% cache product do %>
      <%= render product %>
    <% end %>
  <% end %>
<% end %>

The models — touch: true propagates cache invalidation up the chain:

class Category < ApplicationRecord
  has_many :products
end

class Product < ApplicationRecord
  belongs_to :category, touch: true
end

When a product updates → its updated_at changes → touch: true updates category's updated_at → both cache fragments expire.

Without touch: true, the outer cache serves stale data — updating a product won't change the category's updated_at, so the category fragment never expires. Every belongs_to in a Russian doll chain needs touch: true.

Multi-level touch chains:

class Store < ApplicationRecord
  has_many :categories
end

class Category < ApplicationRecord
  belongs_to :store, touch: true
  has_many :products
end

class Product < ApplicationRecord
  belongs_to :category, touch: true
  # Updating a product touches category, which touches store
end

Step 6: Low-Level Caching with Rails.cache

Use fetch — it handles read + write in one call:

class Product < ApplicationRecord
  def competing_price
    Rails.cache.fetch("#{cache_key_with_version}/competing_price", expires_in: 12.hours) do
      Competitor::API.find_price(id)
    end
  end
end

Key rules for low-level caching:

1. Use cache_key_with_version for AR records — it auto-invalidates when the record updates
2. Set expires_in — unbounded caches accumulate stale data that's hard to debug
3. Cache IDs or primitives, not Active Record objects — cached AR objects become stale and break on code reloads

# Avoid — cached AR objects go stale and break on code reloads
Rails.cache.fetch("super_admins", expires_in: 12.hours) do
  User.super_admins.to_a
end

# Better — cache IDs, re-query fresh
ids = Rails.cache.fetch("super_admin_ids", expires_in: 12.hours) do
  User.super_admins.pluck(:id)
end
User.where(id: ids)

Other cache operations:

Rails.cache.read("key")                    # Returns nil on miss
Rails.cache.write("key", value, expires_in: 1.hour)
Rails.cache.delete("key")
Rails.cache.exist?("key")
Rails.cache.increment("counter")
Rails.cache.decrement("counter")

# Delete by pattern (not supported by all stores)
Rails.cache.delete_matched("products/*")

# fetch_multi for batch operations
Rails.cache.fetch_multi("key1", "key2", "key3", expires_in: 1.hour) do |key|
  expensive_computation(key)
end

Step 7: HTTP Conditional GET (stale? / fresh_when)

This is the most impactful cache — it prevents the response from being generated at all.

Use stale? when you have custom response logic:

class ProductsController < ApplicationController
  def show
    @product = Product.find(params[:id])

    if stale?(@product)
      respond_to do |format|
        format.html
        format.json { render json: @product }
      end
    end
    # If fresh, Rails auto-sends 304 Not Modified
  end
end

Use fresh_when for simple cases (default template rendering):

class ProductsController < ApplicationController
  def show
    @product = Product.find(params[:id])
    fresh_when @product
    # That's it — renders template if stale, 304 if fresh
  end

  def index
    @products = Product.all
    fresh_when @products
  end
end

With explicit options:

fresh_when last_modified: @product.updated_at.utc, etag: @product.cache_key_with_version

For content that never changes:

def show
  http_cache_forever(public: true) do
    render
  end
end

When to use conditional GET:

• Show pages for individual records
• Index pages with predictable update patterns
• API endpoints where clients cache responses
• Static-ish pages (about, terms, etc.)

Step 8: Configure Cache Store

Solid Cache (Rails 8 Default — Use This)

Database-backed caching using SSDs. No extra infrastructure needed.

# config/environments/production.rb
config.cache_store = :solid_cache_store

# config/database.yml
production:
  primary:
    <<: *default
    database: storage/production.sqlite3
  cache:
    <<: *default
    database: storage/production_cache.sqlite3
    migrations_paths: db/cache_migrate

# config/cache.yml
default: &default
  store_options:
    max_age: <%= 60.days.to_i %>
    max_size: <%= 256.megabytes %>
    namespace: <%= Rails.env %>

Why Solid Cache:

• Zero infrastructure — it's just your database
• Larger storage than RAM-based stores (SSDs are cheap)
• FIFO eviction with configurable max_age
• Supports encryption for sensitive cached data
• Supports sharding for horizontal scaling

Other stores: Redis (:redis_cache_store), Memory (:memory_store), Null (:null_store). See references/cache-stores.md for Redis production configuration with error handling, and full store comparison.

Step 9: Enable Caching in Development

bin/rails dev:cache    # Toggle caching on/off

This creates/removes tmp/caching-dev.txt and toggles perform_caching.

To use Solid Cache in development:

# config/environments/development.rb
config.cache_store = :solid_cache_store

And add the cache database to config/database.yml under development.

Step 10: Test Cache Behavior

Test that caching works correctly, not that Rails caching works:

class ProductTest < ActiveSupport::TestCase
  test "competing_price is cached" do
    product = products(:widget)

    # First call hits the API
    assert_equal 29.99, product.competing_price

    # Verify cache was written
    assert Rails.cache.exist?("#{product.cache_key_with_version}/competing_price")
  end

  test "touch propagation invalidates parent cache" do
    category = categories(:electronics)
    product = products(:widget)

    old_updated_at = category.updated_at
    product.update!(name: "New Name")

    assert_operator category.reload.updated_at, :>, old_updated_at
  end
end

In request tests, verify conditional GET:

class ProductsControllerTest < ActionDispatch::IntegrationTest
  test "returns 304 when product unchanged" do
    product = products(:widget)

    get product_path(product)
    assert_response :success
    etag = response.headers["ETag"]

    get product_path(product), headers: { "HTTP_IF_NONE_MATCH" => etag }
    assert_response :not_modified
  end
end

Common Mistakes

1. Caching User-Specific Content Without User Key

<%# Wrong: all users see the same cached content %>
<% cache @dashboard do %>
  Welcome, <%= current_user.name %>!
  <%= render @dashboard.widgets %>
<% end %>

<%# Fixed: include user in cache key %>
<% cache [current_user, @dashboard] do %>
  Welcome, <%= current_user.name %>!
  <%= render @dashboard.widgets %>
<% end %>

2. Missing touch on Associations

# Problem: updating a comment doesn't expire the post cache
class Comment < ApplicationRecord
  belongs_to :post
end

# Fixed: touch propagates invalidation to parent
class Comment < ApplicationRecord
  belongs_to :post, touch: true
end

3. Caching Active Record Objects

# Problem: cached AR objects become stale and break on code reload
Rails.cache.fetch("featured") { Product.featured.to_a }

# Fixed: cache IDs, re-query fresh
ids = Rails.cache.fetch("featured_ids", expires_in: 1.hour) { Product.featured.pluck(:id) }
Product.where(id: ids)

4. No Expiration on Low-Level Cache

# Problem: cache lives forever, exchange rates go stale silently
Rails.cache.fetch("exchange_rates") { ExchangeRateAPI.current }

# Fixed: set expires_in so data refreshes
Rails.cache.fetch("exchange_rates", expires_in: 1.hour) { ExchangeRateAPI.current }

See references/fragment-caching.md for additional common mistakes including cache key dependency issues and helper digest gotchas.

Quick Reference: Cache Key Methods

Reference

For detailed patterns, edge cases, and advanced configuration, see the references/ directory:

• references/fragment-caching.md — Cache keys, composite keys, collection caching, dependencies
• references/russian-doll.md — Nested caching, touch propagation, counter caches
• references/low-level-caching.md — Rails.cache.fetch, fetch_multi, atomic operations
• references/conditional-get.md — stale?, fresh_when, ETags, HTTP caching
• references/cache-stores.md — Store comparison, Solid Cache, Redis, Memcached config
• references/performance.md — N+1 cache calls, warming, stampede prevention, debugging