Mastering Multi‑Layer Caching in Rails with Nginx, Redis, and MySQL

Using cache is a crucial step to improve application performance. This article summarizes various caching techniques for dynamic content using Nginx, Ruby on Rails, MySQL and Redis.

1. Client‑side cache – leverage HTTP 304 Not Modified and Rails' fresh_when method. Example:

class ArticlesController
  def show
    @article = Article.find(params[:id])
    fresh_when last_modified: @article.updated_at.utc, etag: @article
  end
end

Include user‑specific data in the ETag to avoid stale personalized pages.

2. Nginx cache – cache static‑like resources. Two approaches:

A. Turn dynamic responses into static files via an after_filter.

class CategoriesController < ActionController::Base
  after_filter :generate_static_file, only: [:index]

  def index
    @categories = Category.all
  end

  def generate_static_file
    File.open(Rails.root.join('public', 'categories'), 'w') { |f| f.write response.body }
  end
end

Delete the file on updates.

class Category < ActiveRecord::Base
  after_save :delete_static_file
  after_destroy :delete_static_file

  def delete_static_file
    File.delete Rails.root.join('public', 'categories')
  end
end

B. Store rendered pages in Redis and let Nginx fetch them.

upstream redis {
  server redis_server_ip:6379;
}
location /categories {
  set $redis_key $uri;
  default_type text/html;
  redis_pass redis;
  error_page 404 = @httpapp;
}
location @httpapp {
  proxy_pass http://ruby_backend;
}

Use redis_cache.set and redis_cache.setex to manage cache entries.

3. Full‑page cache – cache entire responses with varying parameters, e.g., pagination.

class CategoriesController
  caches_action :index, expires_in: 1.day, cache_path: proc { "categories/index/#{params[:page].to_i}" }
end

4. Fragment cache – the most versatile. Examples for ads, article bodies, complex pages, conditional caching, and touch‑based invalidation.

- cache "adverts/#{request.controller_name}/#{request.action_name}", expires_in: 1.day do
  - if advert = Advert.where(name: request.controller_name + request.action_name, enable: true).first
    div.ad = advert.content
end

- cache "articles/#{@article.id}/#{@article.updated_at.to_i}" do
  div.article = @article.content.markdown2html
end

def cache_if(condition, name = {}, cache_options = {}, &block)
  if condition
    cache(name, cache_options, &block)
  else
    yield
  end
end

5. Query cache – Rails query cache for a single request and gems like identity_cache or second_level_cache for cross‑request caching.

class User < ActiveRecord::Base
  include IdentityCache
end

class User < ActiveRecord::Base
  acts_as_cached version: 1, expires_in: 1.week
end

6. Database cache – summarises the six layers of caching from client to server.