Dynamic Octopus configuration for master/slave horizontal DB scaling with a Rails application on the Heroku stack. See the wiki page for more info: https://github.com/tchandy/octopus/wiki/Replication-with-Rails-on-Heroku. Props to Heroku for the idea, gleaned from the dynamic database.yml they inject at build-time for rails apps.

shards.yml

<%
require 'cgi'
require 'uri'

def attribute(name, value, force_string = false)
  if value
    value_string =
      if force_string
        '"' + value + '"'
      else
        value
      end
    "#{name}: #{value_string}"
  else
    ""
  end
end

configs = case Rails.env
when 'development', 'test'
  # use dev and test DB as feaux 'follower'
  Array.new(2){YAML::load_file(File.open("config/database.yml"))[Rails.env]}
else
  # staging, production, etc with Heroku config vars for follower DBs
  master_url = ENV['DATABASE_URL']
  slave_keys = ENV.keys.select{|k| k =~ /HEROKU_POSTGRESQL_.*_URL/}
  slave_keys.delete_if{ |k| ENV[k] == master_url }

  slave_keys.map do |env_key|
    config = {}
    begin
      uri = URI.parse(ENV["#{env_key}"])
    rescue URI::InvalidURIError
      raise "Invalid DATABASE_URL"
    end

    raise "No RACK_ENV or RAILS_ENV found" unless ENV["RAILS_ENV"] || ENV["RACK_ENV"]
    config['color'] = env_key.match(/HEROKU_POSTGRESQL_(.*)_URL/)[1].downcase
    config['adapter'] = uri.scheme
    config['adapter'] = "postgresql" if config['adapter'] == "postgres"

    config['database'] = (uri.path || "").split("/")[1]

    config['username'] = uri.user
    config['password'] = uri.password

    config['host'] = uri.host
    config['port'] = uri.port

    config['params'] = CGI.parse(uri.query || "")
    config
  end
end

whitelist = ENV['SLAVE_ENABLED_FOLLOWERS'].downcase.split(', ') rescue nil
blacklist = ENV['SLAVE_DISABLED_FOLLOWERS'].downcase.split(', ') rescue nil

configs.delete_if do |c|
  ( whitelist && !c['color'].in?(whitelist) ) || ( blacklist && c['color'].in?(blacklist) )
end
%>
octopus:
  replicated: true
  fully_replicated: false
  environments:
  <% if configs.present? %>
    <%= "- #{ENV["RAILS_ENV"] || ENV["RACK_ENV"] || Rails.env}" %>
  <%= ENV["RAILS_ENV"] || ENV["RACK_ENV"] || Rails.env %>:
    followers:
      <% configs.each_with_index do |c, i| %>
      <%= c.has_key?('color') ? "#{c['color']}_follower" : "follower_#{i + 1}" %>:
        <%= attribute "adapter",  c['adapter'] %>
        <%= attribute "database", c['database'] %>
        <%= attribute "username", c['username'] %>
        <%= attribute "password", c['password'], true %>
        <%= attribute "host",     c['host'] %>
        <%= attribute "port",     c['port'] %>
        <% (c['params'] || {}).each do |key, value| %>
        <%= key %>: <%= value.first %>
        <% end %>

      <% end %>
  <% else %>
    - none
  <% end %>

@jamesfzhang Isn't that because fully_replicated is set to false?

Does this still work? I can't seem to get reads to go to slave unless I specify the query with Octopus.using(:follower).

It works like that because fully_replicated is set to false.