Use database follower with Sidekiq

read_only_middleware.rb

require 'delegate'

module SidekiqExt
  # Use read-only database if :db option is :read_only,
  # otherwise use the default connection.
  #
  # Example:
  #
  #   class MyWorker
  #     include Sidekiq::Worker
  #
  #     sidekiq_options db: :read_only
  #
  #     def perform
  #       ...
  #     end
  #   end
  class ReadOnlyMiddleware
    # Creates a connection pool for read-only database
    class ReadOnlyConnectionPoolOwner < ActiveRecord::Base
      self.abstract_class = true
      establish_connection("follower_#{Rails.env}")
    end

    class ReadOnlySwitcherConnectionHandler < SimpleDelegator
      def initialize(connection_handler)
        super

        # Proxy the `retrieve_connection_pool` calls and return
        # the properly pool connection
        # IMPORTANT: This must be done here since SimpleDelegator doesn't
        # wrap internal calls.
        def connection_handler.retrieve_connection_pool(klass)
          if Thread.current.thread_variable_get(:use_read_only_connection_pool)
            super(ReadOnlyConnectionPoolOwner)
          else
            super
          end
        end
      end
    end

    def initialize(*args)
      connection_handler = ActiveRecord::Base.connection_handler

      unless connection_handler.instance_of?(ReadOnlySwitcherConnectionHandler)
        ActiveRecord::Base.connection_handler =
          ReadOnlySwitcherConnectionHandler.new(connection_handler)
      end
    end

    def call(worker, job, queue, &block)
      db_option = (job['db'] || :master).to_sym

      if db_option == :read_only
        execute_using_read_only_connection(&block)
      else
        yield
      end
    end

    private

    def execute_using_read_only_connection(&block)
      begin
        Thread.current.thread_variable_set(:use_read_only_connection_pool, true)
        yield
      ensure
        Thread.current.thread_variable_set(:use_read_only_connection_pool, false)
      end
    end
  end
end

Kudos to @tlewin.