Stream data as a file in Rails as they are written to IO.

io_streaming_response_body.rb

class IOStreamingResponseBody

  def self.render(options = {}, &block)
    instance = new(options)
    instance.define_singleton_method :render do |dummy_io|
      block.call(dummy_io)
    end
    instance
  end

  def initialize(options = {})
    @options = options
  end

  def each(&block)
    return to_enum(__method__) unless block_given?
    dummy_io = DummyIO.new(@options) do |buffer|
      block.call(buffer)
    end
    render(dummy_io)
    self
  end

  def render(_dummy_io)
    raise 'abstract'
  end

  class DummyIO

    def initialize(options = {}, &callback)
      @chunk_size = options.fetch(:chunk_size, 100.kilobytes)
      @callback = callback
      @buffer = ''
    end

    def <<(str)
      @buffer << str
      flush if @buffer.bytesize > @chunk_size * 0.9
      self
    end

    def flush
      @callback.call(@buffer)
      @buffer.clear
    end

    def close
      flush
    end

  end

end

streaming_csv_example_controller.rb

class StreamingCsvExamplesController < ApplicationController    
    
  def example_csv
    @filename = "example-#{Time.now.utc.strftime('%Y%m%dT%H%M%SZ')}.csv"
  
    headers['Cache-Control'] = 'no-cache'
    headers['X-Accel-Buffering'] = 'no'
    headers.delete('Content-Length')
    send_file_headers!(type: Mime::CSV, filename: @filename)
  
    self.response_body = IOStreamingResponseBody.render do |io|
      Example.output_csv_to_io(io)
    end
    response.status = :ok
  end

end