backend.rb

module Backend
  CHUNKSIZE = 65536

  def self.save(tmp_file, new_file)
    md5 = Digest::MD5.new
    STDERR.puts "COPYING!!"

    open(tmp_file, "rb") do |tmp|
      open(new_file, "wb") do |new|
        until tmp.eof?
          chunk = tmp.read(CHUNKSIZE)
          new << chunk
          md5.update chunk
        end
      end
    end
    md5.hexdigest
  end
end

post_image.rb

class PostImage < Goliath::API
  use Goliath::Rack::Render, 'json'

  def on_headers(env, headers)
    env['headers'] = headers
  end

  # receive large file in chunks and use a tempfile as buffer
  def on_body(env, data)
    env['body'] ||= Tempfile.open('tmp-image', encoding: 'ascii-8bit')
    env['body'] << data
  end

  def response(env)
    id = pull_metadata_from_custom_headers(env['headers'])[:_id]
    body = env['body']

    begin
      meta = upload_and_update(id, body)
    # 400 rescue example only, many more not included to make this gist slim
    rescue => e
      return [400, {}, {message: e.message}] 
    ensure
      body.unlink
    end unless body.nil?

    [200, {}, {image: meta}]
  end

  # Update image status and launch upload
  def upload_and_update(id, body)
    db.put_image(id, status: 'uploading')
    checksum = do_upload(body)
    # only after complete update, update its meta on db
    db.put_image(id, status: 'available', checksum: checksum)
  end

  # Upload of received file (in this case, its just a copy to other hd location)
  def do_upload(body)
    new = File.open('some file')
    Backend.save(body, new)
  end
end