title: Scrapy download_delay vs. max_concurrent_requests_per_domain
body: <p>I'm very confused about the differences and interactions between <code>DOWNLOAD_DELAY</code> and <code>MAX_CONCURRENT_REQUESTS_PER_DOMAIN</code> in Scrapy.</p>
  
  <p>Does download delay affect the maximum number of concurrent requests per domain, e.g., if I set a delay of 10 seconds but allow 8 concurrent requests per domain, will those concurrent requests not be fired concurrently but staggered according to the download delay, or will they be fired concurrently but the downloading of the responses be staggered? Is there any reason <code>DOWNLOAD_DELAY</code> isn't called <code>REQUEST_DELAY</code>?</p>
  
  <p>For example, what would the back-of-the-envelope throughput calculation be in the following scenario:</p>
  
  <ul>
  <li><code>start_urls</code> holds 100 URLs for a given domain</li>
  <li><code>MAX_CONCURRENT_REQUESTS_PER_DOMAIN = 8</code></li>
  <li><code>DOWNLOAD_DELAY = 3</code></li>
  <li>assume the server takes 2 seconds to generate a response</li>
  <li>assume we don't generate any more URLs than what's already in <code>start_urls</code></li>
  </ul>
  
  <p>How long would it take the associated spider to process this queue?</p>