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Downloader

Downloader is an integral part of Scrapy's architecture. It's responsible for fetching web pages and delivering them to the Scrapy engine, which then sends them to spiders for parsing and extracting data. It's also where middleware like robots.txt and HTTP caching come into play.

What is a Downloader in Scrapy?​

A Downloader in Scrapy is a component that retrieves web pages from the internet and passes them onto the Scrapy engine. It takes requests generated from a Spider, fetches the response from the web, and sends it back to the Scrapy Engine.

Downloader Middleware​

Downloader Middleware is a system of hooks into Scrapy's request/response processing. It's a framework of hooks into Scrapy's request/response processing, which allows you to plug in custom functionality and extensions. This middleware is specific to the Downloader component and is used to modify, add or drop requests/responses as needed.

How does the Downloader work?​

The Downloader receives requests from the Scrapy Engine, which were generated by a Spider. It then makes a network call to the server hosting the web page and fetches the page content. Once the page has been retrieved, the Downloader sends the data back to the Scrapy Engine in the form of a Response.

If the Downloader encounters any issues while fetching the page (like a network error or an invalid URL), it will report the error back to the Scrapy Engine.

The Downloader also respects the DOWNLOAD_DELAY and CONCURRENT_REQUESTS_PER_DOMAIN settings to control the rate of requests.

Understanding Downloader Settings​

There are several settings that control the behavior of the Downloader:

  1. DOWNLOAD_DELAY: This setting defines the delay between consecutive requests. It's used to limit the rate at which Scrapy fetches pages, so as not to overburden the server.

  2. CONCURRENT_REQUESTS_PER_DOMAIN: This setting limits the number of concurrent requests that will be performed to any single domain. It's used to avoid overwhelming a server with too many requests at once.

  3. USER_AGENT: The value of this setting is used as the user-agent string when Scrapy makes requests.

Downloader Middleware Examples​

There are several built-in middlewares in Scrapy that you can use or take inspiration from. Here are a few examples:

  1. RobotsTxtMiddleware: This middleware allows Scrapy to respect the robots.txt policies on websites.

  2. HttpCacheMiddleware: This middleware provides a caching layer that can save time and bandwidth.

  3. RetryMiddleware: This middleware is used to retry requests that failed due to temporary issues like a connection timeout.

  4. RedirectMiddleware: This middleware handles redirection of requests.

  5. CookiesMiddleware: This middleware provides support for receiving and sending cookies.

In conclusion, the Downloader in Scrapy is a powerful and flexible component. It fetches pages from the web, handles errors, and respects rate limits. With the help of Downloader Middleware, you can customize its behavior to suit your needs.