How to Use Scrapy to Perform Web Crawling

Scrapy is a tool for requesting, processing, and extracting data from web pages. It is used by organizations for a wide range of purposes.

There are several ways to use Scrapy in order to perform web crawling, but one of the most useful features is the ability to export scraped data in a variety of formats. This includes XML, CSV, and JSON files.

In order to use Scrapy, you need to create a project. This encapsulates all the spiders, utilities, and deployment configurations in one place, making it easy to manage them.

The project will also have a spiders folder where you can store all the spiders you want to use. These spiders are responsible for generating requests, executing them, and yielding requests and/or data points to a callback function that processes the responses.

Requests are generated from a URL and the callback function is invoked when a response is received. As mentioned above, callback functions are a major part of how scrapy works asynchronously.

Using XPath and CSS Selectors

In addition to the standard XPath and CSS selectors, Scrapy can also be configured to use regular expressions to find specific elements on a page. This makes it easier to build reusable spiders that can crawl multiple sites without having to manually write code for each.

This feature is especially important for sites that aren’t easily accessible by humans or computers, such as Facebook and Twitter. It can also be useful for automated data mining applications that need to crawl large amounts of data.

AutoThrottle is a feature that automatically adjusts the tool to an ideal crawling speed. It is a set of settings that control how quickly the tool can reach its target number of requests, based on several factors such as latencies and load on remote servers.

For example, if you have a site with a very high page loading speed, it’s likely that your server will be overloaded and Scrapy can send more than its set limit of concurrent requests. With AutoThrottle, the crawler will try to approach the suggested value of the setting and if it gets too close, it will slow down the crawling process until it hits a safe speed.

The setting also limits the number of concurrent requests per domain and IP, which is a good idea for security reasons. However, this setting is not a hard limit and can be adjusted dynamically.

To make this more polite, AutoThrottle also sets a delay between requests and tries to reduce the total number of concurrent requests per domain/IP if it reaches the maximum set value, by adjusting the delay according to the average number of requests each website should receive at that time. This makes it very conservative and allows the tool to run at its best even in situations where latencies are high, but it isn’t a good idea for a site that is prone to overloading.