How do I set up Colly to scrape websites with different domains?

Colly is a popular scraping framework for Go developers, providing a clean and efficient way to scrape data from websites. To set up Colly to scrape websites with different domains, you need to create a Colly collector and configure it to visit URLs from the various domains you're interested in.

Here's a step-by-step guide to setting up Colly for scraping multiple domains:

1. Install Colly: First, you need to have Go installed on your machine. Then you can install Colly by running the following command:

   go get -u github.com/gocolly/colly/v2
   

2. Import Colly in Your Go Program: Start your Go program by importing the Colly package.

   package main
   
   import (
       "fmt"
       "github.com/gocolly/colly/v2"
   )
   

3. Create a New Colly Collector: Instantiate a new Colly collector. You can set various options on the collector, such as the AllowedDomains if you want to restrict the scraping to a list of domains.

   func main() {
       // Instantiate default collector
       c := colly.NewCollector(
           // Optionally, specify allowed domains
           colly.AllowedDomains("example.com", "example.org", "anotherdomain.net"),
       )
   
       // ... setup callbacks and options
   }
   

4. Set Up Callbacks: Define the callbacks for the events you are interested in, such as OnHTML for scraping HTML elements or OnResponse for handling raw responses.

   c.OnHTML("a[href]", func(e *colly.HTMLElement) {
       link := e.Attr("href")
       fmt.Printf("Found link: %s\n", link)
       // Visit link found on page
       // Only those links are visited which are in AllowedDomains
       e.Request.Visit(link)
   })
   

5. Start Scraping: Begin by visiting the URLs you are interested in. Colly will handle the crawling process according to the rules you've set.

   c.Visit("http://example.com")
   

6. Handle Cross-Domain Scraping: If you have not set AllowedDomains (or if you want to visit a domain not listed in AllowedDomains), you can still manually control the navigation using callbacks.

   c.OnHTML("a[href]", func(e *colly.HTMLElement) {
       link := e.Attr("href")
       // Implement logic to determine if the link should be visited
       // Example: Check if the link matches a certain pattern or if it contains a certain domain
   
       // Assuming `shouldVisit(link)` is a function that decides if you should visit the link
       if shouldVisit(link) {
           e.Request.Visit(link)
       }
   })
   

   Make sure to implement a custom function like shouldVisit(link) to decide whether a link should be visited based on your scraping logic.

7. Limitations and Respectfulness: Always be respectful of the websites you are scraping. Avoid hammering servers with too many requests in a short period. You can configure rate limits and implement polite scraping features using Colly's configuration options.

   c.Limit(&colly.LimitRule{
       DomainGlob:  "*.*",
       Parallelism: 2,
       Delay:       5 * time.Second,
   })
   

This setup allows Colly to scrape multiple domains effectively. Remember to respect robots.txt directives and website terms of service when scraping. It's also good practice to identify yourself by setting a custom User-Agent with c.UserAgent = "your-custom-user-agent" so that website owners can identify the source of the traffic.