Can Colly handle scraping multiple pages in parallel?

Yes, Colly, a popular scraping framework for Go (Golang), can handle scraping multiple pages in parallel. Colly provides a simple and efficient way to do concurrent web scraping while managing request delays and limits. The framework has built-in support for asynchronous I/O and concurrent task execution, which can be utilized to scrape multiple pages at the same time.

Here's a basic example of how you can use Colly to scrape multiple pages in parallel:

package main

import (
    "fmt"
    "log"
    "sync"

    "github.com/gocolly/colly"
)

func main() {
    // Create a Collector
    c := colly.NewCollector(
        colly.Async(true), // Enable asynchronous requests
    )

    // Limit the number of concurrent requests
    c.Limit(&colly.LimitRule{
        DomainGlob:  "*",
        Parallelism: 2, // Adjust the number of concurrent requests
    })

    // A WaitGroup will help us to wait for all goroutines to finish their work
    var wg sync.WaitGroup

    // Callback for when a visited page is scrapped
    c.OnHTML("title", func(e *colly.HTMLElement) {
        fmt.Println("Page Title:", e.Text)
        wg.Done() // Indicate that a goroutine has finished
    })

    // List of URLs to scrape
    urls := []string{
        "http://example.com/page1",
        "http://example.com/page2",
        "http://example.com/page3",
        // Add more URLs as needed
    }

    for _, url := range urls {
        wg.Add(1) // Add a count to the WaitGroup for each URL we're about to visit
        c.Visit(url)
    }

    // Wait for the asynchronous visit calls to finish
    wg.Wait()

    // Start scraping in asynchronous mode
    c.Wait()
}

func main() {
    scrapeParallelPages()
}

In this code example, we:

1. Create a new Colly collector and set it to run asynchronously by passing colly.Async(true).
2. Set a limit rule to control the number of concurrent requests using Parallelism.
3. Use a sync.WaitGroup to wait for all goroutines initiated by the Visit method calls to complete.
4. Visit multiple URLs by calling c.Visit(url) inside a loop.
5. Wait for all visits to complete using wg.Wait() before exiting the program.

Keep in mind that when you are scraping websites in parallel, you should always be respectful of the website's terms and conditions, as well as its robots.txt file. Also, ensure that you set appropriate limits to avoid overwhelming the site's servers. Adjust the Parallelism value according to the website's capacity and the politeness policy you want to adhere to.