Is it possible to scrape asynchronously with Colly?

Yes, it is possible to scrape asynchronously with Colly, which is a popular Go library for web scraping. Colly supports asynchronous operations out of the box, allowing concurrent requests to be made to speed up the scraping process. This is particularly useful when you need to scrape a large amount of data from websites that can handle concurrent connections.

Here's an example of how to use Colly asynchronously:

package main

import (
    "fmt"
    "log"
    "sync"

    "github.com/gocolly/colly"
)

func main() {
    // Instantiate the collector
    c := colly.NewCollector(
        colly.Async(true), // Enable asynchronous requests
    )

    // Limit the number of threads started by colly to two
    // This means that colly will use at most two go routines while scraping
    c.Limit(&colly.LimitRule{DomainGlob: "*", Parallelism: 2})

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

    // On every a element which has href attribute call callback
    c.OnHTML("a[href]", func(e *colly.HTMLElement) {
        link := e.Attr("href")
        // Visit link found on page on a new thread (asynchronously)
        wg.Add(1) // Increment the WaitGroup counter
        go func() {
            defer wg.Done() // Decrement the counter when the goroutine completes
            e.Request.Visit(link)
        }()
    })

    // Before making a request print "Visiting ..."
    c.OnRequest(func(r *colly.Request) {
        fmt.Println("Visiting", r.URL.String())
    })

    // Start scraping on https://example.com
    c.Visit("https://example.com")

    // Wait for the asynchronous tasks to complete
    wg.Wait()
}

In this example, we enable asynchronous requests by passing colly.Async(true) to the collector. We use a sync.WaitGroup to wait for all go routines started by c.Visit calls to finish. Each callback for the a[href] selector starts a new goroutine to visit links concurrently, incrementing the WaitGroup before starting the goroutine and decrementing it when the goroutine has finished its work.

Remember that when scraping asynchronously, you need to be respectful to the target web servers. Do not overload them with too many concurrent requests, and obey the robots.txt file and scraping policies of the website. It's also good practice to set rate limits and respect Retry-After headers to avoid being blocked and to maintain a good relationship with the web servers you're scraping from.

Always review a website's terms of service before scraping to ensure that you're in compliance with their policies, and consider the ethical implications of your scraping activities.