Simple example of goroutines and channels

concurrency.go

package main

import (
	"fmt"
	"net/http"
)

const maxConcurrentRequests = 100

type Response struct {
	url        string
	httpStatus int
}

func asyncGET(urls []string) []*Response {
	var responses []*Response
	semaphore := make(chan int, maxConcurrentRequests)
	results := make(chan *Response)
	client := http.Client{}

	for _, url := range urls {
		go func(url string) {
			fmt.Printf("QUEUE %s\n", url)

			// Try to add a value to the semaphore channel
			// If channel is full, then this code will block
			semaphore <- 0

			// Run HTTP GET request
			resp, _ := client.Get(url)
			defer resp.Body.Close()

			// Return response to channel
			results <- &Response{url, resp.StatusCode}

			// Pull one item from semaphore channel, to open up space for
			// next request
			<-semaphore
		}(url)
	}

	for {
		select {
		case r := <-results:
			// Read a Response from the results channel
			fmt.Printf("HTTP %d %s\n", r.httpStatus, r.url)
			responses = append(responses, r)
			if len(responses) == len(urls) {
				return responses
			}
		}
	}
}

func generateURLs(count int) []string {
	urls := make([]string, count)
	base := "https://duckduckgo.com/"
	for idx := range urls {
		urls[idx] = fmt.Sprintf("%s?q=%d", base, idx)
	}
	return urls
}

func main() {
	urls := generateURLs(100)
	results := asyncGET(urls)
	fmt.Printf("DONE. Got %d responses!", len(results))
}