Callback Hell (2016)(callbackhell.com)

  0.0s - [1] Client 1: Apply coupon
  0.0s - [2] Client 2: Apply coupon
  0.1s - [1] Server to database: Is coupon marked as used?
  0.1s - [2] Server to database: Is coupon marked as used?
  0.2s - [1] Database to server: Nope! All good.
  0.2s - [2] Database to server: Nope! All good.*

  0.3s - [1] Server to database: Update coupon as used.
  0.3s - [2] Server to database: Update coupon as used.*

         * Coupon has been used twice :(

    func main() {
        documentURLs := []string{
            "http://example.org/foo.txt",
            "http://example.net/bar.txt",
            "http://example.com/quux.txt",
        }
        for _, url := range documentURLs {
            response, err := http.Get(url)
            body, err := ioutil.ReadAll(response.Body)
            fmt.Print(string(body))
        }
    }

    func main() {
        documentURLs := []string{
            "http://example.org/foo.txt",
            "http://example.net/bar.txt",
            "http://example.com/quux.txt",
        }

        // `documents` is a channel of values of type `string`.
        // A channel is a safe FIFO queue.
        documents := make(chan string)

        for _, url := range documentURLs {
            go func() {
                response, err := http.Get(url)
                body, err := ioutil.ReadAll(response.Body)
                documents <- string(body)
            }()
        }

        for doc := range documents {
            fmt.Print(doc)
        }
    }

    ln, err := net.Listen("tcp", ":8080")
    for {
        conn, err := ln.Accept()
        go handleConnection(conn)
    }