Goroutines and Channels

Concurrency is Go's headline feature. A goroutine is a lightweight, independently-running function, and a channel is a typed pipe for goroutines to safely communicate through, instead of sharing memory directly.

Goroutines

func sayHi(name string) {
    fmt.Println("Hi,", name)
}

func main() {
    go sayHi("Ada")    // runs concurrently, doesn't block main
    go sayHi("Grace")
    time.Sleep(100 * time.Millisecond) // crude wait, channels below do this properly
}

The go keyword launches a function as a goroutine, thousands of them can run concurrently with very little overhead, far lighter-weight than an OS thread.

Channels

Without coordination, main might exit before the goroutines above even run. Channels solve this properly:

func fetchStatus(url string, results chan<- string) {
    // pretend this makes a real HTTP request
    results <- url + ": OK"   // send a value into the channel
}

func main() {
    urls := []string{"a.com", "b.com", "c.com"}
    results := make(chan string)

    for _, url := range urls {
        go fetchStatus(url, results)
    }

    for range urls {
        fmt.Println(<-results)   // receive a value, blocks until one arrives
    }
}

results <- value sends into the channel, <-results receives from it, and receiving blocks until a value is available, this is exactly what replaces time.Sleep guesswork with real synchronization: main waits precisely as long as it needs to, no more, no less.