Goroutines

• A goroutine is a simple way to make several function calls simultaneously. The work gets split up among your CPU cores, and they all work on it at the same time.
• A Go program starts with a single goroutine, which runs the main function

package main

import (
    "fmt"
    "time"
)

func longTask() {
    fmt.Println("Starting long task")
    time.Sleep(3 * time.Second)
    fmt.Println("Long task finished")
}

func main() {
    longTask()
    longTask()
    longTask()
}

• The first longTask call runs, sleeps for 3 seconds, and then returns. Then the second longTask call runs, sleeps for 3 seconds, and so on. Because the calls to longTask run one at a time, the program takes just over 9 seconds to complete.
• Prepend the go keyword to a function call to launch another goroutine, which runs alongside the first.

    go longTask()
    go longTask()
    go longTask()

• Now calls to longTask run in separate goroutines. After each goroutine kicks off, it goes back to the main goroutine and launches the next goroutine.
• The problem is that as soon as the main goroutine finishes, the program exits. So the other goroutines don't get a chance to do anything. So just as a quick fix, we'll add a call to Sleep to the main function:

    go longTask()
    go longTask()
    go longTask()
    time.Sleep(4 * time.Second)

More info

• Rob Pike's talk: "Concurrency is not Parallelism"
• Goroutines and channels in the Go Tour