Select

Go’s select lets you wait on multiple channel operations. Combining goroutines and channels with select is a powerful feature of Go.

package main

import (
    "fmt"
    "time"
)

func main() {

    // For our example we’ll select across two channels.
    c1 := make(chan string)
    c2 := make(chan string)

    // Each channel will receive a value after some amount of time
    // to simulate e.g. blocking RPC operations executing in concurrent goroutines.
    go func() {
        time.Sleep(1 * time.Second)
        c1 <- "one"
    }()
    go func() {
        time.Sleep(2 * time.Second)
        c2 <- "two"
    }()

    // We’ll use select to await both of these values simultaneously
    // printing each one as it arrives.
    for i := 0; i < 2; i++ {
        select {
        case msg1 := <-c1:
            fmt.Println("received", msg1)
        case msg2 := <-c2:
            fmt.Println("received", msg2)
        }
    }
}

We receive the values "one" and then "two" as expected.

$ time go run select.go 
received one
received two

real    0m2.245s

Note that the total execution time is only ~2 seconds since both the 1 and 2 second Sleeps execute concurrently.