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Go Concurrency

Go select and Timeouts

Wait on several channels at once, give up after a timeout, or check without waiting.

  • Advanced
  • 4 min read
  • 1 runnable example
What you'll learn
  • select with several cases
  • time.After
  • default
Run code →
select
case <-fastready after 10 ms
case <-slowready after 200 ms
case <-time.After(50ms)timeout

The first ready channel wins

select waits for the first of several channel operations that is ready. It is how a goroutine listens to more than one channel, adds a timeout, or checks a channel without waiting.

The first select receives from fast because it is ready first. In the second, time.After delivers a value after 50 milliseconds, before slow is ready, so the timeout case runs. A default case runs at once when no channel is ready.

Key points

  1. select runs the first case that is ready.
  2. time.After adds a timeout.
  3. default runs when nothing is ready.

Runnable Go example

Go
Run code →
main.go
package main

import (
	"fmt"
	"time"
)

func main() {
	fast := make(chan string)
	slow := make(chan string)

	go func() {
		time.Sleep(10 * time.Millisecond)
		fast <- "fast result"
	}()
	go func() {
		time.Sleep(200 * time.Millisecond)
		slow <- "slow result"
	}()

	select {
	case message := <-fast:
		fmt.Println(message)
	case message := <-slow:
		fmt.Println(message)
	}

	select {
	case message := <-slow:
		fmt.Println(message)
	case <-time.After(50 * time.Millisecond):
		fmt.Println("timed out waiting for slow")
	}

	ready := make(chan int)
	select {
	case value := <-ready:
		fmt.Println(value)
	default:
		fmt.Println("nothing ready, moving on")
	}
}
Get a Run button for this example Result of running the code
fast result
timed out waiting for slow
nothing ready, moving on

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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