Table of Contents

Overview

Timeout in select can be achieved by using After() function of time package. Below is the signature of After() function.

func After(d Duration) <-chan Time

The After function waits for d duration to finish and then it returns the current time on a channel -

https://golang.org/pkg/time/#Time.After

Let's see a program

Code

package main

import (
	"fmt"
	"time"
)

func main() {
	ch1 := make(chan string)
	go goOne(ch1)

	select {
	case msg := <-ch1:
		fmt.Println(msg)
	case <-time.After(time.Second * 1):
		fmt.Println("Timeout")
	}
}

func goOne(ch chan string) {
	time.Sleep(time.Second * 2)
	ch <- "From goOne goroutine"
}

Output

Timeout

In the above select statement we are waiting for for the receive operation to complete on ch1. In other case statement we have time.After with the duration of 1 second. So essentially this select statement will wait for at least 1 second for receive operation to complete on ch1 and after that the time.After case statement will be executed. We have put a timeout of more than 1 seconds in the goOne function and hence we see the time.After statement getting executed and

Timeout

getting printed as output.

So time.After() is a channel operation that gets unblocked after some time.

Timeout with infinite for loop outside select statement

We can have infinite for loop outside a select statement. This will cause the select statement to execute indefinite number of times.So when using for statement with infinite loop outside the select statement, we need to have a way yo break out of the for loop. One of the use case of having infinite for look outside select statement could be that you are waiting for multiple operations to receive on a particular channel for a certain time.

See below example

package main

import (
	"fmt"
	"time"
)

func main() {
	news := make(chan string)
	go newsFeed(news)

	printAllNews(news)
}

func printAllNews(news chan string) {
	for {
		select {
		case n := <-news:
			fmt.Println(n)
		case <-time.After(time.Second * 1):
			fmt.Println("Timeout: News feed finished")
			return
		}
	}
}

func newsFeed(ch chan string) {
	for i := 0; i < 2; i++ {
		time.Sleep(time.Millisecond * 400)
		ch <- fmt.Sprintf("News: %d", i+1)
	}
}

Output

News: 1
News: 2
Timeout: News feed finished

In the above program, we have created a channel named news which will hold data of string type. Then we pass this channel to the newsfeed function which is pushing the news feed to this channel . In the select statement, we are receiving the news feed from the news channel. This select statement is inside an infinite for loop so the select statement will be executed multiple times until we exit out of for loop . We also have time.After with a duration for 1 second as one of the case statements. So this set up will receive all the news from the news channel for 1 second and then exit.

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