GeezerVison

Timeouts are important for programs that connect to external resources or that otherwise need to bound execution time. Implementing timeouts in Go is easy and elegant thanks to channels and select.

package main

import (
    "fmt"
    "time"
)

func main() {

For our example, suppose we’re executing an external call that returns its result on a channel c1 after 2s. Note that the channel is buffered, so the send in the goroutine is nonblocking. This is a common pattern to prevent goroutine leaks in case the channel is never read.

    c1 := make(chan string, 1)
    go func() {
        time.Sleep(2 * time.Second)
        c1 <- "result 1"
    }()

Here’s the select implementing a timeout. res := <-c1 awaits the result and <-time.After awaits a value to be sent after the timeout of 1s. Since select proceeds with the first receive that’s ready, we’ll take the timeout case if the operation takes more than the allowed 1s.

    select {
    case res := <-c1:
        fmt.Println(res)
    case <-time.After(1 * time.Second):
        fmt.Println("timeout 1")
    }

If we allow a longer timeout of 3s, then the receive from c2 will succeed and we’ll print the result.

    c2 := make(chan string, 1)
    go func() {
        time.Sleep(2 * time.Second)
        c2 <- "result 2"
    }()
    select {
    case res := <-c2:
        fmt.Println(res)
    case <-time.After(3 * time.Second):
        fmt.Println("timeout 2")
    }
}

Running this program shows the first operation timing out and the second succeeding.

$ go run timeouts.go 
timeout 1
result 2