CodeUtility
Making your developer's life easier!
Go Concurrency

Go context: Cancellation and Timeouts

Stop work that takes too long or is no longer needed with context.WithTimeout and WithCancel.

  • Advanced
  • 3 min read
  • 1 runnable example
What you'll learn
  • context.WithTimeout
  • ctx.Done()
  • context.WithCancel
Run code →
Timeout of 50 ms
5 ms of worknil, finished in time
500 ms of workcontext deadline exceeded
after stop()context canceled

A cancellation signal passed down

A context carries a cancellation signal through your program. Functions that may take a long time accept a context and stop when it is cancelled or its deadline passes.

WithTimeout creates a context that is cancelled after 50 milliseconds. slowOperation waits for either its work or ctx.Done(), whichever comes first, and returns ctx.Err() when the context wins. WithCancel returns a function that cancels the context by hand.

Key points

  1. A context is cancelled by a deadline or by hand.
  2. ctx.Done() is a channel that closes on cancel.
  3. ctx.Err() says why.

Runnable Go example

Go
Run code →
main.go
package main

import (
	"context"
	"fmt"
	"time"
)

func slowOperation(ctx context.Context, duration time.Duration) error {
	select {
	case <-time.After(duration):
		return nil
	case <-ctx.Done():
		return ctx.Err()
	}
}

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
	defer cancel()

	fmt.Println("quick:", slowOperation(ctx, 5*time.Millisecond))
	fmt.Println("slow:", slowOperation(ctx, 500*time.Millisecond))

	manual, stop := context.WithCancel(context.Background())
	stop()
	fmt.Println("cancelled:", slowOperation(manual, time.Second))
}
Get a Run button for this example Result of running the code
quick: <nil>
slow: context deadline exceeded
cancelled: context canceled

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.

Finished this lesson?

Mark it as learned to track your progress.

Practice with a challenge

Use what this lesson covers to solve a real problem. Each challenge runs in your browser with instant tests and an AI review.

All challenges