Misunderstanding Go contexts

A Context carries a deadline, a cancellation signal, and other values across API boundaries.

Deadline

A deadline is a specific point in time:

If a deadline is met then any ongoing activities using that context should be stopped.

func (h publishHandler) publishPosition(position flight.Position) error {
  // context with a deadline of 4 seconds
  ctx, cancel := context.WithTimeout(context.Background(), 4*time.Second)
  defer cancel() // safeguard to cancel the context goroutine

  return h.pub.Publish(ctx, position)
}

Cancellation signals

func main() {
  ctx, cancel := context.WithCancel(context.Background())
  defer cancel() // this cancel signal gets called when main exits

  go func() {
    CreateFileWatcher(ctx, "foo.txt")
  }()
  // ...
}

Context values

Key value pairs can also be passed with a context

// best practice for context keys is to use a custom, un-exported type
// this avoids any possible collisions
type key string
const myCustomKey key = "key"

ctx := context.WithValue(parentCtx, myCustomKey, "value")
fmt.Println(ctx.Value("key"))

Use cases

Context values have wide use cases such as tracing IDs or middle-ware

type key string

const isValidHostKey key = "isValidHost"

func checkValid(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    validHost := r.Host == "acme"
    ctx := context.WithValue(r.Context(), isValidHostKey, validHost)

    next.ServeHTTP(w, r.WithContext(ctx))
  })
}

Catching a context cancellation

The context.Context type exports a Done method that returns a receive only notification channel <-chan struct{}. This closes when:

context.Context also exports an Err method that returns nil if the Done channel is not closed. When it is not nil, it is either context.Canceled or context.DeadlineExceeded.

func handler(ctx context.Context, ch chan Message) error {
  for {
    select {
      // keep reading messages
      case msg := <-ch:
        // ...
      // context is done
      case <-ctx.Done():
        return ctx.Err()
    }
  }
}

References