select, Timeouts and context

Wait on several operations

select waits on several channel operations and proceeds with whichever is ready. Combined with context.Context, it implements timeouts and cancellation: a context created with context.WithTimeout or WithCancel closes its Done() channel when it expires or is cancelled, and ctx.Err() says why. Pass ctx as the first parameter through call chains (HTTP handlers, database calls) so a cancelled request stops all its work.

A query racing a deadline, run

I ran this with Go 1.27.1 (go run . in a module named demo, standard library only). With a 50 ms deadline, a 10 ms query returns rows, but a 200 ms query is abandoned when the context expires, returning context.DeadlineExceeded.

package main

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

func slowQuery(ctx context.Context, d time.Duration) (string, error) {
	select {
	case <-time.After(d):
		return "rows", nil
	case <-ctx.Done(): // cancelled or deadline exceeded
		return "", ctx.Err()
	}
}

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
	defer cancel()
	res, err := slowQuery(ctx, 10*time.Millisecond)
	fmt.Println("fast:", res, err)
	res, err = slowQuery(ctx, 200*time.Millisecond)
	fmt.Println("slow:", res, err, errors.Is(err, context.DeadlineExceeded))
}

Output:

fast: rows <nil>
slow:  context deadline exceeded true

Always defer cancel()

Calling the cancel function releases the context's resources even when the work finishes early.

Quick check: What does ctx.Done() return?

• A boolean
• An error value
• A channel that is closed when the context is cancelled or times out
• The remaining time