The `context` Package – Cancellation & Timeouts

Context

This lesson explains the main concept clearly with beginner-friendly examples and background so you can learn why the code works.

context.Context carries deadlines, cancellation signals, and request‑scoped values.

func fetch(ctx context.Context, url string) ([]byte, error) {
    req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
    resp, err := http.DefaultClient.Do(req)
    if err != nil { return nil, err }
    defer resp.Body.Close()
    return io.ReadAll(resp.Body)
}

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
    defer cancel()
    data, err := fetch(ctx, "https://example.com")
    if err != nil { log.Fatal(err) }
    fmt.Println(string(data))
}

Passing values via context

type key int
const requestIDKey key = 0

ctx = context.WithValue(ctx, requestIDKey, "abc-123")
id := ctx.Value(requestIDKey).(string)

Best practices

• Never store large data structures in a Context.
• Use context for cancellation and deadlines, not for passing optional parameters.