Go: What is context?

1// Create contexts
2ctx := context.Background()                    // root context
3ctx, cancel := context.WithCancel(parent)       // manual cancel
4ctx, cancel := context.WithTimeout(parent, 5s)  // timeout
5ctx, cancel := context.WithDeadline(parent, t)  // deadline
6ctx = context.WithValue(parent, "key", "val")  // add value
7
8// Always call cancel!
9defer cancel()

1select {
2case <-ctx.Done():
3    // Context cancelled or deadline exceeded
4    err := ctx.Err()
5    if errors.Is(err, context.Canceled) {
6        // Manual cancellation
7    } else if errors.Is(err, context.DeadlineExceeded) {
8        // Timeout/deadline
9    }
10case result := <-ch:
11    // Normal operation
12}

1// Use custom key types to avoid collisions
2type contextKey struct{}
3
4func WithRequestID(ctx context.Context, id string) context.Context {
5    return context.WithValue(ctx, contextKey{}, id)
6}
7
8func GetRequestID(ctx context.Context) string {
9    if id, ok := ctx.Value(contextKey{}).(string); ok {
10        return id
11    }
12    return ""
13}