Where should business validation (e.g., date checks, booking limit of 10 days) be located? In the handler or in the service?

Business validation, such as checking dates or limiting bookings to 10 days, should be in the service layer, not in the handler. The handler is responsible for receiving and passing data, handling HTTP requests, while the service handles business logic and rules. This allows for centralizing business rules, simplifying testing, and reusing logic.

Example:

func (s *BookingService) CreateBooking(req BookingRequest) error {
    if req.EndDate.Sub(req.StartDate).Hours()/24 > 10 {
        return errors.New("booking cannot exceed 10 days")
    }
    // other business logic
    return s.repo.Save(req)
}

// In the handler, simply call the service
func (h *Handler) CreateBookingHandler(w http.ResponseWriter, r *http.Request) {
    var req BookingRequest
    // parse request
    err := h.service.CreateBooking(req)
    if err != nil {
        // handle error
    }
}