Advanced Promotion Engine with Pre-Order Validation Logic
For a complex B2B e-commerce platform I developed a fully custom promotion system that supports multiple types of dynamic discounts and gift-based offers (e.g., “Buy N, get X free”, “Minimum cart value – receive a gift”, “Buy one, get % off second item” etc.).
A key architectural challenge was the decoupling of cart submission and order approval. In this system, users often place pre-orders that are subject to later approval (by managers, departments, or other user roles).
Promotions are not frozen at the moment of cart submission. Between the time a pre-order is saved and the moment it is approved, the commercial context may change — products can be removed, discounts updated, or new promotions may appear.
To ensure consistency and fairness:
Each pre-order saves a snapshot of applicable promotions and resulting effects (e.g., gifts received, prices after discount).
Upon approval, a validation engine checks for any discrepancies:
Price increases → require review.
Gifts or discounts no longer available → require review.
New promotions that apply but weren’t present before → ignored unless they affect user-selected items.
For cross-product promotions (e.g., “Buy A, get % off B”), the system uses a greedy pairing algorithm that dynamically distributes discounts to maximize customer value, while retaining full traceability.
The design balances flexibility for the admin (who can add/change/remove promotions at any time) with predictability for the user, who is always notified if their pre-order needs to be reviewed.
✦ The logic supports dozens of concurrent promotions, handles edge cases like overlapping rules, and ensures that no promotion ever applies silently to the user's disadvantage.
