Microservices fail when their boundaries are wrong. Domain Driven Design (DDD) provides the blueprint for drawing those boundaries based on business logic, not technical convenience.
A 'Product' in a Warehouse context means dimensions and weight. A 'Product' in a Sales context means price and marketing description. Instead of one giant `Product` class, DDD creates two separate models in two separate **Bounded Contexts**. This prevents 'Big Ball of Mud' architectures.
An Aggregate is a cluster of objects treated as a single unit for data changes. The **Aggregate Root** (e.g., an `Order`) is the only gatekeeper. You never modify an `OrderItem` directly; you ask the `Order` to add it. This ensures business rules (invariants) are always enforced.
Q: "How do you handle relationships BETWEEN microservices without 'Join' queries?"
Architect Answer: "We use **Data Duplication** and **Eventual Consistency**. If the 'Shipping' service needs the user's name, it shouldn't call the 'Identity' service every time. Instead, it listens for a `UserCreated` event and stores a local copy of the ID and Name. We trade storage for performance and availability. The source of truth remains with Identity, but Shipping has what it needs to work independently."