Domain-Driven Design

By the end, you can

• Explain what ubiquitous language is and why it matters in DDD.
• Distinguish an entity from a value object and classify real-world examples of each.
• Describe the role of an aggregate root as a consistency boundary.
• State the three aggregate rules of thumb (small, reference by identity, one per transaction).
• Explain why mutable overwrite loses history and what problem event sourcing solves.
• Trace how current state is derived by replaying an append-only event log.
• Perform the replay arithmetic: fold a sequence of events to compute derived state.
• List the three benefits and three honest costs of event sourcing.
• Describe the CQRS write-side flow (command → aggregate → event append) and the read-side role of projections.
• Explain eventual consistency and why it arises when projections run asynchronously.
• Identify which domains are good and bad candidates for these patterns.
• Confirm that event sourcing and CQRS are independent patterns that can be used separately.