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Quiz

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Domain-Driven Design

Q1. What is the primary purpose of a Bounded Context in DDD?

  • A. It limits the number of entities a single service can manage
  • B. It defines the boundary within which a domain model is valid — the same term can mean different things in different contexts
  • C. It enforces a maximum size for aggregates within the domain
  • D. It specifies which database schema a service is allowed to access
Answer

B — Defines the boundary where a domain model is valid.

A Customer in the Sales context (credit limit, region) is a completely different model from Customer in Billing (tax ID, billing address). Bounded contexts allow each team to use the term differently without creating one bloated class that satisfies none of them well. In microservices, a bounded context typically maps to one service.

Q2. Which DDD building block is the sole entry point for all modifications to the objects within it?

  • A. Domain Service
  • B. Repository
  • C. Value Object
  • D. Aggregate Root
Answer

D — Aggregate Root.

The Aggregate Root is the only object external code holds a reference to. All state changes to child entities and value objects inside the aggregate must go through the root, which enforces invariants. For example, Order.addItem() checks the "max 50 items" rule before allowing a new OrderItem to be added.

Q3. What distinguishes an Entity from a Value Object?

  • A. Entities are immutable; Value Objects change state over time
  • B. An Entity has a persistent identity that survives attribute changes; a Value Object is defined entirely by its attributes and has no identity
  • C. Entities exist only in memory; Value Objects are persisted to the database
  • D. Entities are shared between bounded contexts; Value Objects are local to one context
Answer

B — Entity has identity; Value Object is defined by its attributes.

Two Order objects with the same ID are the same order even if their status differs (entity). Two Money(10.00, BRL) objects are identical and interchangeable because they have the same attributes (value object). Value objects should be immutable — operations return new instances.

Q4. In Event Storming, what do Domain Events (orange sticky notes) represent?

  • A. Commands that users send to the system to trigger state changes
  • B. External API calls that the system makes to third-party services
  • C. Things that happened in the domain, expressed in past tense, used to trigger reactions in other bounded contexts
  • D. Business rules that enforce invariants within an aggregate
Answer

C — Things that happened, past tense.

OrderPlaced, PaymentCharged, ItemShipped — domain events describe facts. They are immutable records that other bounded contexts (Inventory, Notifications, Analytics) subscribe to and react to independently. This decouples contexts without requiring direct calls between them.

Q5. According to the DDD-to-microservices mapping, what is the natural starting point for a microservice boundary?

  • A. A database table — each table becomes one microservice
  • B. A use case — each user story maps to one service
  • C. A team — Conway's Law dictates that one team = one service
  • D. A Bounded Context — the ubiquitous language and model are internally consistent within that boundary
Answer

D — A Bounded Context.

The bounded context defines a conceptual seam where the model is self-consistent and the team owns the ubiquitous language. Starting there (rather than by table or use case) produces services that can evolve independently. However, a bounded context is a starting point — operational or team constraints may lead to merging or splitting.

Microservice Architecture

Q6. What is the primary role of an API Gateway in a microservices architecture?

  • A. It stores shared configuration for all microservices
  • B. It acts as a message broker between services using async events
  • C. It is the single external entry point — handling routing, authentication, rate limiting, and SSL termination on behalf of all services
  • D. It manages the lifecycle of containers and restarts failed services
Answer

C — Single external entry point.

The gateway is the only service exposed to the internet. Internal services never directly face external traffic. By centralising cross-cutting concerns (auth, rate limiting, SSL, logging), the gateway lets business microservices focus entirely on their domain logic.

Q7. Why does each microservice own its own database (database-per-service pattern)?

  • A. It reduces infrastructure cost by using smaller, cheaper databases
  • B. It is required by the OpenAPI specification
  • C. It ensures services can evolve their schema independently without coupling to other services' data models
  • D. It prevents services from reading each other's data, improving security
Answer

C — Independent schema evolution.

A shared database creates a hidden coupling: changing a table that Service A owns might break Service B, which also reads it. With database-per-service, each service's schema is its private implementation detail. Services communicate through APIs, never through shared tables.

Q8. What is the key trade-off between synchronous (HTTP) and asynchronous (messaging) inter-service communication?

  • A. Sync is more secure; async is less secure
  • B. Sync gives immediate results but creates temporal coupling; async decouples services but the caller cannot wait for a result
  • C. Async is always better — sync should never be used in microservices
  • D. Sync uses more CPU; async uses more memory
Answer

B — Sync = immediate result, temporal coupling; async = decoupled, no immediate result.

If the Order service calls the Inventory service synchronously and Inventory is down, Order is also broken. With async messaging, Order publishes an OrderPlaced event and continues; Inventory processes it when it recovers. Use sync when the caller needs the result immediately (e.g., auth validation); use async when strict consistency is not required (e.g., sending confirmation email).

Q9. What problem does Service Discovery solve in a dynamic microservice environment?

  • A. It prevents two services from being deployed with the same name
  • B. It allows services to find each other's current network locations dynamically, since IP addresses change as instances scale up or down
  • C. It monitors health checks and restarts unhealthy services
  • D. It routes external traffic to the correct microservice based on the URL path
Answer

B — Finding current network locations of dynamic instances.

In a containerised environment, instances are created and destroyed constantly, and their IP addresses change on every restart. A service registry (Consul, Eureka, Kubernetes DNS) maintains the current location of all instances. Services register on startup and deregister on shutdown, and callers query the registry to find them.

Q10. In the course's reference architecture, who is responsible for validating JWT tokens on each request?

  • A. Each business microservice validates the token independently on every request
  • B. The identity provider re-validates the token on every request via a callback
  • C. The Gateway delegates validation to the Auth Service; business microservices trust the claims in the forwarded JWT without re-validating
  • D. The load balancer validates tokens before passing requests to the gateway
Answer

C — Gateway delegates to Auth Service; business services trust forwarded claims.

The gateway intercepts every request, calls the Auth Service to validate the JWT, and then forwards the resolved identity (e.g., X-Account-Id header) to the downstream service. Business microservices trust these headers without calling Auth Service themselves, keeping each service focused on its domain.