Design Patterns.

Software design patterns and principles. Covers 12 topics.

• Design Patterns · 1 of 3

  Event Sourcing & CQRS

  An event-sourced shopping cart handles these commands in order: add item A1, add item B2, remove item B2. How many events does the event store hold afterward?

  ATwo: the removal event deletes the earlier event that added item B2BThree: removing B2 appends one more event, and both earlier events are keptCZero: the store drops each event after the read model has applied it to the cartDOne: the store keeps only the events for items that are still in the cart

• Design Patterns · 2 of 3

  Structural Patterns (Adapter, Decorator, Proxy)

  Which structural pattern provides a simplified interface over a complex subsystem?

  AAdapterBProxyCBridgeDFacade

• Design Patterns · 3 of 3

  Event Sourcing & CQRS

  Reviewing an ES design, a senior engineer flags 'this isn't really event sourcing, it's just an audit log.' Per current framing, the missing capability that makes the difference is which of these?

  AEvents are emitted to a durable broker on every state mutationBAggregate state is rebuilt by replaying events, not stored directlyCAn async consumer mirrors mutations into a separate analytics storeDEach command writes one row to a chronologically ordered table