The delivery tier combines derived views, durable work and user-facing freshness. These fresh scenarios ask you to choose a boundary rather than simply add more servers. A CDN hit may bypass origin authorization. A live socket may skip an offline message. A search hit may describe inventory that is already gone.
Reuse the contracts you have practiced: immutable versions for public objects, an authoritative check before a scarce resource is claimed, and a stable identity across retries. Keep arithmetic explicit when converting a minute rate to a backlog or calculating origin traffic. The board tasks focus on the property that prevents a stated failure. You don't need to build an unrelated part of the system to demonstrate that decision.