Text-based stimulus: A simple network diagram with three labeled boxes: Client App on the left, Local Cache below the client, and Server on the right. Solid arrow from Client App to Server is labeled 'request'. Dashed arrow from Client App to Local Cache is labeled 'fallback'. A red X appears on the Server box labeled 'temporarily unavailable'. The text-based stimulus describes a client app, a local cache, and a server. The server is temporarily unavailable. Which structure prevents a crash while keeping the user informed?