Design real-time delivery tracking with async updates

## Scenario Design a **scalable, fault-tolerant** backend system to track deliveries in real time. ### Context - Your company provides a tracking platform. - **Third-party delivery providers** (3P) perform the actual delivery and emit events (e.g., accepted, picked up, in transit, delivered, failed). - Users/clients need to: 1. **Query** the latest delivery status (low latency). 2. Optionally **subscribe** to real-time updates (e.g., WebSocket/SSE/push). ### Core requirement (most important) Handle **asynchronous task execution** and ensure correct **state updates + queries** even with: - Out-of-order events - Duplicate events (at-least-once delivery) - Provider downtime and retries - Partial failures and regional outages ### Deliverables Propose an architecture and explain: - APIs (ingest events, query status, subscriptions) - Data model (deliveries, events, status timeline) - How you process events asynchronously and update the “current status” - Ordering/idempotency strategy - Fault tolerance, scaling, and observability - Tradeoffs (consistency vs availability, storage choices)