Delivery System OOD - Design a Package Delivery Tracker

Question

## Problem

Design a delivery tracking system. A delivery moves through states: `CREATED -> PICKED_UP -> IN_TRANSIT -> OUT_FOR_DELIVERY -> DELIVERED` (or `FAILED`). Implement:

- `create_delivery(package_id, origin, destination) -> Delivery`
- `update_status(delivery_id, new_status, location, timestamp)`
- `get_status(delivery_id) -> DeliveryStatus`
- `get_history(delivery_id) -> List[StatusEvent]`
- `deliveries_by_driver(driver_id) -> List[Delivery]`

```python
class Delivery:
    id: str
    package_id: str
    origin: str
    destination: str
    driver_id: Optional[str]
    current_status: str

class StatusEvent:
    status: str
    location: str
    timestamp: datetime

class DeliverySystem:
    def create_delivery(self, package_id: str, origin: str, destination: str) -> Delivery: ...
    def update_status(self, delivery_id: str, status: str, location: str, ts: datetime): ...
    def get_status(self, delivery_id: str) -> str: ...
    def get_history(self, delivery_id: str) -> List[StatusEvent]: ...
```

## Follow-ups

1. How do you enforce state machine transitions (reject invalid state changes)?
2. How would you notify customers in real time when status changes?
3. If a driver handles 200 deliveries/day, how do you query their active deliveries efficiently?
4. How would you handle clock skew if status updates arrive out of order?

Delivery System OOD - Design a Package Delivery Tracker

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