Order Management: Design an Order State Machine with Status Transitions and Validation

Question

## Problem

Design an `OrderManager` system that manages the lifecycle of orders. An order moves through states: `PENDING -> CONFIRMED -> SHIPPED -> DELIVERED`. It can also be `CANCELLED` from `PENDING` or `CONFIRMED` only. Invalid transitions should raise an error.

```python
from enum import Enum

class OrderStatus(Enum):
    PENDING = "PENDING"
    CONFIRMED = "CONFIRMED"
    SHIPPED = "SHIPPED"
    DELIVERED = "DELIVERED"
    CANCELLED = "CANCELLED"

class OrderManager:
    def __init__(self): ...
    def create_order(self, order_id: str, items: list[str]) -> None: ...
    def transition(self, order_id: str, new_status: OrderStatus) -> None: ...
    def get_status(self, order_id: str) -> OrderStatus: ...
```

**Example:**
```
om = OrderManager()
om.create_order("ORD-1", ["shirt", "jeans"])
om.transition("ORD-1", OrderStatus.CONFIRMED)  # OK
om.transition("ORD-1", OrderStatus.DELIVERED)   # raises InvalidTransitionError
om.transition("ORD-1", OrderStatus.SHIPPED)     # OK
```

## Follow-ups

1. How would you store transition history (audit log) with timestamps for each state change?
2. What design pattern formalizes this kind of state machine, and how does it differ from a simple `if/elif` chain?
3. How would you add hooks (callbacks) that fire when an order enters a specific state (e.g., send email on `SHIPPED`)?
4. If this system must handle thousands of concurrent orders, what persistence layer and locking strategy would you choose?

Order Management: Design an Order State Machine with Status Transitions and Validation

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