Order Processing - Design a Multi-Step Order Fulfillment Pipeline

Question

## Round 1 - Coding

## Problem

Design an order processing pipeline for a food delivery platform. Orders go through: payment validation, restaurant notification, dasher assignment, and status tracking. Implement the pipeline with error handling at each stage.

```python
class OrderProcessor:
    def process(
        self,
        order: dict
    ) -> dict:
        # order = {"id": str, "customer_id": str, "items": list, "total": float, "payment_token": str}
        # Returns {"status": str, "order_id": str, "error": str | None}
        ...

def validate_payment(self, order: dict) -> bool: ...
    def notify_restaurant(self, order: dict) -> bool: ...
    def assign_dasher(self, order: dict) -> str | None:  # returns dasher_id or None
        ...
    def update_status(self, order_id: str, status: str) -> None: ...
```

```
Example:
processor = OrderProcessor()
result = processor.process({
    "id": "ORD-001",
    "customer_id": "USR-42",
    "items": [{"name": "Burger", "qty": 1}],
    "total": 15.99,
    "payment_token": "tok_valid"
})
# result -> {"status": "ACCEPTED", "order_id": "ORD-001", "error": None}
```

## Follow-ups

1. If restaurant notification fails, should you refund immediately or retry? Design the retry policy.
2. How do you make this pipeline idempotent (safe to reprocess the same order)?
3. How would you implement a dead-letter queue for permanently failed orders?
4. What observability would you add (metrics, traces, alerts) to monitor this pipeline?

Order Processing - Design a Multi-Step Order Fulfillment Pipeline

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