Design a Configurable Order Validator with DAG Dependencies

Problem Statement

Design an OrderValidator system that checks if an incoming order is valid. An order is a dictionary of properties (e.g., item_id, price, user_id, timestamp).

Validation logic is composed of individual rules. Some rules depend on the outcome of others. For example, the PriceValidator should only run if the ItemAvailabilityValidator has already confirmed the item exists. If any rule fails, the validation process should stop and return the failure reason.

Requirements

1. Define an interface for a ValidatorRule and a way to register dependencies between rules.
2. Implement a validate(order) method that executes the rules in a valid order (topological sort).
3. Ensure the system is extensible so new rules can be added without modifying existing code (Open/Closed Principle).

Example

Rules:

• A: Check if item exists.
• B: Check if price is positive (Depends on A).
• C: Check if order timestamp is not in the future (No dependencies).

Execution flow: Either (C -> A -> B) or (A -> C -> B) are valid topological orderings.

Constraints

• The system must detect circular dependencies during initialization.
• Each rule should return a boolean is_valid and an optional error_message.
• Your solution should handle a list of rules provided at runtime.

class OrderValidator:
    def add_rule(self, name, rule_func, dependencies=[]):
        # Logic to register rules
        pass

    def validate(self, order):
        # Logic to execute rules in order
        pass