Design an Extensible Expense Validation Engine

Question

Design an extensible expense validation engine that evaluates business trip expenses against various rules to determine compliance.

Codemia · Accepted Answer

To design the expense validation engine, we need to identify the core requirements. The system should validate expenses based on various rules, such as:
1. Total trip amount should not exceed a specified limit.
2. Individual expenses must adhere to their respective limits.
3. Certain types of expenses, like meals, have specific maximum amounts.
4. Some expense types may not be allowed to coexist in a single trip.

Understanding these requirements helps in structuring the validation logic.

The architecture of the expense validation engine can be divided into several components:
- **Expense Class**: Represents an individual expense with attributes like amount, type, and description.
- **Validator Interface**: A common interface for all validators that defines a method for validation.
- **Concrete Validators**: Implementations of the Validator interface for each specific rule (e.g., TotalAmountValidator, SingleExpenseValidator).
- **RuleEngine**: Orchestrates the validation process by applying all registered validators to a list of expenses and collecting violations.

Each validator will implement the validation logic specific to its rule. For example:
- **TotalAmountValidator** checks if the sum of all expenses exceeds the limit.
- **SingleExpenseValidator** ensures that each individual expense does not exceed its defined limit.
- **MealExpenseValidator** validates meal expenses against the maximum allowed amount.

Using a strategy pattern allows us to easily add new validators as business rules evolve.

When designing the system, we need to consider trade-offs:
- **Performance**: Running multiple validators on a large set of expenses can be computationally expensive. We can mitigate this by short-circuiting validation when a violation is found early.
- **Extensibility**: The use of an interface for validators allows for easy addition of new rules without modifying existing code, adhering to the Open/Closed Principle.

As the system scales, we may implement caching for frequently validated expenses or introduce a rules engine that can dynamically load rules from a configuration file.

Design an Extensible Expense Validation Engine

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