Introduction

Validation is essential in any software system — but not all validation is the same. Different types of validation belong in different layers, depending on their responsibility and characteristics.

In this document, we categorize validations and explain which layer should handle each type.

Validation Locations

For simplicity, let's consider three main types of validations:

• Input value / Required value
• Use case conditions
• Domain invariant conditions

1. Input value / Required value → Presentation Layer

• Definition: Data entered by the user, and fields that must be filled in.
• Why here: Closest to the user, so feedback can be given immediately.
• Examples: String length, regex patterns, required fields, number ranges, date formats.

Typical roles in architectures:

• Hexagonal → Adapter
• Layered → Controller
• Clean → Interface Adapter

Example:

// JavaScript client/server validation
if (!email.includes("@")) {
  alert("Invalid email address");
}

// ASP.NET model validation
[Required]
[StringLength(20)]
public string UserName { get; set; }

2. Use case conditions → Application Layer

• Definition: Checks related to system operations from the user's perspective.
• Why here: Ensures the correct sequence or eligibility before involving the domain layer.
• Examples: Login attempt limits, permission checks, precondition validation.

Typical roles in architectures:

• Hexagonal → Application Service / Port
• Layered → Service
• Clean → Use Case Layer

Example:

if (loginAttemptService.isExceeded(userId)) {
    throw new LoginLimitExceededException();
}
user.login(password);

3. Domain invariant conditions → Domain Layer

• Definition: Business rules that must always hold true in the domain.
• Why here: Protects the integrity of domain objects.
• Examples: "Orders can only be canceled before delivery", "Price must be positive."

Typical roles in architectures:

• Hexagonal → Domain Model
• Layered → Domain Layer Entity / VO
• Clean → Entity Layer

Example:

public class Product {
    private Money price;

    public Product(Money price) {
        if (price.isNegativeOrZero()) {
            throw new InvalidPriceException();
        }
        this.price = price;
    }
}