Business Logic Layer (BLL) (extra)

What is Business Logic Layer?

Business Logic Layer (BLL) is the part of the application that contains:

• Core rules of the system
• Validation logic
• Decision making
• Calculations
• Workflows

It is the brain of the application.

---

Where It Sits in Architecture

Typical layered architecture:

• Controller Layer (API Layer)
• Business Logic Layer
• Data Access Layer (Database Layer)

Flow:

Request → Controller → Business Logic → Database → Response

---

What Should Be Inside BLL?

1. Business rules
2. Data validation (domain level)
3. Calculations
4. Orchestration between multiple services
5. Transaction handling

Example:

Creating an order:

• Check if user exists
• Check if product is in stock
• Calculate total price
• Apply discount
• Save order
• Reduce inventory

All this belongs in Business Logic Layer.

---

What Should NOT Be Inside BLL?

• HTTP request parsing
• Response formatting
• Database queries (raw SQL)
• Framework-specific code

BLL should not depend on web framework.

---

Why Separate Business Logic?

1. Reusability
2. Testability
3. Clean architecture
4. Easy maintenance
5. Separation of concerns

---

Example (Simple Structure)

Controller:

• Accept request
• Call service method
• Return response

Service (Business Logic):

• Validate rules
• Perform operations
• Call repository

Repository:

• Handle database queries

---

Key Principle

Controllers should be thin.

Business logic should be inside services.

If logic grows inside controller, move it to BLL.