# N-Tier Architecture Design for Enterprise Systems

For internal enterprise systems such as ERP, CRM, or HRM platforms, adopting an **N-Tier architecture** is a common and effective practice. It enables clear separation of concerns, modular design, and strong scalability. This article outlines a five-layer N-Tier architecture tailored for enterprise systems, explaining the responsibilities and characteristics of each tier.

## Overview of the Five-Tier Architecture

| Layer Name | Description |
|--------------------------|-------------|
| Presentation Layer | Provides the user interface (UI), such as WinForm, Web UI, or mobile app UI. |
| API Connector Layer | Encapsulates API calling logic on the client side, supporting serialization, encryption, and compression. Acts as the external interface of the client. |
| API Service Layer | Provides a unified API endpoint for external systems, handling authentication and parameter parsing. |
| Business Logic Layer (BLL)| Processes internal business logic and flow control; core components include `BusinessObjects`. |
| Data Access Layer (DAL) | Abstracts database access; data operations are managed via `DbAccess` without exposing connection strings. |

### 📌 Architecture Flow Diagram

flowchart TD
    A[Presentation Layer]
    B[API Connector Layer]
    C[API Service Layer]
    D[Business Logic Layer]
    E[Data Access Layer]
    F1[Local Call : In-Process]
    F2[Remote Call : JSON-RPC]

    A --> B
    B --> F1
    B --> F2
    F1 --> C
    F2 --> C
    C --> D
    D --> E

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## Layer-by-Layer Explanation

### 1. Presentation Layer

- Designed for **internal users** who are familiar with business workflows.
- UI is **standardized and unified**, ensuring consistent user experience and reduced training costs.
- Uses `FormLayout.xml` to dynamically define UI fields and behaviors, which can auto-generate interfaces for WinForm, Web, and mobile apps.
- Customizations can be made by modifying the layout configuration without altering source code.

### 2. API Connector Layer

- Abstracts client-to-server communication details, allowing developers to focus on business logic.
- Supports switching between **local and remote API calls**:
- **Local**: used in development for easier debugging.
- **Remote**: used in production deployment.
- Handles **serialization, encryption, and compression** for secure and efficient data transmission.
- Centralizes API invocation logic on the client, minimizing redundancy.

### 3. API Service Layer

- Provides a **unified API endpoint**—the primary external access point for the backend system.
- Built-in support for **authorization, parameter parsing, exception handling**, and more.
- Supports two types of payload formats:
- **JSON**: for integration with third-party systems.
- **Encoded** (serialized/encrypted/compressed): optimized for internal client usage.
- When implementing new features, developers only need to write the business logic method. API-level handling is automatically supported.

### 4. Business Logic Layer (BLL)

- The **core of the enterprise system**, responsible for processing all business operations such as:
- Inventory management
- Purchase planning
- Cost calculations
- Each business function is represented by a `BusinessObject` that unifies behavior and data handling:
- Multiple frontends (Web, App, WinForm) all interact with the same business logic component.
- Third-party API calls also route through the same object to **ensure consistency and maintainability**.
- Supports **customizable logic via inheritance**:
- Developers can inherit from base `BusinessObject` and override specific methods as needed.
- Example: override `DoBeforeSave` for pre-save validation or `DoAfterSave` for post-save posting logic.
- This enables flexibility without full rewrites, enhancing maintainability and extensibility.

### 5. Data Access Layer (DAL)

- Abstracts all database interaction details.
- Developers access databases via `DatabaseID` without managing connection strings.
- Utilizes a custom ORM framework, **XORM**:
- Manages schema definitions and generates CRUD logic automatically.
- Table structure is defined in `DbTable.xml`, supporting multiple DBMSs (e.g., SQL Server, MySQL).
- UI field behavior and layout are defined in `FormDefine.xml` for each form (e.g., employee, department).
- Relationships are modeled at the form level, **not directly tied to physical database schema**—promoting decoupling.

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## Conclusion

Adopting a five-tier N-Tier architecture allows enterprise systems to achieve:

- **Modular maintenance**: responsibilities are clearly separated.
- **Scalable extensibility**: new features can be added by expanding the appropriate layer.
- **Unified development pattern**: reduces training cost and development errors.
- **High maintainability and reusability**: especially within the business logic and API layers.

This architecture has been successfully applied to real-world ERP and HRM systems, proving its scalability and long-term maintainability.

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