Application architecture models - Learn Tech From Zero

3.4 Explain methods of application architecture and delivery models.

📘CompTIA ITF+ (FC0-U61)

Application architecture describes how software applications are structured and how different parts of the application communicate with each other. Choosing the right architecture affects performance, scalability, maintenance, and how users access the application. There are four main models: one-tier, two-tier, three-tier, and n-tier.

1. One-Tier Architecture (Single-Tier)

Definition:
All parts of the application run on a single system or device. This includes the user interface, business logic, and data storage.
Key Features:
- Everything happens on one computer.
- No network is needed.
- Simple to install and use.
IT Example:
- A desktop word processor like Microsoft Word where the document is saved locally.
- A local database application installed on a single machine where users enter and retrieve data.
Advantages:
- Easy to set up.
- No network dependency.
Disadvantages:
- Difficult to scale for multiple users.
- Data is not shared easily across multiple devices.

2. Two-Tier Architecture (Client-Server)

Definition:
Application is split into two layers:
1. Client layer (front-end): The interface users interact with.
2. Server layer (back-end): Handles data storage and sometimes business logic.
Key Features:
- Client sends requests to the server.
- Server processes requests and sends back results.
- Usually requires a network connection.
IT Example:
- A desktop accounting program where the client software runs on employee computers and connects to a central database server to store financial data.
- Email clients like Outlook connecting to an Exchange server to send and receive emails.
Advantages:
- Centralized data storage improves consistency.
- Easier maintenance compared to one-tier.
Disadvantages:
- Server becomes a bottleneck if too many clients connect.
- Limited scalability for large systems.

3. Three-Tier Architecture

Definition:
Application is split into three layers:
1. Presentation layer: User interface (UI).
2. Logic layer (business layer): Handles the rules, calculations, and processes.
3. Data layer: Stores and retrieves data from a database.
Key Features:
- Each layer can run on a different server.
- Layers communicate over a network.
- Common for web applications and enterprise software.
IT Example:
- A web-based e-commerce site:
  - Presentation layer = Web browser interface for customers.
  - Logic layer = Web server running shopping cart calculations and order processing.
  - Data layer = Database server storing products, orders, and customer information.
Advantages:
- Better scalability and performance than two-tier.
- Easier to update one layer without affecting others.
- More secure: sensitive data can be isolated on the data layer.
Disadvantages:
- More complex to design and maintain.
- Network dependency for all three layers.

4. N-Tier Architecture (Multi-Tier)

Definition:
Similar to three-tier, but with more than three layers, depending on the application’s complexity.
- Each layer is specialized for a specific purpose (e.g., caching, authentication, reporting).
Key Features:
- Flexible and highly scalable.
- Layers can be distributed across multiple servers or locations.
- Ideal for large enterprise applications and cloud-based services.
IT Example:
- Large online banking systems:
  - Presentation layer = Web or mobile app.
  - Application layer = Business logic servers for transactions and validations.
  - Data layer = Database servers for account info.
  - Additional layers = Security/authentication servers, logging servers, and reporting servers.
Advantages:
- Very scalable and maintainable.
- Each layer can be upgraded or replaced independently.
- Supports many users simultaneously.
Disadvantages:
- More expensive to build and maintain.
- Complex to troubleshoot due to multiple layers.

Quick Comparison Table

Architecture	Layers	Example in IT	Pros	Cons
One-tier	1	Desktop app with local database	Simple, no network	Hard to share, limited scalability
Two-tier	2	Client software + database server	Centralized data, easier maintenance	Server bottleneck, limited scalability
Three-tier	3	Web app (browser + web server + database)	Scalable, secure, modular	More complex, network required
N-tier	4+	Online banking system	Highly scalable, maintainable	Expensive, complex

💡 Exam Tip:

Remember that “tiers” = layers. More tiers generally mean better scalability, security, and maintainability, but more complexity.
Focus on identifying which part handles UI, logic, and data, and be ready to recognize examples for each type.