Layered architecture is the most common pattern for organizing software. It groups related functionality into distinct horizontal layers, with each layer having a specific responsibility.

Think of a Postal Service:

1. Presentation Layer: You write the letter and put it in the mailbox. You only care about the address.
2. Transport Layer: The truck picks up the mail and moves it between cities. It doesn't care what's inside the letter.
3. Sorting Layer: The heavy machines at the post office scan the zip codes.
4. Delivery Layer: The local mailman puts it in the final house.
5. Decoupling: If the post office buys a new sorting machine, you don't have to change how you write your letters!

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The Standard 4 Layers

1. Presentation (UI): Handles the user interface and browser requests.
2. Business (Logic): Where the "magic" happens. Calculates totals, verifies rules, and processes data.
3. Persistence (Data Access): The layer above the database (DAO/Repository) that knows how to save and load objects.
4. Database: The physical storage layer.

Rule of the Layer

A layer should only talk to the layer directly below it.

• The UI should never talk directly to the Database. It must go through the Business Logic first. This ensures you can change your Database without breaking your UI.

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Layered vs. Microservices

Layered architecture can be part of a Monolith or a Microservice. Most Microservices are internally built using a "Layered" or "Clean" architecture.

Benefits

• Testability: You can test the "Business Logic" layer without needing a real UI or a real Database (by using Mocks).
• Maintainability: Clear separation of concerns.

Top Questions

• "What happens if a layer becomes too thick (The Fat Service problem)?"
• "Compare Layered Architecture vs Circular Dependencies."
• "What is N-Tier architecture?" (Answer: N-Tier is the physical version of Layered architecture, where layers are on different servers). 🏢