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In a typical **web application**, the architecture is split into several layers, each playing its own role - from the user interface to the database.

Here is the structure and examples:

---

### **1. Presentation Layer**

**Purpose:** interaction with the user.
**What it does:** displays data and accepts input.
**Examples:**

- HTML, CSS, JavaScript
- Frameworks: React, Vue, Angular
- Controllers and routers (for example, in Express.js, Django Views, [ASP.NET](http://ASP.NET) Controllers)
- Forms, buttons, pages, an API interface

---

### **2. Business Logic Layer**

**Purpose:** implementing the rules and logic of the application.
**What it does:** decides *what exactly* happens when the user acts.
**Examples:**

- Service classes, managers, handlers (Service, Manager, UseCase)
- Modules that perform calculations, checks, validations
- Frameworks: Spring Services, Django Models/Services, NestJS Services
- Example functions: calculating a price, applying a discount, checking authorization

---

### **3. Data Access Layer**

**Purpose:** access to the database.
**What it does:** saves, retrieves, and updates data.
**Examples:**

- SQL queries, ORM models (Django ORM, SQLAlchemy, Hibernate)
- Repositories and DAOs (Data Access Objects)
- Connections to PostgreSQL, MySQL, MongoDB, and others

---

### **4. Infrastructure Layer**

**Purpose:** keeps supporting systems running.
**What it does:** manages external resources, logs, files, networks.
**Examples:**

- Configuration, API clients, queues (RabbitMQ, Kafka)
- Notification services, caching (Redis), logging
- Authentication and authorization (JWT, OAuth)

---

### **5. (Sometimes) API or Integrations Layer**

**Purpose:** the link between the internal logic and external systems.
**Examples:**

- REST / GraphQL / gRPC controllers
- API adapters for third-party services (payments, email, geolocation)

---

**Summary:**
The typical structure of a web application looks like this:

```javascript
[Presentation Layer] → [Business Logic] → [Data Layer]
              ↑                     ↓
       [API / Integrations]   [Infrastructure]
```

This separation makes the system understandable, modular, and easy to maintain.

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