Monday, 22 December 2025

FastAPI vs Flask vs Django: Which One Should You Choose?

In 2025, choosing the right Python web framework can make or break your backend development workflow. FastAPI, Flask, and Django are the most popular options—but which one is right for you? In this post, we’ll compare them based on performance, ease of use, scalability, and use cases.

What is FastAPI?

FastAPI is a modern Python framework designed for building high-performance APIs. It uses:

Python type hints for automatic request validation
Pydantic for data parsing and validation
ASGI for asynchronous support

Key Features:

Lightning-fast performance
Automatic Swagger and ReDoc documentation
Built-in validation and type safety
Async support for high-concurrency APIs

Best Use Cases: Microservices, API gateways, ML/AI APIs, integrations.

What is Flask?

Flask is a lightweight Python web framework that gives you full control over your application structure. It is often called a “micro-framework” because it provides the basics only, letting you add libraries as needed.

Key Features:

Minimal and flexible
Easy to learn for beginners
Large community and mature ecosystem
Supports extensions for database, authentication, and more

Best Use Cases: Small projects, prototypes, single-page applications, and microservices with simple APIs.

What is Django?

Django is a full-featured Python web framework that follows the “batteries-included” philosophy. It’s designed for building complex, large-scale web applications.

Key Features:

ORM for database management
Built-in authentication and admin interface
Strong security features
Scalable and maintainable

Best Use Cases: E-commerce, social media platforms, enterprise applications, content management systems.

FastAPI vs Flask vs Django: Detailed Comparison

Feature	FastAPI	Flask	Django
Performance	⭐⭐⭐⭐⭐	⭐⭐⭐	⭐⭐
Learning Curve	Easy	Easy	Medium-High
Async Support	✅ Native	❌	⚠️ (channels)
Automatic Docs	✅ Swagger / ReDoc	❌	❌
Validation	✅ Pydantic	❌	❌
Project Size	Small to Medium	Small	Medium to Large
Community	Growing	Large	Very Large
Ideal For	APIs, Microservices, ML	Prototypes, Microservices	Enterprise Apps, Full Web Apps

When to Choose FastAPI

✅ You want high-performance APIs
✅ You are building microservices
✅ Your project needs automatic validation and docs
✅ You are using Python async features
✅ You want fast development with minimal boilerplate

When to Choose Flask

✅ You want a flexible, lightweight framework
✅ You are building small apps or prototypes
✅ You want full control over libraries
✅ You are okay writing manual validation

When to Choose Django

✅ You need a full-stack solution
✅ You want built-in admin and auth
✅ You are building complex applications
✅ You want scalable and maintainable code

FastAPI vs Flask vs Django: Use Case Summary

FastAPI: Best for APIs, microservices, AI/ML endpoints
Flask: Best for small projects, prototypes, MVPs
Django: Best for large, full-featured web applications

Final Thoughts

Choosing the right framework depends on:

Project type: API vs full web app
Performance needs: High concurrency vs simple apps
Team expertise: Python async knowledge vs classic web development
Maintenance and scalability

✅ Pro tip: Many companies use FastAPI for APIs and Django for the main web application, combining speed and full-featured power.

FastAPI Microservices Setup

Below is a real-world FastAPI microservices setup, explained step by step, and mapped to how companies actually do it in production. I’ll also relate it to Spring Boot microservices, so it’s easy to grasp.

FastAPI Microservices Setup (Production-Ready Guide)

1️⃣ What Is a FastAPI Microservices Architecture?

Instead of one large backend, you create multiple small FastAPI services, each responsible for one business capability.

Example services:

Auth Service
User Service
Notification Service
Event / Integration Service
Reporting Service

Each service:

Has its own codebase
Has its own database (recommended)
Can be deployed and scaled independently

2️⃣ High-Level Architecture


Frontend (Angular / React / Mobile)
          ↓
      API Gateway
          ↓
 ┌────────┬─────────┬──────────┐
 | Auth   | Users   | Events   |  FastAPI services
 |Service | Service | Service  |
 └────────┴─────────┴──────────┘
          ↓
 Databases / Queues / Caches

3️⃣ Example Microservices Folder Structure


microservices/
│
├── api-gateway/
│   └── app/
│       └── main.py
│
├── auth-service/
│   └── app/
│       ├── main.py
│       └── api/
│
├── user-service/
│   └── app/
│       ├── main.py
│       └── api/
│
├── event-service/
│   └── app/
│       ├── main.py
│       └── api/
│
├── docker-compose.yml
└── README.md

👉 Each service is a separate FastAPI app.

4️⃣ Service-to-Service Communication

Option 1: REST (Most Common)

HTTP calls between services
Use async HTTP clients (httpx)


User Service → Auth Service

Option 2: Message Queue (Recommended for scale)

Kafka / RabbitMQ / AWS SQS
Event-driven communication


Event Service → Queue → Notification Service

5️⃣ API Gateway (Very Important)

Companies use:

Nginx
Kong
FastAPI Gateway

Responsibilities:

Routing
Authentication
Rate limiting
Logging

Simple FastAPI Gateway Example


@app.api_route("/{path:path}", methods=["GET", "POST"])
async def proxy(path: str, request: Request):
    async with httpx.AsyncClient() as client:
        response = await client.request(
            request.method,
            f"http://user-service/{path}",
            headers=request.headers.raw,
            content=await request.body()
        )
    return Response(response.content)

6️⃣ Authentication Strategy

Central Auth Service (Recommended)

Flow:

User logs in → Auth Service
JWT token issued
Other services validate JWT

Each service:

Validates token
Does not store auth logic

📌 Same idea as Spring Security + JWT.

7️⃣ Database Strategy

Best practice:

One database per service

Examples:

Auth Service → PostgreSQL
User Service → MongoDB
Event Service → NoSQL / Kafka

❌ Avoid shared databases between services.

8️⃣ Async & Performance Setup

Run services using:


Gunicorn + Uvicorn workers

Example:


gunicorn app.main:app -k uvicorn.workers.UvicornWorker -w 4

9️⃣ Docker Setup (Per Service)

Dockerfile (Common for all services)


FROM python:3.11-slim

WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt

COPY app app
CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]

🔟 docker-compose (Local Development)


version: "3.8"
services:
  auth-service:
    build: ./auth-service
    ports:
      - "8001:8000"

  user-service:
    build: ./user-service
    ports:
      - "8002:8000"

  event-service:
    build: ./event-service
    ports:
      - "8003:8000"

1️⃣1️⃣ Configuration Management

Use:

.env files
Environment variables
Secrets manager (in prod)

Example:


AUTH_SERVICE_URL=http://auth-service:8000

1️⃣2️⃣ Logging & Monitoring

Production services include:

Structured logging
Request tracing (request ID)
Metrics (Prometheus)
Central log aggregation

1️⃣3️⃣ Scaling Strategy

Stateless services
Horizontal scaling
Kubernetes HPA
Load balancing via ingress

1️⃣4️⃣ FastAPI vs Spring Boot Microservices

Aspect	FastAPI	Spring Boot
Startup time	Very fast	Slower
Resource usage	Low	Higher
Async	Simple	Complex
Enterprise tooling	Moderate	Excellent

Many companies use:


Spring Boot → Core services
FastAPI     → Integration & API services

1️⃣5️⃣ Common Mistakes to Avoid

❌ Sharing databases
❌ Tight coupling between services
❌ Missing API gateway
❌ Blocking calls in async routes
❌ No centralized auth

Final Thoughts

FastAPI microservices are:

Lightweight
Fast
Easy to scale
Perfect for integration-heavy systems

For someone with your background:

Spring Boot → Core business logic
FastAPI → Microservices, integrations, ML APIs

This combo is very powerful in modern architectures.

FastAPI Project Structure

Production Best Practices

1️⃣ Recommended Folder Structure


fastapi-app/
│
├── app/
│   ├── main.py              # Entry point (like SpringBootApplication)
│   │
│   ├── core/                # Core configs (security, settings)
│   │   ├── config.py        # App settings (env based)
│   │   ├── security.py      # Auth, JWT, OAuth2
│   │   └── logging.py
│   │
│   ├── api/                 # API layer (Controllers)
│   │   ├── v1/
│   │   │   ├── router.py    # API router aggregation
│   │   │   └── endpoints/
│   │   │       ├── users.py
│   │   │       ├── auth.py
│   │   │       └── health.py
│   │
│   ├── models/              # DB models (Entities)
│   │   ├── user.py
│   │   └── base.py
│   │
│   ├── schemas/             # Pydantic schemas (DTOs)
│   │   ├── user.py
│   │   └── auth.py
│   │
│   ├── services/            # Business logic (Service layer)
│   │   ├── user_service.py
│   │   └── auth_service.py
│   │
│   ├── repositories/        # DB access layer
│   │   └── user_repo.py
│   │
│   ├── db/                  # Database setup
│   │   ├── session.py
│   │   └── init_db.py
│   │
│   ├── middlewares/         # Custom middleware
│   │   └── request_log.py
│   │
│   ├── utils/               # Utilities/helpers
│   │   └── date_utils.py
│   │
│   └── tests/               # Unit & integration tests
│       └── test_users.py
│
├── requirements.txt
├── Dockerfile
├── docker-compose.yml
├── .env
└── README.md

2️⃣ Mapping to Spring Boot (So It Feels Familiar)

Spring Boot	FastAPI
@SpringBootApplication	`main.py`
Controller	`api/endpoints`
Service	`services`
Repository	`repositories`
Entity	`models`
DTO	`schemas`
application.yml	`config.py` / `.env`
Filters	`middlewares`

3️⃣ `main.py` – Application Entry Point


from fastapi import FastAPI
from app.api.v1.router import api_router

app = FastAPI(title="FastAPI App")

app.include_router(api_router, prefix="/api/v1")

➡ Similar to:


@SpringBootApplication
public class App { }

4️⃣ Router Aggregation (Like Controller Scanning)

`api/v1/router.py`


from fastapi import APIRouter
from app.api.v1.endpoints import users, auth, health

api_router = APIRouter()
api_router.include_router(users.router, prefix="/users", tags=["Users"])
api_router.include_router(auth.router, prefix="/auth", tags=["Auth"])
api_router.include_router(health.router, prefix="/health", tags=["Health"])

5️⃣ Controller Layer (Endpoints)

`api/v1/endpoints/users.py`


from fastapi import APIRouter
from app.schemas.user import UserCreate

router = APIRouter()

@router.post("/")
def create_user(user: UserCreate):
    return {"message": "User created"}

6️⃣ Schema (DTO)

`schemas/user.py`


from pydantic import BaseModel

class UserCreate(BaseModel):
    name: str
    email: str

7️⃣ Service Layer (Business Logic)

`services/user_service.py`


def create_user(user):
    # business rules here
    return user

8️⃣ Repository Layer (DB Access)

`repositories/user_repo.py`


def save(user):
    pass

9️⃣ Dependency Injection (FastAPI Style)


from fastapi import Depends

def get_user_service():
    return UserService()

@router.get("/{id}")
def get_user(id: int, service=Depends(get_user_service)):
    return service.find(id)

➡ Similar concept to @Autowired.

🔟 Environment Configuration

`.env`


DB_URL=mongodb://localhost:27017/app
JWT_SECRET=secret

`config.py`


from pydantic import BaseSettings

class Settings(BaseSettings):
    DB_URL: str
    JWT_SECRET: str

settings = Settings()

1️⃣1️⃣ Testing Structure


tests/
 ├── test_users.py
 └── conftest.py

Uses:

pytest
TestClient

1️⃣2️⃣ Production-Grade Tips

✅ Keep business logic out of controllers
✅ Use async DB drivers
✅ Version your APIs (/v1)
✅ Use dependency injection properly
✅ Add middleware for logging
✅ Use Docker for consistency

1️⃣3️⃣ Minimal Structure (For Small Services)


app/
 ├── main.py
 ├── routes.py
 ├── schemas.py
 └── services.py

Use this only for small microservices.

Final Thoughts

FastAPI doesn’t force structure like Spring Boot, but teams must enforce discipline.

If you follow the above structure:

Code stays clean
Team collaboration improves
Scaling becomes easy

FastAPI Interview Questions and Answers (Beginner to Advanced)

FastAPI has quickly become one of the most popular Python frameworks for building high-performance APIs. As a result, FastAPI interview questions are now common in backend, microservices, and AI/ML roles.

This post covers commonly asked FastAPI interview questions with simple and clear answers, from basic concepts to advanced topics.

1️⃣ What is FastAPI?

FastAPI is a modern, high-performance Python web framework used to build REST APIs quickly.
It is built on Starlette for web handling and Pydantic for data validation.

Key features:

Fast performance
Automatic API documentation
Built-in request validation
Async support

2️⃣ Why is FastAPI faster than Flask or Django?

FastAPI is faster because:

It uses ASGI instead of WSGI
Supports async/await
Uses Uvicorn (high-performance server)
Minimizes blocking IO

This makes FastAPI ideal for high-concurrency APIs.

3️⃣ What is ASGI and how is it different from WSGI?

WSGI	ASGI
Synchronous	Asynchronous
One request per thread	Event-loop based
Used by Flask, Django	Used by FastAPI

FastAPI uses ASGI, enabling better scalability.

4️⃣ What are Pydantic models?

Pydantic models are used for:

Request body validation
Response validation
Type enforcement

Example:


class User(BaseModel):
    name: str
    age: int

FastAPI automatically validates incoming JSON against this model.

5️⃣ How does FastAPI generate Swagger documentation?

FastAPI automatically generates OpenAPI (Swagger) documentation using:

Python type hints
Route definitions
Pydantic models

Available at:

/docs (Swagger UI)
/redoc (ReDoc)

6️⃣ What is dependency injection in FastAPI?

Dependency Injection (DI) allows reusable components like:

Database sessions
Authentication logic
Common validations

Example:


Depends(get_db)

It improves code reusability and testability.

7️⃣ How do you handle authentication in FastAPI?

FastAPI supports:

OAuth2
JWT
API Keys
Basic Auth

It provides built-in security utilities like:


OAuth2PasswordBearer

8️⃣ What is the difference between `def` and `async def` in FastAPI?

def	async def
Blocking	Non-blocking
CPU-bound tasks	IO-bound tasks
Simpler	High concurrency

Use async def when calling:

Databases
External APIs
Message queues

9️⃣ How does FastAPI handle request validation errors?

FastAPI:

Automatically validates requests
Returns HTTP 422 (Unprocessable Entity)
Provides clear error messages

No manual validation code is required.

🔟 How do you connect FastAPI to a database?

FastAPI supports:

SQLAlchemy (SQL)
Motor (MongoDB)
Async ORMs

Typically:

Create a DB session dependency
Inject it using Depends()

1️⃣1️⃣ How do you write unit tests in FastAPI?

FastAPI uses:

pytest
TestClient

Example:


def test_health():
    response = client.get("/health")
    assert response.status_code == 200

1️⃣2️⃣ How is FastAPI deployed in production?

Typical production setup:


Nginx
 ↓
Gunicorn + Uvicorn workers
 ↓
FastAPI app

Often containerized using Docker and orchestrated with Kubernetes.

1️⃣3️⃣ How does FastAPI compare to Spring Boot?

FastAPI	Spring Boot
Python	Java
Async-first	Thread-based
Minimal config	Heavy config
Fast development	Enterprise stability

Many companies use both together.

1️⃣4️⃣ What are background tasks in FastAPI?

Background tasks allow execution after response is sent.

Used for:

Sending emails
Logging
Notifications

Example:


BackgroundTasks

1️⃣5️⃣ What are common FastAPI interview mistakes?

❌ Using def instead of async def
❌ Blocking calls inside async routes
❌ Ignoring dependency injection
❌ Not handling exceptions properly
❌ Poor project structure

Advanced FastAPI Interview Questions (For Experienced Devs)

How does FastAPI handle concurrency internally?
How do you implement rate limiting?
How do you use middleware in FastAPI?
How do you secure APIs using OAuth2 + JWT?
How do you scale FastAPI horizontally?
How do you integrate FastAPI with Kafka / RabbitMQ?

Final Thoughts

FastAPI is not just a framework, it’s a modern API philosophy:

Clean code
High performance
Strong typing
Developer-friendly

FastAPI vs Spring Boot

FastAPI vs Spring Boot: Which One Should You Choose?

In the world of backend development, Spring Boot has been the gold standard for years—especially in enterprise Java applications.
But recently, FastAPI has gained massive popularity in the Python ecosystem for building modern APIs.

So the big question is:

👉 FastAPI vs Spring Boot — which one is better?
👉 Should Java developers care about FastAPI?

Let’s compare them from a real-world, production perspective.

What is Spring Boot?

Spring Boot is a Java-based framework that simplifies building enterprise-grade applications.

It provides:

Dependency Injection
Security
Transaction management
Database integration
Production-ready tooling

Spring Boot is widely used in:

Banking
Insurance
Government systems
Large enterprises

What is FastAPI?

FastAPI is a modern Python framework designed specifically for building high-performance APIs.

It focuses on:

Speed
Simplicity
Automatic validation
Automatic documentation

FastAPI is widely used in:

Microservices
Startups
AI / ML platforms
Integration-heavy systems

Core Comparison: FastAPI vs Spring Boot

1️⃣ Language & Ecosystem

Aspect	FastAPI	Spring Boot
Language	Python	Java
Ecosystem	Lightweight, fast	Enterprise-grade
Verbosity	Very low	High

Verdict:
Spring Boot is structured and strict. FastAPI is flexible and fast to develop.

2️⃣ Development Speed

	FastAPI	Spring Boot
Boilerplate	Minimal	Heavy
API setup time	Minutes	30–60 minutes
Configuration	Very less	Extensive

FastAPI allows teams to ship APIs much faster.

3️⃣ Performance & Concurrency

	FastAPI	Spring Boot
Concurrency model	Async (event loop)	Thread-based
Best for	IO-heavy APIs	CPU-heavy logic
Latency	Lower	Slightly higher

FastAPI shines in:

Webhooks
Event ingestion
High-concurrency APIs

4️⃣ Validation & API Contracts

	FastAPI	Spring Boot
Request validation	Automatic	Annotations
OpenAPI spec	Auto-generated	Manual setup
Error messages	Clear	Verbose

FastAPI’s validation feels like:

DTO + Validation + Swagger — without extra configuration

5️⃣ API Documentation (Major Difference)

FastAPI provides:

Swagger UI at /docs
ReDoc at /redoc

No extra setup needed.

In Spring Boot:

Swagger/OpenAPI requires manual configuration
Docs often go out of sync

📌 Frontend teams love FastAPI for this reason alone.

6️⃣ Async Support

	FastAPI	Spring Boot
Async support	Native	WebFlux
Complexity	Simple	High
Learning curve	Easy	Steep

Many Java teams avoid WebFlux due to complexity, while FastAPI makes async natural.

7️⃣ Enterprise Features

Feature	FastAPI	Spring Boot
Security	Good	Excellent
Transactions	Manual	Strong
AOP	❌	✅
Dependency Injection	Basic	Advanced

Spring Boot clearly wins in enterprise governance and safety.

8️⃣ Database & ORM Support

	FastAPI	Spring Boot
ORM	SQLAlchemy	Hibernate
MongoDB	Motor	Spring Data Mongo
Maturity	Medium	Very High

Spring Data is more mature for large teams and long-term projects.

When to Use FastAPI?

Choose FastAPI if you are building:

Microservices
API gateways
Integration platforms
Event-driven systems
AI / ML APIs
Backend for React / Angular apps

When to Use Spring Boot?

Choose Spring Boot if you are building:

Enterprise systems
Banking or finance apps
Complex transactional workflows
Long-term systems (10–15 years lifespan)
Large, regulated platforms

Real-World Architecture (Very Common)

Many companies use both together:


Spring Boot → Core business logic
FastAPI     → API layer, integrations, ML services

This hybrid approach offers stability + speed.

Final Verdict

FastAPI is not a replacement for Spring Boot.
It is a complement.

Spring Boot = Enterprise backbone
FastAPI = Fast, modern API layer

For experienced Java developers, learning FastAPI adds a powerful new skill and opens doors to modern architectures and AI-driven systems.

Conclusion

If you want:

Faster development
Cleaner APIs
Better API documentation
Modern async architecture

👉 FastAPI is worth learning.

If you want:

Enterprise reliability
Strong governance
Long-term maintainability

👉 Spring Boot remains unbeatable.

Pages

Monday, 22 December 2025

FastAPI vs Flask vs Django: Which One Should You Choose?

What is FastAPI?

What is Flask?

What is Django?

FastAPI vs Flask vs Django: Detailed Comparison

When to Choose FastAPI

When to Choose Flask

When to Choose Django

FastAPI vs Flask vs Django: Use Case Summary

Final Thoughts

FastAPI Microservices Setup

FastAPI Microservices Setup (Production-Ready Guide)

1️⃣ What Is a FastAPI Microservices Architecture?

2️⃣ High-Level Architecture

3️⃣ Example Microservices Folder Structure

4️⃣ Service-to-Service Communication

Option 1: REST (Most Common)

Option 2: Message Queue (Recommended for scale)

5️⃣ API Gateway (Very Important)

Simple FastAPI Gateway Example

6️⃣ Authentication Strategy

Central Auth Service (Recommended)

7️⃣ Database Strategy

8️⃣ Async & Performance Setup

9️⃣ Docker Setup (Per Service)

Dockerfile (Common for all services)

🔟 docker-compose (Local Development)

1️⃣1️⃣ Configuration Management

1️⃣2️⃣ Logging & Monitoring

1️⃣3️⃣ Scaling Strategy

1️⃣4️⃣ FastAPI vs Spring Boot Microservices

1️⃣5️⃣ Common Mistakes to Avoid

Final Thoughts

FastAPI Project Structure

Production Best Practices

1️⃣ Recommended Folder Structure

2️⃣ Mapping to Spring Boot (So It Feels Familiar)

3️⃣ main.py – Application Entry Point

4️⃣ Router Aggregation (Like Controller Scanning)

api/v1/router.py

5️⃣ Controller Layer (Endpoints)

api/v1/endpoints/users.py

6️⃣ Schema (DTO)

schemas/user.py

7️⃣ Service Layer (Business Logic)

services/user_service.py

8️⃣ Repository Layer (DB Access)

repositories/user_repo.py

9️⃣ Dependency Injection (FastAPI Style)

🔟 Environment Configuration

.env

config.py

1️⃣1️⃣ Testing Structure

1️⃣2️⃣ Production-Grade Tips

1️⃣3️⃣ Minimal Structure (For Small Services)

Final Thoughts

FastAPI Interview Questions and Answers (Beginner to Advanced)

1️⃣ What is FastAPI?

2️⃣ Why is FastAPI faster than Flask or Django?

3️⃣ What is ASGI and how is it different from WSGI?

4️⃣ What are Pydantic models?

5️⃣ How does FastAPI generate Swagger documentation?

6️⃣ What is dependency injection in FastAPI?

7️⃣ How do you handle authentication in FastAPI?

8️⃣ What is the difference between def and async def in FastAPI?

9️⃣ How does FastAPI handle request validation errors?

🔟 How do you connect FastAPI to a database?

1️⃣1️⃣ How do you write unit tests in FastAPI?

1️⃣2️⃣ How is FastAPI deployed in production?

1️⃣3️⃣ How does FastAPI compare to Spring Boot?

1️⃣4️⃣ What are background tasks in FastAPI?

1️⃣5️⃣ What are common FastAPI interview mistakes?

Advanced FastAPI Interview Questions (For Experienced Devs)

Final Thoughts

FastAPI vs Spring Boot

FastAPI vs Spring Boot: Which One Should You Choose?

What is Spring Boot?

What is FastAPI?

3️⃣ `main.py` – Application Entry Point

`api/v1/router.py`

`api/v1/endpoints/users.py`

`schemas/user.py`

`services/user_service.py`

`repositories/user_repo.py`

`.env`

`config.py`

8️⃣ What is the difference between `def` and `async def` in FastAPI?