DEV Community

Haripriya Veluchamy
Haripriya Veluchamy

Posted on

Why Do We Need WSGI for Python Web Apps? (And Why Flask Uses Gunicorn)

#python #flask #fastapi #production

If you're new to deploying Python web applications, one confusing question always pops up:

“I can run my Flask app using python app.py, then why do I need something like Gunicorn, uWSGI, or Waitress?”

This is a common turning point for developers who move from local development to real production environments.

This post explains WSGI in the simplest way possible and why production-grade servers exist for Python.

🚀 1. What Actually Happens When You Run python app.py?

When you run:

python app.py
Enter fullscreen mode Exit fullscreen mode

Flask starts its development server, which is meant for:

  • local testing
  • debugging
  • auto-reload
  • simple workflows

But this server is NOT designed for production because it:

❌ Can’t handle heavy traffic
❌ Can’t manage multiple worker processes
❌ Can’t gracefully recover if a request hangs
❌ Isn’t optimized for concurrency
❌ Has security limitations

So… we need something stronger.

🧩 2. What is WSGI?

WSGI stands for:

Web Server Gateway Interface

It is a standard interface that connects Python web apps to real production web servers.

Think of WSGI as a bridge between:

HTTP Requests (from browser)
           ↓
WSGI Server (Gunicorn / uWSGI / Waitress)
           ↓
Your Flask App (Python)
           ↓
HTTP Response
Enter fullscreen mode Exit fullscreen mode

WSGI solves one major problem:

➡️ Python was never designed to directly speak HTTP like a web server.

WSGI enables Python apps to run behind efficient web servers.

⚙️ 3. Why Do We Need a WSGI Server?

A WSGI server handles:

✔️ Multiple worker processes

To serve many requests at the same time.

✔️ Concurrency & load

Gunicorn/uWSGI can spawn:

  • multiple processes
  • multiple threads
  • async workers

So your app does not freeze during load.

✔️ Security

The dev server is not hardened or sandboxed.

✔️ Reverse proxy compatibility

Works properly behind:

  • Nginx
  • Apache
  • Azure App Service
  • AWS ALB
  • GCP Load Balancers

✔️ Crash handling

If one worker dies, others continue.

🏗️ 4. Different WSGI Servers Used Across Operating Systems

Here are the popular choices and when they're used:

🟢 Linux/macOS

  • Gunicorn → most popular for Flask/Django
  • uWSGI → extremely powerful, configurable
  • mod_wsgi → Apache module for Python
  • Hypercorn (ASGI + WSGI)

🔵 Windows

Windows does not natively support Gunicorn.

So developers use:

  • Waitress → pure Python WSGI server (great for Windows)
  • IIS with wfastcgi (enterprise Windows environments)
  • mod_wsgi-express for limited use

This OS-level difference is one big reason WSGI servers vary across systems.

🐎 5. Why Flask Uses Gunicorn (or Another WSGI Server)

Because Flask itself is not a web server.

Flask is only:

  • a micro-framework
  • routing system
  • request handler
  • template/render engine

It does not manage multi-process serving or handle HTTP at scale.

Gunicorn (or others) fills that gap.

🔥 6. Example: Running Flask with Gunicorn 

gunicorn -w 4 -b 0.0.0.0:8000 app:app
Enter fullscreen mode Exit fullscreen mode

This means:

  • 4 worker processes
  • binding to all interfaces
  • load balanced workers

Now your Flask app is production-ready.

🖥️ 7. Why Local Development Feels “Too Easy”

Because Python’s dev server hides the complexity.

In production:

  • concurrency
  • scaling
  • fault-tolerance
  • OS process management

…are ALL handled by WSGI servers.

This is why every cloud provider expects WSGI servers for Python apps.

🧠 Final Takeaway

✔️ Flask’s built-in server = for development
❌ Not for real production
✔️ WSGI = standard that lets Python apps talk to web servers
✔️ Gunicorn/uWSGI/Waitress = production WSGI servers
✔️ Different OS = different supported WSGI servers

If you want your Flask app to be:

  • scalable
  • secure
  • stable
  • production-grade

You must use a WSGI server.

Top comments (0)