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

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

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

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.