WSGI servers (Gunicorn, uWSGI) in Flask - Step-by-Step Execution

Concept Flow - WSGI servers (Gunicorn, uWSGI)

Start Flask App

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WSGI Server Receives HTTP Request

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WSGI Server Calls Flask App

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Flask App Processes Request

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Flask App Returns Response

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WSGI Server Sends HTTP Response

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Client Receives Response

The WSGI server acts as a middleman between the web and the Flask app, receiving requests, passing them to Flask, then sending back responses.

Execution Sample

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello from Flask!'

# Run with: gunicorn app:app

A simple Flask app that returns a greeting, served by Gunicorn as the WSGI server.

Execution Table

Step	Action	WSGI Server State	Flask App State	Response Sent
1	WSGI server starts and listens	Listening for requests	Not called yet	No response
2	HTTP request arrives	Request received	Not called yet	No response
3	WSGI server calls Flask app with request	Processing request	Running home() route	No response yet
4	Flask app returns 'Hello from Flask!'	Waiting to send response	Response ready	No response yet
5	WSGI server sends HTTP response to client	Idle, waiting for next request	Idle	Response sent: 'Hello from Flask!'
6	Client receives response	Idle	Idle	Response received by client

💡 Request cycle ends after response is sent and client receives it

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
WSGI Server State	Idle	Processing request	Waiting to send response	Idle	Idle
Flask App State	Not called	Running home()	Response ready	Idle	Idle
Response Sent	No	No	No	Yes	Yes

Key Moments - 3 Insights

Why does the WSGI server call the Flask app instead of handling the request itself?

The WSGI server acts like a waiter taking orders; it passes the request to Flask (the kitchen) to prepare the response. See execution_table step 3 where the server calls the Flask app.

When does the Flask app actually run the code to create the response?

The Flask app runs when the WSGI server calls it with the request, shown in step 3 and 4 of the execution_table where the home() function runs and returns the response.

What happens if the WSGI server does not send the response back to the client?

The client waits and sees no result. The cycle ends only after the server sends the response, as shown in step 5 and 6 where the response is sent and received.

Visual Quiz - 3 Questions

Look at the execution table, what is the WSGI server state at step 4?

A Listening for requests

B Idle

C Waiting to send response

D Request received

💡 Hint

Check the 'WSGI Server State' column at step 4 in the execution_table.

At which step does the Flask app finish processing and have the response ready?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Flask App State' column to see when the response is ready.

If the WSGI server never sends the response, what would the 'Response Sent' variable show after step 5?

A No

B Maybe

C Yes

D Error

💡 Hint

Refer to the 'Response Sent' variable in variable_tracker after step 5.

Concept Snapshot

WSGI servers like Gunicorn or uWSGI receive HTTP requests from clients.
They call the Flask app with the request.
Flask processes and returns a response.
The server sends this response back to the client.
This cycle repeats for each request.
WSGI servers act as the middleman between web and Flask.

Full Transcript

WSGI servers such as Gunicorn and uWSGI serve as intermediaries between the web and Flask applications. When a client sends an HTTP request, the WSGI server receives it and calls the Flask app with this request. The Flask app processes the request by running the appropriate route function and returns a response string. The WSGI server then sends this response back to the client. This process repeats for every incoming request, allowing Flask to focus on application logic while the WSGI server handles communication with the web. The execution table shows each step from server start, request receipt, Flask processing, to response delivery. Variables track the server and app states and when the response is sent. Understanding this flow helps beginners see how Flask apps run in real web environments using WSGI servers.