Nginx as reverse proxy in Flask - Step-by-Step Execution

Concept Flow - Nginx as reverse proxy

Client sends HTTP request

↓

Nginx receives request

↓

Nginx forwards request to Flask app

↓

Flask app processes request

↓

Flask app sends response

↓

Nginx receives response

↓

Nginx sends response back to client

The client sends a request to Nginx, which forwards it to the Flask app. Flask processes and replies, then Nginx sends the response back to the client.

Execution Sample

Flask

server {
    listen 80;
    location / {
        proxy_pass http://127.0.0.1:5000;
    }
}

Nginx listens on port 80 and forwards all requests to the Flask app running on localhost port 5000.

Execution Table

Step	Action	Request/Response	Source	Destination
1	Client sends HTTP GET /	GET /	Client	Nginx
2	Nginx receives request	GET /	Nginx	Nginx
3	Nginx forwards request	GET /	Nginx	Flask app (127.0.0.1:5000)
4	Flask app processes request	GET /	Flask app	Flask app
5	Flask app sends response	200 OK with content	Flask app	Nginx
6	Nginx receives response	200 OK with content	Nginx	Nginx
7	Nginx sends response back	200 OK with content	Nginx	Client
8	Client receives response	200 OK with content	Client	Client

💡 Request cycle completes after client receives the response.

Variable Tracker

Variable	Start	After Step 3	After Step 5	Final
Request URL	/	/	/	/
Request Source	Client	Nginx	Flask app	Client
Response Status	None	None	200 OK	200 OK

Key Moments - 3 Insights

Why does Nginx forward the request to 127.0.0.1:5000?

Because the proxy_pass directive tells Nginx to send incoming requests to the Flask app running locally on port 5000, as shown in execution_table step 3.

Does the client communicate directly with the Flask app?

No, the client only talks to Nginx. Nginx acts as a middleman forwarding requests and responses, as seen in execution_table steps 1, 3, and 7.

What happens if the Flask app is down?

Nginx will fail to get a response and usually returns an error to the client. This is because Nginx depends on Flask to handle the request after forwarding it.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Nginx forward the request to Flask?

A Step 1

B Step 3

C Step 5

D Step 7

💡 Hint

Check the 'Action' column for forwarding requests in execution_table row 3.

According to variable_tracker, what is the Response Status after Flask app processes the request?

A None

B 404 Not Found

C 200 OK

D 500 Internal Server Error

💡 Hint

Look at the 'Response Status' row after Step 5 in variable_tracker.

If Nginx did not send the response back to the client, which step would be missing in execution_table?

A Step 7

B Step 2

C Step 4

D Step 6

💡 Hint

Step 7 shows Nginx sending the response back to the client.

Concept Snapshot

Nginx as reverse proxy:
- Nginx listens on a public port (e.g., 80)
- Forwards client requests to Flask app (proxy_pass)
- Flask processes and replies
- Nginx sends Flask's response back to client
- Keeps client unaware of Flask's internal address
- Improves security and load management

Full Transcript

This visual trace shows how Nginx acts as a reverse proxy for a Flask app. The client sends a request to Nginx, which listens on port 80. Nginx forwards the request to the Flask app running locally on port 5000. The Flask app processes the request and sends back a response. Nginx receives this response and sends it back to the client. Variables like request URL and response status change as the request moves through these steps. Key points include that the client never talks directly to Flask, and Nginx manages the communication. If Flask is down, Nginx cannot fulfill the request. This setup helps keep the Flask app hidden and secure behind Nginx.