Request/response transformation in Nginx - Step-by-Step Execution

Process Flow - Request/response transformation

Client sends request

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Nginx receives request

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Apply request transformations

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Forward transformed request to backend

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Backend sends response

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Apply response transformations

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Send transformed response to client

Nginx intercepts client requests and backend responses, modifies them as configured, then forwards them along.

Execution Sample

Nginx

server {
  listen 8080;
  location / {
    proxy_set_header X-Original-URI $request_uri;
    proxy_pass http://backend;
    sub_filter 'Hello' 'Hi';
    sub_filter_once off;
  }
}

This config adds a header to the request and replaces 'Hello' with 'Hi' in the response body.

Process Table

Step	Action	Input	Transformation	Output
1	Receive client request	GET /hello HTTP/1.1	None yet	Request: GET /hello HTTP/1.1
2	Apply request transformation	Request headers	Add header X-Original-URI: /hello	Request headers with X-Original-URI
3	Forward request to backend	Transformed request	None	Backend receives GET /hello with extra header
4	Backend sends response	Response body: 'Hello World!'	None	Response body: 'Hello World!'
5	Apply response transformation	Response body	Replace 'Hello' with 'Hi'	Response body: 'Hi World!'
6	Send response to client	Transformed response	None	Client receives 'Hi World!'
7	End	N/A	N/A	Request/response cycle complete

💡 Request and response transformations applied; cycle ends after sending response to client.

Status Tracker

Variable	Start	After Step 2	After Step 5	Final
request_uri	/hello	/hello	/hello	/hello
request_headers	No X-Original-URI	Includes X-Original-URI: /hello	Includes X-Original-URI: /hello	Includes X-Original-URI: /hello
response_body	Hello World!	Hello World!	Hi World!	Hi World!

Key Moments - 2 Insights

Why does the request header X-Original-URI appear only after step 2?

Because the header is added during the request transformation step (step 2), before forwarding to backend (step 3). Before that, the request has no such header.

Why is the response body changed from 'Hello World!' to 'Hi World!' at step 5?

Because the response transformation uses sub_filter to replace 'Hello' with 'Hi' in the response body before sending it to the client.

Visual Quiz - 3 Questions

Look at the execution table at step 2. What change happens to the request?

A A new header X-Original-URI is added

B The request method changes from GET to POST

C The request URI changes from /hello to /hi

D The request body is modified

💡 Hint

Check the 'Transformation' column at step 2 in the execution table.

At which step does the response body get modified?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look for the step where 'Replace Hello with Hi' happens in the transformation column.

If sub_filter_once was set to 'on' instead of 'off', how would the response transformation change?

A All occurrences of 'Hello' would be replaced

B No replacements would happen

C Only the first occurrence of 'Hello' would be replaced

D The request header would be removed

💡 Hint

Refer to the sub_filter_once directive behavior in the execution_sample code.

Concept Snapshot

Nginx request/response transformation:
- Modify requests with directives like proxy_set_header
- Modify responses with sub_filter
- Transformations happen between client and backend
- Use sub_filter_once off to replace all matches
- Useful for header injection and content rewriting

Full Transcript

This visual execution shows how Nginx handles request and response transformations. First, the client sends a request. Nginx adds a header X-Original-URI to the request before forwarding it to the backend. The backend responds with 'Hello World!'. Nginx then replaces 'Hello' with 'Hi' in the response body before sending it back to the client. Variables like request_uri remain unchanged, headers get added, and response body content is modified. Key moments include when headers are added and when response text is replaced. The quiz tests understanding of these steps and configuration effects.