REST API serving with FastAPI in MLOps - Step-by-Step Execution

Process Flow - REST API serving with FastAPI

Start FastAPI app

↓

Receive HTTP request

↓

Match request path & method

↓

Call corresponding function

↓

Process input data

↓

Generate response data

↓

Send HTTP response

↓

Wait for next request

This flow shows how FastAPI starts, receives a request, matches it to a function, processes data, and sends back a response.

Execution Sample

MLOps

from fastapi import FastAPI
app = FastAPI()

@app.get("/hello")
def say_hello():
    return {"message": "Hello, world!"}

This code creates a FastAPI app with one GET endpoint '/hello' that returns a greeting message.

Process Table

Step	Action	Input	Function Called	Output	Response Sent
1	Start FastAPI app	N/A	N/A	App running on localhost:8000	N/A
2	Receive HTTP GET request	GET /hello	say_hello()	Returns {"message": "Hello, world!"}	HTTP 200 with JSON body
3	Send response	N/A	N/A	Response sent to client	Client receives JSON message
4	Wait for next request	N/A	N/A	Idle, listening	N/A

💡 No exit; server runs continuously waiting for requests.

Status Tracker

Variable	Start	After Request 1	After Request 2	Final
app	FastAPI instance created	Running	Running	Running
request	None	GET /hello	GET /hello	Depends on incoming requests
response	None	{"message": "Hello, world!"}	{"message": "Hello, world!"}	Depends on function output

Key Moments - 3 Insights

Why does the function say_hello() not take any parameters?

Because the endpoint '/hello' does not require any input data, so FastAPI calls say_hello() without arguments as shown in execution_table step 2.

How does FastAPI know which function to call for the GET /hello request?

FastAPI matches the HTTP method and path to the decorated function @app.get('/hello'), as shown in the flow and execution_table step 2.

Why does the server keep running after sending the response?

Because FastAPI is designed to serve multiple requests continuously, it waits for new requests after sending each response, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of the function say_hello() at step 2?

A Empty response

B HTTP 404 Not Found

C {"message": "Hello, world!"}

D Error message

💡 Hint

Check the 'Output' column in execution_table row for step 2.

At which step does the server send the HTTP response back to the client?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for 'Send response' action in execution_table.

If you add a POST endpoint, how would the execution_table change?

A Add new rows for POST requests with corresponding function calls

B Replace GET /hello with POST /hello

C Remove step 4

D No change needed

💡 Hint

Consider how new request types appear in execution_table rows.

Concept Snapshot

FastAPI creates a web server to handle HTTP requests.
Use @app.get('/path') to define GET endpoints.
Functions return data that FastAPI sends as JSON responses.
Server runs continuously, waiting for requests.
No parameters needed if endpoint has no input.

Full Transcript

This visual execution shows how FastAPI serves a REST API. First, the FastAPI app starts and listens on a port. When a client sends a GET request to '/hello', FastAPI matches this to the say_hello() function. The function returns a JSON message. FastAPI sends this response back to the client with HTTP status 200. After sending, the server waits for more requests. Variables like app, request, and response change state during this process. Key points include how FastAPI matches routes to functions and why the server keeps running. The quiz tests understanding of outputs, response timing, and adding new endpoints.