Health check endpoints in FastAPI - Step-by-Step Execution

Concept Flow - Health check endpoints

Start FastAPI app

↓

Define /health endpoint

↓

Receive HTTP GET request at /health

↓

Run health check logic

↓

Return JSON response with status

↓

Client receives status response

The app starts, defines a /health endpoint, listens for GET requests, runs checks, and returns a status JSON.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/health")
async def health_check():
    return {"status": "ok"}

Defines a simple health check endpoint that returns a JSON with status 'ok' when accessed.

Execution Table

Step	Action	Input	Process	Output
1	Start FastAPI app	N/A	App initializes and listens for requests	App ready
2	Receive GET request	GET /health	Route matches /health endpoint	Invoke health_check function
3	Execute health_check	No parameters	Return JSON {"status": "ok"}	{"status": "ok"}
4	Send response	{"status": "ok"}	HTTP 200 with JSON body	Client receives JSON
5	End	N/A	Request handled successfully	Ready for next request

💡 Request completes after sending JSON response with status 'ok'

Variable Tracker

Variable	Start	After Request	Final
app	FastAPI instance created	Running and listening	Running and listening
request	None	GET /health received	Handled and response sent
response	None	None	{"status": "ok"}

Key Moments - 2 Insights

Why does the endpoint return a JSON with {"status": "ok"} instead of plain text?

The execution_table row 3 shows the function returns a JSON dictionary, which FastAPI automatically converts to JSON response for clients.

What happens if a request is sent to a different path than /health?

The app will not match the route, so no health_check function runs. This is implied by execution_table row 2 where only GET /health triggers the function.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 3?

A Plain text 'ok'

B HTTP 404 Not Found

C {"status": "ok"}

D Empty response

💡 Hint

Check the 'Output' column in row 3 of the execution_table

At which step does the app receive the GET /health request?

A Step 2

B Step 4

C Step 1

D Step 5

💡 Hint

Look at the 'Action' and 'Input' columns in the execution_table

If the health_check function returned {"status": "fail"}, how would the output at step 3 change?

A {"status": "ok"}

B {"status": "fail"}

C HTTP 500 Internal Server Error

D No output

💡 Hint

Refer to the 'Process' and 'Output' columns in step 3 of the execution_table

Concept Snapshot

FastAPI health check endpoint:
- Define with @app.get('/health')
- Async function returns JSON status
- Client sends GET /health
- Server responds with JSON {"status": "ok"}
- Used to verify app is running

Full Transcript

This visual execution shows how a FastAPI app defines a health check endpoint at /health. When the app starts, it listens for HTTP requests. Upon receiving a GET request at /health, it runs the health_check function which returns a JSON response with status 'ok'. This response is sent back to the client, confirming the app is running properly. Variables like app, request, and response change state during this process. Key points include understanding the JSON response and route matching. The quiz tests knowledge of the output and request handling steps.