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

Concept Flow - Health check endpoints

Start Server

↓

Receive HTTP Request

↓

Check if URL is /health

↓

Send 200 OK

↓

End Request

The server starts and listens. When a request comes, it checks if the URL is '/health'. If yes, it sends a 200 OK response. Otherwise, it passes the request to other routes.

Execution Sample

Express

const express = require('express');
const app = express();
app.get('/health', (req, res) => res.send('OK'));
app.listen(3000);

This code creates a simple Express server with a health check endpoint at '/health' that responds with 'OK'.

Execution Table

Step	Incoming Request URL	Condition (/health?)	Action	Response Sent
1	/health	Yes	Send 200 OK with 'OK'	OK
2	/api/data	No	Pass to other routes	No response here
3	/health	Yes	Send 200 OK with 'OK'	OK
4	/unknown	No	Pass to other routes	No response here

💡 Requests not matching '/health' are passed to other routes or middleware.

Variable Tracker

Variable	Start	Request 1	Request 2	Request 3	Request 4
req.url	N/A	/health	/api/data	/health	/unknown
res.statusCode	N/A	200	Not set	200	Not set
res.body	N/A	'OK'	N/A	'OK'	N/A

Key Moments - 2 Insights

Why does the server respond only to '/health' and not other URLs?

Because the route handler is defined only for '/health' (see execution_table rows 1 and 2). Other URLs are passed to other routes or middleware.

What happens if a request URL is '/health' but the method is POST?

The handler is defined for GET requests only, so POST requests to '/health' will not match and will be passed on (similar to execution_table row 2).

Visual Quiz - 3 Questions

Look at the execution table, what response is sent when the request URL is '/health'?

A 'OK' with status 200

B No response

C 404 Not Found

D 'Error' message

💡 Hint

Check rows 1 and 3 in the execution_table where URL is '/health'.

At which request does the condition '/health?' evaluate to No?

A Request 3

B Request 1

C Request 2

D Request 4

💡 Hint

Look at the 'Condition (/health?)' column in execution_table.

If we add a POST handler for '/health', how would the response change for a POST request to '/health'?

A It would cause a server error

B It would respond with 200 OK for POST requests too

C It would still pass to other routes

D It would respond with 404 Not Found

💡 Hint

Think about how route handlers match method and path in Express.

Concept Snapshot

Health check endpoints in Express:
- Use app.get('/health', handler) to define a GET endpoint.
- When a request matches '/health', send a 200 OK response.
- Other URLs pass to other routes or middleware.
- Useful for monitoring server status simply.
- Keep response lightweight like 'OK'.

Full Transcript

This visual execution shows how an Express server handles health check endpoints. The server listens for HTTP requests. When a request comes in, it checks if the URL path is '/health'. If yes, it sends a 200 OK response with body 'OK'. If not, it passes the request to other routes or middleware. The execution table traces requests with different URLs and shows which get a response. Variables like request URL and response status are tracked. Key moments clarify why only '/health' matches and what happens with other methods. The quiz tests understanding of response behavior and routing. The snapshot summarizes how to set up a health check endpoint in Express simply and effectively.