Health check endpoints in Node.js - Step-by-Step Execution

Concept Flow - Health check endpoints

Start Server

↓

Receive HTTP Request

↓

Check if URL is /health

↓

Run Health Check

↓

Return Status 200 and JSON

↓

End Request

The server listens for requests, checks if the URL is /health, runs a health check, and returns a status response.

Execution Sample

Node.js

import http from 'http';

const server = http.createServer((req, res) => {
  if (req.url === '/health') {
    res.writeHead(200, {'Content-Type': 'application/json'});
    res.end(JSON.stringify({status: 'ok'}));
  } else {
    res.writeHead(404);
    res.end();
  }
});

server.listen(3000);

This code creates a simple HTTP server that responds with a JSON status when /health is requested.

Execution Table

Step	Request URL	Condition (req.url === '/health')	Action	Response Status	Response Body
1	/health	true	Send 200 with JSON {status: 'ok'}	200	{"status":"ok"}
2	/other	false	Send 404 with empty body	404

💡 Requests not matching /health get 404; /health requests return 200 with status JSON.

Variable Tracker

Variable	Start	After Step 1	After Step 2
req.url	undefined	/health	/other
res.statusCode	undefined	200	404
res.body	undefined	{"status":"ok"}

Key Moments - 2 Insights

Why does the server return 404 for URLs other than /health?

Because the condition req.url === '/health' is false for other URLs, so the else branch sends a 404 response as shown in execution_table row 2.

What does the server send back when the URL is /health?

It sends a 200 status with a JSON body {"status":"ok"}, as shown in execution_table row 1.

Visual Quiz - 3 Questions

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

A 500

B 404

C 200

D 301

💡 Hint

Check the Response Status column in execution_table row 1.

At which step does the condition req.url === '/health' evaluate to false?

A Step 2

B Step 1

C Both steps

D Never

💡 Hint

Look at the Condition column in execution_table row 2.

If the server code is changed to respond with status 503 on /health, how would the response status change in the table?

A It would stay 200

B It would change to 503

C It would become 404

D It would be empty

💡 Hint

Focus on the Action and Response Status columns for /health requests.

Concept Snapshot

Health check endpoints listen for a specific URL (e.g., /health).
If the request URL matches, respond with status 200 and a JSON status.
If not, respond with 404 or other error.
This helps external systems check if the server is running.
Use simple HTTP server code to implement.
Always return JSON for easy parsing.

Full Transcript

A health check endpoint is a special URL on a server that tells if the server is working. When the server gets a request, it checks if the URL is '/health'. If yes, it sends back a 200 status code and a JSON message {"status":"ok"}. If the URL is different, it sends a 404 status code. This lets other systems or users know if the server is alive and healthy. The example code uses Node.js's http module to create this simple server. The execution table shows two cases: one where the URL is '/health' and one where it is not. Variables like req.url and res.statusCode change accordingly. This pattern is common in web services to monitor uptime and readiness.

Practice

1. What is the main purpose of a health check endpoint in a Node.js application?

easy

A. To confirm the app is running and responsive

B. To handle user login requests

C. To serve static files like images

D. To connect to the database

Solution

1. Step 1: Understand the role of health check endpoints

   Health check endpoints are designed to verify if the application is alive and can respond to requests.

2. Step 2: Differentiate from other endpoints

   Endpoints for login, static files, or database connections serve different purposes unrelated to health checks.

3. Final Answer:

   To confirm the app is running and responsive -> Option A

4. Quick Check:

   Health check = confirms app is alive [OK]

Hint: Health check endpoints confirm app status quickly [OK]

Common Mistakes:

• Confusing health check with user authentication
• Thinking health check serves static content
• Assuming health check manages database connections

2. Which of the following is the correct way to define a simple health check endpoint using Express in Node.js?

easy

A. app.listen('/health', () => console.log('OK'))

B. app.post('/health', (req, res) => res.send('OK'))

C. app.get('/health', (req, res) => res.send('OK'))

D. app.use('/health', (req, res) => res.send('OK'))

Solution

1. Step 1: Identify the HTTP method for health checks

   Health checks typically use GET requests to allow simple status queries.

2. Step 2: Check Express syntax for route definition

   Using app.get with a path and a callback sending a response is the correct pattern.

3. Final Answer:

   app.get('/health', (req, res) => res.send('OK')) -> Option C

4. Quick Check:

   GET route with res.send('OK') = correct syntax [OK]

Hint: Health checks use GET routes with res.send [OK]

Common Mistakes:

• Using POST instead of GET for health checks
• Using app.use which matches all methods
• Confusing app.listen with route handling

3. Given this Express code snippet, what will the response be when a client requests /health?

const express = require('express');
const app = express();
app.get('/health', (req, res) => {
  res.json({ status: 'ok', uptime: process.uptime() });
});
app.listen(3000);

medium

A. No response because the route is not defined

B. A plain text 'ok' message

C. An error because res.json is not a function

D. A JSON object with status 'ok' and uptime in seconds

Solution

1. Step 1: Analyze the route handler

   The route '/health' uses res.json to send a JSON object with keys 'status' and 'uptime'.

2. Step 2: Understand process.uptime()

   process.uptime() returns the number of seconds the Node.js process has been running, a number value.

3. Final Answer:

   A JSON object with status 'ok' and uptime in seconds -> Option D

4. Quick Check:

   res.json sends JSON = JSON object response [OK]

Hint: res.json sends JSON object, not plain text [OK]

Common Mistakes:

• Thinking res.json is undefined
• Expecting plain text instead of JSON
• Assuming route is missing or broken

4. Identify the error in this health check endpoint code and choose the fix:

const express = require('express');
const app = express();
app.get('/health', (req, res) => {
  res.sendStatus(200).send('OK');
});
app.listen(3000);

medium

A. Replace res.sendStatus(200).send('OK') with res.status(200).send('OK')

B. Change app.get to app.post

C. Remove res.send('OK') and keep only res.sendStatus(200)

D. Add next parameter and call next() after res.sendStatus

Solution

1. Step 1: Understand res.sendStatus behavior

   res.sendStatus(200) sends the status and ends the response, so chaining .send('OK') causes an error.

2. Step 2: Use correct method chaining

   res.status(200).send('OK') sets status and sends body correctly without ending response prematurely.

3. Final Answer:

   Replace res.sendStatus(200).send('OK') with res.status(200).send('OK') -> Option A

4. Quick Check:

   res.status().send() chains properly, res.sendStatus() ends response [OK]

Hint: Use res.status().send() to set status and send body [OK]

Common Mistakes:

• Chaining send after sendStatus causes error
• Changing HTTP method unnecessarily
• Calling next() after sending response

5. You want to create a health check endpoint that returns JSON with status as 'ok' only if the database connection is alive, otherwise status is 'error'. Which approach correctly implements this in Node.js with Express?

hard

A. app.get('/health', (req, res) => {
checkDbConnection().then(() => res.json({ status: 'ok' }));
});

B. app.get('/health', async (req, res) => {
const dbAlive = await checkDbConnection();
res.json({ status: dbAlive ? 'ok' : 'error' });
});

C. app.get('/health', (req, res) => {
const dbAlive = checkDbConnection();
res.json({ status: dbAlive ? 'ok' : 'error' });
});

D. app.get('/health', (req, res) => {
res.json({ status: 'ok' });
});

Solution

1. Step 1: Recognize async database check

   Checking database connection is asynchronous, so await or then() must be used to get result.

2. Step 2: Validate correct async usage

   app.get('/health', async (req, res) => { const dbAlive = await checkDbConnection(); res.json({ status: dbAlive ? 'ok' : 'error' }); }); uses async/await properly to wait for checkDbConnection result before sending JSON response.

3. Final Answer:

   Use async function with await to check DB and respond accordingly -> Option B

4. Quick Check:

   Async/await needed for async DB check [OK]

Hint: Use async/await to handle async DB checks in health endpoint [OK]

Common Mistakes:

• Calling async function without await or then
• Ignoring DB status and always returning 'ok'
• Not handling promise results properly