Health check endpoints in Node.js - Mini Project: Build & Apply

Health Check Endpoints

📖 Scenario: You are building a simple Node.js server to monitor the health of your web service. Health check endpoints help other systems know if your service is running well.

🎯 Goal: Create a Node.js server with two health check endpoints: /health and /ready. The /health endpoint returns a simple status message. The /ready endpoint returns a readiness message.

📋 What You'll Learn

Create a Node.js server using the built-in http module.

Create a variable called port set to 3000.

Add a request listener function that checks the URL path.

Respond with JSON messages for /health and /ready endpoints.

Start the server listening on the specified port.

💡 Why This Matters

🌍 Real World

Health check endpoints are used by monitoring tools and load balancers to check if your service is running and ready to receive traffic.

💼 Career

Knowing how to create health check endpoints is important for backend developers and DevOps engineers to ensure reliable and maintainable services.

1

Create the server and port variable

Create a variable called port and set it to 3000. Then create a server using http.createServer() and assign it to a variable called server.

Node.js

const http = require('http');

// Your code here

Hint

Use const port = 3000; and const server = http.createServer(); to start.

2

Add a request listener function

Add a request listener function to server using server.on('request', (req, res) => { ... }). Inside the function, create a variable called url and set it to req.url.

Node.js

const http = require('http');

const port = 3000;
const server = http.createServer();

// Your code here

Hint

Use server.on('request', (req, res) => { const url = req.url; }) to handle requests.

3

Add core logic for health and ready endpoints

Inside the request listener, use if and else if statements to check if url is '/health' or '/ready'. For /health, respond with status code 200 and JSON {"status": "ok"}. For /ready, respond with status code 200 and JSON {"ready": true}. For other URLs, respond with status code 404 and JSON {"error": "Not found"}. Remember to set the Content-Type header to application/json before sending the response.

Node.js

const http = require('http');

const port = 3000;
const server = http.createServer();

server.on('request', (req, res) => {
  const url = req.url;
  // Your code here
});

Hint

Use if and else if to check url and respond with JSON and status codes.

4

Start the server listening on the port

Add a line to start the server listening on the port variable using server.listen(port).

Node.js

const http = require('http');

const port = 3000;
const server = http.createServer();

server.on('request', (req, res) => {
  const url = req.url;

  res.setHeader('Content-Type', 'application/json');

  if (url === '/health') {
    res.statusCode = 200;
    res.end(JSON.stringify({ status: 'ok' }));
  } else if (url === '/ready') {
    res.statusCode = 200;
    res.end(JSON.stringify({ ready: true }));
  } else {
    res.statusCode = 404;
    res.end(JSON.stringify({ error: 'Not found' }));
  }
});

// Your code here

Hint

Use server.listen(port) to start the server.

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