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.

Practice

1. What is the main purpose of a health check endpoint in an Express app?

easy

A. To serve static files like images

B. To confirm the app is running and responsive

C. To handle user login requests

D. To connect to the database

Solution

1. Step 1: Understand what a health check endpoint does

   A health check endpoint is a simple route that tells if the app is alive and working.

2. Step 2: Compare options to this purpose

   Only To confirm the app is running and responsive matches this purpose. Other options are unrelated to health checks.

3. Final Answer:

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

4. Quick Check:

   Health check = Confirm app running [OK]

Hint: Health check endpoints just say if app is alive [OK]

Common Mistakes:

• Thinking health check handles user data
• Confusing health check with static file serving
• Assuming it manages database connections

2. Which of the following is the correct syntax to create a simple health check GET route in Express?

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 check

   Health checks use GET requests to check status, so app.get is correct.

2. Step 2: Check syntax correctness

   app.get('/health', (req, res) => res.send('OK')); uses app.get with path '/health' and sends 'OK' response correctly.

3. Final Answer:

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

4. Quick Check:

   GET route with res.send('OK') = app.get('/health', (req, res) => res.send('OK')); [OK]

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

Common Mistakes:

• Using POST instead of GET
• Using app.use instead of app.get
• Trying to listen on a route path

3. What will be the output when a client sends a GET request to the following Express route?

app.get('/health', (req, res) => {
  res.status(200).send('Service is up');
});

medium

A. HTTP 200 OK with body 'Service is up'

B. HTTP 404 Not Found

C. HTTP 500 Internal Server Error

D. No response, request times out

Solution

1. Step 1: Analyze the route handler

   The route listens for GET on '/health' and sends status 200 with text 'Service is up'.

2. Step 2: Determine the HTTP response

   The client receives status 200 OK and the string 'Service is up' in the response body.

3. Final Answer:

   HTTP 200 OK with body 'Service is up' -> Option A

4. Quick Check:

   Status 200 + send text = HTTP 200 OK with body 'Service is up' [OK]

Hint: res.status(200).send('text') sends 200 OK with that text [OK]

Common Mistakes:

• Assuming 404 if route exists
• Confusing status codes
• Expecting no response or error

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

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

medium

A. Remove res.send('OK') because res.sendStatus ends response

B. Change res.sendStatus(200) to res.status(200)

C. Add next() call after res.sendStatus(200)

D. Change app.get to app.post

Solution

1. Step 1: Understand res.sendStatus behavior

   res.sendStatus(200) sends status and ends the response immediately.

2. Step 2: Identify why res.send('OK') causes error

   Calling res.send after response ended causes an error. So remove the second send.

3. Final Answer:

   Remove res.send('OK') because res.sendStatus ends response -> Option A

4. Quick Check:

   One response per request = Remove res.send('OK') because res.sendStatus ends response [OK]

Hint: Only send one response per request [OK]

Common Mistakes:

• Sending multiple responses in one route
• Confusing res.sendStatus and res.status
• Adding next() unnecessarily

5. You want your Express app's health check endpoint to return JSON with status and uptime in seconds. Which code correctly implements this?

hard

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

B. app.get('/health', (req, res) => {
res.send('status: ok, uptime: ' + process.uptime());
});

C. app.get('/health', (req, res) => {
res.status(200).send({ status: 'ok', uptime: process.uptime() });
});

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

Solution

1. Step 1: Understand JSON response method

   res.json sends a JSON object with correct headers automatically.

2. Step 2: Check each option's response format

   app.get('/health', (req, res) => { res.json({ status: 'ok', uptime: process.uptime() }); }); sends a JSON object with keys 'status' and 'uptime' correctly. app.get('/health', (req, res) => { res.send('status: ok, uptime: ' + process.uptime()); }); sends a string, not JSON. app.get('/health', (req, res) => { res.status(200).send({ status: 'ok', uptime: process.uptime() }); }); sends an object with res.send but may not set JSON headers properly. app.get('/health', (req, res) => { res.json('status: ok, uptime: ' + process.uptime()); }); sends a string inside res.json, which is incorrect.

3. Final Answer:

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

4. Quick Check:

   Use res.json with object for JSON response [OK]

Hint: Use res.json({key: value}) to send JSON response [OK]

Common Mistakes:

• Sending JSON as string instead of object
• Using res.send with object without headers
• Confusing res.json and res.send