Performance monitoring basics in Express - Mini Project: Build & Apply

Performance monitoring basics

📖 Scenario: You are building a simple Express.js server. You want to monitor how long each request takes to process. This helps you understand your server's performance and find slow parts.

🎯 Goal: Create an Express.js server that tracks the time taken to handle each request and logs it to the console.

📋 What You'll Learn

Create an Express.js app with a single GET route at '/'

Add a middleware to record the start time of each request

Calculate the duration after the response is sent

Log the request method, URL, and duration in milliseconds

💡 Why This Matters

🌍 Real World

Monitoring request durations helps developers find slow endpoints and improve user experience by making servers faster.

💼 Career

Performance monitoring is a key skill for backend developers and DevOps engineers to ensure applications run smoothly and efficiently.

1

Set up Express server

Create an Express app by requiring express and calling express(). Then create a GET route at '/' that sends the text 'Hello World' as the response.

Express

const express = require('express')
const app = express()

// Your code here

Hint

Use require('express') to import Express. Then call express() to create the app. Use app.get('/', ...) to define the route.

2

Add start time middleware

Add a middleware using app.use that records the current time in milliseconds in req.startTime before passing control to the next middleware.

Express

const express = require('express')
const app = express()

app.use((req, res, next) => {
  // Your code here
})

app.get('/', (req, res) => {
  res.send('Hello World')
})

Hint

Use Date.now() to get the current time in milliseconds. Store it in req.startTime. Call next() to continue.

3

Calculate and log request duration

Add a listener for the 'finish' event on res inside the middleware. When the response finishes, calculate the duration by subtracting req.startTime from the current time. Then log the request method, URL, and duration in milliseconds using console.log.

Express

const express = require('express')
const app = express()

app.use((req, res, next) => {
  req.startTime = Date.now()
  res.on('finish', () => {
    // Your code here
  })
  next()
})

app.get('/', (req, res) => {
  res.send('Hello World')
})

Hint

Use res.on('finish', callback) to run code after response ends. Calculate duration with Date.now() - req.startTime. Use a template string to log method, URL, and duration.

4

Start server and test logging

Add app.listen to start the server on port 3000. Then add a console.log to print 'Server running on port 3000'. Run the server and visit http://localhost:3000/ to see the log of the request duration.

Express

const express = require('express')
const app = express()

app.use((req, res, next) => {
  req.startTime = Date.now()
  res.on('finish', () => {
    const duration = Date.now() - req.startTime
    console.log(`${req.method} ${req.url} - ${duration}ms`)
  })
  next()
})

app.get('/', (req, res) => {
  res.send('Hello World')
})

// Your code here

Hint

Use app.listen(3000, callback) to start the server. Inside the callback, print the message with console.log.

Practice

1. What is the main purpose of performance monitoring in an Express app?

easy

A. To measure how fast the app responds to requests

B. To add new features to the app

C. To change the app's user interface

D. To store user data securely

Solution

1. Step 1: Understand performance monitoring

   Performance monitoring tracks how quickly the app handles requests.

2. Step 2: Identify the main goal

   The goal is to find slow parts and improve speed, not to add features or change UI.

3. Final Answer:

   To measure how fast the app responds to requests -> Option A

4. Quick Check:

   Performance monitoring = measure response speed [OK]

Hint: Performance monitoring = measuring app speed [OK]

Common Mistakes:

• Confusing performance monitoring with adding features
• Thinking it changes the user interface
• Assuming it stores user data

2. Which Express middleware code snippet correctly measures request duration?

easy

A. app.use((req, res, next) => { const start = Date.now(); res.on('finish', () => { console.log(Date.now() - start); }); next(); });

B. app.use((req, res, next) => { const start = Date.now(); next(); console.log(Date.now() - start); });

C. app.use((req, res) => { console.log('Request received'); });

D. app.use((req, res, next) => { console.log('Start'); next(); console.log('End'); });

Solution

1. Step 1: Check middleware timing logic

   app.use((req, res, next) => { const start = Date.now(); res.on('finish', () => { console.log(Date.now() - start); }); next(); }); starts timer, waits for response finish event, then logs duration correctly.

2. Step 2: Compare other options

   app.use((req, res, next) => { const start = Date.now(); next(); console.log(Date.now() - start); }); logs too early before response ends. Another option lacks next() call. The remaining option logs 'Start' and 'End' immediately, not duration.

3. Final Answer:

   app.use((req, res, next) => { const start = Date.now(); res.on('finish', () => { console.log(Date.now() - start); }); next(); }); -> Option A

4. Quick Check:

   Use 'res.on("finish")' to measure full request time [OK]

Hint: Use 'res.on("finish")' event to log after response ends [OK]

Common Mistakes:

• Logging time before response finishes
• Forgetting to call next() in middleware
• Not using response events to measure duration

3. Given this middleware, what will be logged when a request takes 150ms?

app.use((req, res, next) => {
  const start = Date.now();
  res.on('finish', () => {
    console.log(`Response time: ${Date.now() - start}ms`);
  });
  next();
});

medium

A. No output logged

B. Response time: 0ms

C. Response time: NaN

D. Response time: 150ms

Solution

1. Step 1: Understand timer start and finish

   The timer starts at request start and logs when response finishes, so it measures full duration.

2. Step 2: Calculate logged time

   If request takes 150ms, logged time is approximately 150ms.

3. Final Answer:

   Response time: 150ms -> Option D

4. Quick Check:

   Timer logs actual elapsed time on 'finish' event [OK]

Hint: Logs time difference on response finish event [OK]

Common Mistakes:

• Assuming log happens immediately (0ms)
• Confusing timer start and end
• Expecting no output without console.log

4. Why does this middleware fail to log correct response time?

app.use((req, res, next) => {
  const start = Date.now();
  console.log(Date.now() - start);
  next();
});

medium

A. It logs after response ends, so time is too long

B. It logs before response finishes, so time is near zero

C. It uses wrong event to measure time

D. It does not call next(), so request hangs

Solution

1. Step 1: Analyze when logging happens

   Logging happens immediately after start, before response finishes, so duration is near zero.

2. Step 2: Check middleware flow

   next() is called, so request continues; no wrong event used; logging is too early.

3. Final Answer:

   It logs before response finishes, so time is near zero -> Option B

4. Quick Check:

   Log after response finish for correct timing [OK]

Hint: Log after response finishes, not before [OK]

Common Mistakes:

• Logging too early before response ends
• Forgetting to call next() (not here though)
• Using wrong event to measure time

5. You want to log slow requests taking over 500ms in Express. Which code correctly implements this?

hard

A. app.use((req, res) => { if (Date.now() > 500) console.log('Slow request'); });

B. app.use((req, res, next) => { const start = Date.now(); if (Date.now() - start > 500) console.log('Slow request'); next(); });

C. app.use((req, res, next) => { const start = Date.now(); res.on('finish', () => { if (Date.now() - start > 500) console.log('Slow request'); }); next(); });

D. app.use((req, res, next) => { console.log('Slow request'); next(); });

Solution

1. Step 1: Understand timing and event usage

   app.use((req, res, next) => { const start = Date.now(); res.on('finish', () => { if (Date.now() - start > 500) console.log('Slow request'); }); next(); }); measures time from request start to response finish, then checks if duration > 500ms.

2. Step 2: Check other options

   Other options either check time immediately (always near zero), use wrong conditions like Date.now() > 500, or log unconditionally for all requests.

3. Final Answer:

   app.use((req, res, next) => { const start = Date.now(); res.on('finish', () => { if (Date.now() - start > 500) console.log('Slow request'); }); next(); }); -> Option C

4. Quick Check:

   Check duration on 'finish' event, then log if slow [OK]

Hint: Check time after response finishes, then log if slow [OK]

Common Mistakes:

• Checking time too early (before response ends)
• Logging all requests as slow
• Not using 'finish' event to measure full duration