HTTP caching headers (ETag, Cache-Control) in Express - Mini Project: Build & Apply

HTTP caching headers (ETag, Cache-Control) with Express

📖 Scenario: You are building a simple Express server that serves a JSON response. To improve performance and reduce unnecessary data transfer, you want to add HTTP caching headers: ETag and Cache-Control.This will help browsers cache the response and only request new data when it changes.

🎯 Goal: Create an Express server with one route /data that sends a JSON object. Add Cache-Control header to tell browsers to cache the response for 60 seconds. Add ETag header to allow conditional requests.

📋 What You'll Learn

Create an Express app with a route /data

Send a JSON response with a fixed object

Add a Cache-Control header with max-age 60 seconds

Add an ETag header based on the JSON content

Handle conditional requests using the If-None-Match header

💡 Why This Matters

🌍 Real World

HTTP caching headers help reduce bandwidth and speed up web apps by letting browsers reuse cached data when it hasn't changed.

💼 Career

Understanding and implementing caching headers is important for backend developers to optimize API performance and improve user experience.

1

Set up Express app and JSON data

Create an Express app by requiring express and calling express(). Create a constant called data with the exact JSON object { message: "Hello, world!", version: 1 }.

Express

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

// Your code here

Hint

Use const to declare variables. The JSON object must have message and version keys with exact values.

2

Add Cache-Control header configuration

Create a constant called cacheDuration and set it to 60 to represent 60 seconds cache duration.

Express

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

const data = { message: "Hello, world!", version: 1 };

// Your code here

Hint

Use const cacheDuration = 60; to set the cache time in seconds.

3

Create /data route with Cache-Control and ETag headers

Add a GET route /data to the Express app. Inside the route handler, convert data to a JSON string called jsonData. Create an etag string by hashing jsonData using require('crypto').createHash('md5').update(jsonData).digest('hex'). Set the Cache-Control header to public, max-age=cacheDuration. Set the ETag header to the etag string. Check if the request header If-None-Match matches etag. If yes, respond with status 304 and end. Otherwise, send the JSON response with jsonData.

Express

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

const data = { message: "Hello, world!", version: 1 };

const cacheDuration = 60;

// Your code here

Hint

Use app.get to create the route. Use res.set to add headers. Use req.headers['if-none-match'] to check the ETag sent by the client.

4

Start the Express server

Add code to start the Express server listening on port 3000. Use app.listen(3000).

Express

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

const data = { message: "Hello, world!", version: 1 };

const cacheDuration = 60;

app.get('/data', (req, res) => {
    const jsonData = JSON.stringify(data);
    const etag = require('crypto').createHash('md5').update(jsonData).digest('hex');
    res.set('Cache-Control', `public, max-age=${cacheDuration}`);
    res.set('ETag', etag);
    if (req.headers['if-none-match'] === etag) {
        res.status(304).end();
        return;
    }
    res.send(jsonData);
});

// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

Practice

1. What is the main purpose of the ETag header in Express HTTP responses?

easy

A. To redirect the client to a different URL

B. To specify how long the browser should cache the resource

C. To identify a specific version of a resource for caching validation

D. To indicate the content type of the response

Solution

1. Step 1: Understand the role of ETag

   The ETag header provides a unique identifier for a specific version of a resource, allowing the server to tell if the resource has changed.

2. Step 2: Compare with other headers

   Cache-Control sets caching duration, redirects use Location header, and content type is set by Content-Type header, so these are not related to ETag.

3. Final Answer:

   To identify a specific version of a resource for caching validation -> Option C

4. Quick Check:

   ETag = version ID for caching [OK]

Hint: ETag tags resource versions to avoid unnecessary downloads [OK]

Common Mistakes:

• Confusing ETag with Cache-Control duration
• Thinking ETag redirects clients
• Mixing ETag with content type headers

2. Which of the following is the correct way to set a Cache-Control header in Express to cache a response for 1 hour?

easy

A. res.set('Cache-Control', 'max-age=3600')

B. res.header('Cache-Control', 'no-cache')

C. res.cache('max-age=3600')

D. res.setHeader('Cache-Control', 'max-age=60')

Solution

1. Step 1: Identify Express method to set headers

   Express uses res.set() or res.header() to set headers. Both work, but res.set() is common and correct here.

2. Step 2: Check Cache-Control value for 1 hour

   1 hour = 3600 seconds, so max-age=3600 is correct. res.header('Cache-Control', 'no-cache') uses 'no-cache' which disables caching. res.cache('max-age=3600') is invalid method. res.setHeader('Cache-Control', 'max-age=60') sets max-age to 60 seconds, not 1 hour.

3. Final Answer:

   res.set('Cache-Control', 'max-age=3600') -> Option A

4. Quick Check:

   Cache-Control max-age=3600 sets 1 hour caching [OK]

Hint: Use res.set with 'max-age=seconds' for caching time [OK]

Common Mistakes:

• Using wrong method like res.cache
• Confusing seconds with minutes in max-age
• Setting no-cache when caching is needed

3. Given this Express code snippet, what will the client receive if the request includes header If-None-Match matching the ETag?

app.get('/data', (req, res) => {
  const data = { message: 'Hello' };
  const etag = '12345';
  res.set('ETag', etag);
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
  } else {
    res.json(data);
  }
});

medium

A. A 500 Internal Server Error

B. A 304 Not Modified status with no body

C. A 404 Not Found error

D. A JSON response with { message: 'Hello' }

Solution

1. Step 1: Check If-None-Match header against ETag

   If the request's If-None-Match header equals the ETag '12345', the server responds with status 304 and no body.

2. Step 2: Understand 304 response meaning

   304 means Not Modified, telling the client to use cached data without sending the body again.

3. Final Answer:

   A 304 Not Modified status with no body -> Option B

4. Quick Check:

   If-None-Match matches ETag = 304 response [OK]

Hint: If ETag matches If-None-Match, send 304 no body [OK]

Common Mistakes:

• Sending JSON body with 304 status
• Confusing 304 with 404 or 500 errors
• Ignoring If-None-Match header

4. Identify the bug in this Express code that tries to set Cache-Control and ETag headers:

app.get('/resource', (req, res) => {
  const content = 'data';
  res.setHeader('Cache-Control', 'max-age=600');
  res.set('ETag', 'abc123');
  res.send(content);
});

medium

A. Using both res.setHeader and res.set causes header conflicts

B. ETag header value must be a number, not a string

C. Cache-Control max-age should be in milliseconds, not seconds

D. No bug; code sets headers correctly

Solution

1. Step 1: Check header setting methods

   In Express, res.setHeader and res.set both set headers correctly; using both is allowed and does not conflict.

2. Step 2: Validate header values

   ETag can be any string value. Cache-Control max-age is in seconds, so '600' is valid for 10 minutes.

3. Final Answer:

   No bug; code sets headers correctly -> Option D

4. Quick Check:

   res.setHeader and res.set both valid; ETag string OK [OK]

Hint: res.set and res.setHeader both valid for headers [OK]

Common Mistakes:

• Thinking ETag must be numeric
• Confusing max-age units (seconds vs milliseconds)
• Assuming res.setHeader and res.set conflict

5. You want to implement HTTP caching in Express so that clients cache a JSON response for 5 minutes, but also use ETag to validate changes. Which code snippet correctly combines Cache-Control and ETag headers to achieve this?

hard

A. res.set({ 'Cache-Control': 'max-age=300', ETag: 'xyz789' }); res.json(data);

B. res.set('Cache-Control', 'no-store'); res.set('ETag', 'xyz789'); res.json(data);

C. res.set('Cache-Control', 'max-age=300'); res.removeHeader('ETag'); res.json(data);

D. res.set('Cache-Control', 'max-age=300, no-cache'); res.set('ETag', 'xyz789'); res.json(data);

Solution

1. Step 1: Set Cache-Control for 5 minutes

   5 minutes = 300 seconds, so max-age=300 is correct to allow caching for that duration.

2. Step 2: Include ETag header for validation

   Setting ETag allows clients to check if the cached version is still valid before using it.

3. Step 3: Analyze other options

   res.set('Cache-Control', 'no-store'); res.set('ETag', 'xyz789'); res.json(data); disables caching with 'no-store'. res.set('Cache-Control', 'max-age=300'); res.removeHeader('ETag'); res.json(data); removes ETag, losing validation. res.set('Cache-Control', 'max-age=300, no-cache'); res.set('ETag', 'xyz789'); res.json(data); uses conflicting directives 'max-age=300, no-cache' which disables caching despite max-age.

4. Final Answer:

   res.set({ 'Cache-Control': 'max-age=300', ETag: 'xyz789' }); res.json(data); -> Option A

5. Quick Check:

   Cache-Control max-age + ETag = proper caching [OK]

Hint: Combine max-age with ETag for caching plus validation [OK]

Common Mistakes:

• Using no-store disables caching
• Removing ETag loses validation benefits
• Combining max-age with no-cache cancels caching