HTTP caching headers (ETag, Cache-Control) in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ETag HTTP header?

The ETag header provides a unique identifier for a specific version of a resource. It helps browsers and servers check if the resource has changed, so the browser can use a cached version if it hasn't.

beginner

Explain the role of the Cache-Control header in HTTP caching.

The Cache-Control header tells browsers and proxies how to store and reuse responses. It can specify how long to keep a resource, whether to revalidate it, or if it should not be cached at all.

intermediate

How does Express help you set ETag headers automatically?

Express automatically generates ETag headers for responses by default, based on the response body. This helps with efficient caching without extra code.

beginner

What does the Cache-Control: no-store directive do?

It tells browsers and proxies not to store any part of the response. This means the resource will always be fetched fresh from the server.

beginner

Why is it important to use caching headers like ETag and Cache-Control in web applications?

Using caching headers improves performance by reducing unnecessary data transfer. It saves bandwidth, speeds up page loads, and reduces server load by letting browsers reuse cached resources when possible.

What does the ETag header represent?

A The size of the resource in bytes

B A unique version identifier for a resource

C The expiration time of a cached resource

D The server's IP address

ETag is a unique identifier for a resource version, used to check if the cached copy is still valid.

Which Cache-Control directive tells the browser to always fetch a fresh copy?

A no-cache

B max-age=3600

C public

D private

no-cache means the browser must revalidate with the server before using the cached resource.

In Express, how can you disable automatic ETag generation?

A app.etag = false

B app.disable('etag')

C app.use(etag(false))

D app.set('etag', false)

Use app.set('etag', false) to turn off automatic ETag generation in Express.

What does Cache-Control: max-age=0 mean?

A Cache forever

B Do not cache at all

C Resource is stale immediately and must be revalidated

D Cache for 1 hour

max-age=0 means the cached resource is immediately stale and requires revalidation.

Which header does the browser send to check if the cached resource matches the server's version?

A If-None-Match

B ETag

C Cache-Control

D If-Modified-Since

The browser sends If-None-Match with the ETag value to ask if the resource has changed.

Describe how ETag and Cache-Control headers work together to improve web performance.

Think about how the browser knows when to reuse cached data or ask the server for updates.

Explain how to set caching headers in an Express app to cache a response for 1 hour.

Remember Cache-Control max-age is in seconds.

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