ETag and conditional requests in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an ETag in HTTP?

An ETag (Entity Tag) is a unique identifier assigned by a server to a specific version of a resource. It helps browsers know if the resource has changed since the last time it was fetched.

beginner

How does a conditional request use an ETag?

A client sends the ETag it has in the 'If-None-Match' header. The server compares it with the current ETag. If they match, the server replies with 304 Not Modified, saving bandwidth by not sending the full resource again.

beginner

Which HTTP status code indicates the resource has not changed when using ETag conditional requests?

The status code 304 Not Modified means the resource has not changed and the client can use its cached version.

intermediate

In Node.js, which module is commonly used to handle ETag generation and conditional requests?

The 'http' module can be used directly, but frameworks like Express.js provide middleware (e.g., 'etag' middleware) to automatically generate and check ETags for responses.

beginner

Why are ETags useful for improving web performance?

ETags reduce unnecessary data transfer by letting clients know if their cached copy is still valid. This saves bandwidth and speeds up page loads by avoiding full downloads when content hasn't changed.

What header does a client send to perform a conditional request using an ETag?

A If-Match

B If-None-Match

C ETag

D Cache-Control

The client sends the 'If-None-Match' header with the ETag value to ask the server if the resource has changed.

What does the server respond with if the ETag matches the client's version?

A 304 Not Modified

B 500 Internal Server Error

C 404 Not Found

D 200 OK with full content

A 304 Not Modified response tells the client to use its cached version because the resource has not changed.

Which Node.js framework commonly helps manage ETags automatically?

A Express.js

B React

C Vue.js

D Angular

Express.js has built-in support and middleware to handle ETags and conditional requests.

What is the main benefit of using ETags in web applications?

A Speeds up server processing by caching requests

B Improves security by encrypting data

C Reduces bandwidth by avoiding sending unchanged data

D Allows multiple users to edit the same resource simultaneously

ETags help reduce bandwidth by letting clients reuse cached data when the resource hasn't changed.

If a client sends an ETag that does not match the server's current ETag, what happens?

A Server sends 304 Not Modified

B Server closes the connection

C Server sends 403 Forbidden

D Server sends 200 OK with updated resource

If the ETags don't match, the server sends the full updated resource with a 200 OK status.

Explain how ETags and conditional requests work together to optimize web resource loading.

Think about how your browser checks if a file has changed before downloading it again.

Describe how you would implement ETag support in a Node.js server using Express.

Express has built-in features to help with ETags and conditional requests.

Practice

1. What is the main purpose of using an ETag in HTTP responses in Node.js?

easy

A. To set the cache expiration time

B. To encrypt the response data for security

C. To specify the content type of the response

D. To help browsers check if the content has changed and avoid re-downloading unchanged data

Solution

1. Step 1: Understand what ETag represents

   ETag is a unique identifier for a specific version of a resource, used to detect changes.

2. Step 2: Identify its role in HTTP caching

   Browsers use ETag to check if the resource changed since last download, avoiding unnecessary data transfer.

3. Final Answer:

   To help browsers check if the content has changed and avoid re-downloading unchanged data -> Option D

4. Quick Check:

   ETag purpose = avoid re-download [OK]

Hint: ETag means "same content? skip download" [OK]

Common Mistakes:

• Confusing ETag with encryption
• Thinking ETag sets content type
• Mixing ETag with cache expiration headers

2. Which of the following is the correct way to set an ETag header in a Node.js HTTP response?

easy

A. res.setHeader('ETag', '12345')

B. res.header('etag', 12345)

C. res.etag = '12345'

D. res.set('ETag', 12345)

Solution

1. Step 1: Recall Node.js HTTP response header syntax

   Use res.setHeader(name, value) to set headers correctly.

2. Step 2: Check the correct header name and value type

   Header name is case-insensitive but usually 'ETag', and value must be a string.

3. Final Answer:

   res.setHeader('ETag', '12345') -> Option A

4. Quick Check:

   Set header with res.setHeader('ETag', value) [OK]

Hint: Use res.setHeader('ETag', 'value') to set ETag [OK]

Common Mistakes:

• Using incorrect method names like res.header or res.set
• Passing number instead of string as header value
• Trying to assign ETag directly as a property

3. Consider this Node.js code snippet handling a request with an ETag check:

const etag = 'abc123';
if (req.headers['if-none-match'] === etag) {
  res.statusCode = 304;
  res.end();
} else {
  res.setHeader('ETag', etag);
  res.end('Hello World');
}

What will the server respond if the client sends header If-None-Match: abc123?

medium

A. Respond with status 200 and body 'Hello World'

B. Respond with status 500 Internal Server Error

C. Respond with status 304 and no body

D. Respond with status 404 Not Found

Solution

1. Step 1: Check the condition comparing If-None-Match header

   The code compares req.headers['if-none-match'] to etag which is 'abc123'. If equal, it sets status 304.

2. Step 2: Understand HTTP 304 meaning

   Status 304 means Not Modified, so no body is sent and client uses cached content.

3. Final Answer:

   Respond with status 304 and no body -> Option C

4. Quick Check:

   If-None-Match matches ETag = 304 no body [OK]

Hint: If-None-Match equals ETag means 304 Not Modified [OK]

Common Mistakes:

• Thinking 304 responses include body content
• Confusing 304 with 200 OK
• Ignoring the If-None-Match header check

4. This Node.js code tries to handle ETag conditional requests but has a bug:

const etag = 'xyz789';
if (req.headers['If-None-Match'] === etag) {
  res.statusCode = 304;
  res.end();
} else {
  res.setHeader('etag', etag);
  res.end('Content');
}

What is the bug causing the ETag check to fail?

medium

A. ETag value should be a number, not a string

B. Header name case mismatch: 'If-None-Match' vs 'if-none-match'

C. res.end() is missing a callback function

D. res.statusCode must be set after res.end()

Solution

1. Step 1: Check header name case sensitivity in Node.js

   Node.js lowercases all incoming header names, so req.headers['If-None-Match'] is undefined; correct is req.headers['if-none-match'].

2. Step 2: Confirm impact on ETag comparison

   Because the header is undefined, the condition never matches, so 304 is never sent.

3. Final Answer:

   Header name case mismatch: 'If-None-Match' vs 'if-none-match' -> Option B

4. Quick Check:

   Headers lowercase in Node.js = use 'if-none-match' [OK]

Hint: Node.js headers are lowercase keys [OK]

Common Mistakes:

• Using wrong case for header keys
• Thinking ETag must be numeric
• Misordering res.statusCode and res.end()

5. You want to optimize your Node.js server to use ETags and respond with 304 when content is unchanged. Which approach correctly combines generating an ETag and handling conditional requests?

hard

A. Generate a hash of the response body, set it as ETag header, then compare with req.headers['if-none-match'] to decide 304 or 200

B. Always send status 200 with ETag header, ignoring If-None-Match header

C. Set ETag header only on POST requests and ignore GET requests

D. Use res.setHeader('Cache-Control', 'no-store') to disable caching instead of ETags

Solution

1. Step 1: Understand ETag generation

   ETags should uniquely represent the current content, often by hashing the response body.

2. Step 2: Use conditional request header to decide response

   Compare req.headers['if-none-match'] with generated ETag; if equal, respond 304, else send content with new ETag.

3. Final Answer:

   Generate a hash of the response body, set it as ETag header, then compare with req.headers['if-none-match'] to decide 304 or 200 -> Option A

4. Quick Check:

   Hash body -> set ETag -> compare If-None-Match -> 304 or 200 [OK]

Hint: Hash content, set ETag, compare If-None-Match for 304 [OK]

Common Mistakes:

• Ignoring If-None-Match header
• Sending 200 always without checking ETag
• Misusing Cache-Control to replace ETag logic