ETag and conditional requests in Node.js - Step-by-Step Execution

Concept Flow - ETag and conditional requests

Client sends GET request

↓

Server generates ETag for resource

↓

Server checks If-None-Match header

↓

ETag matches

↓

Respond 304

↓

Client uses cached resource

The client requests a resource, the server creates an ETag, compares it with the client's If-None-Match header, and responds with 304 if unchanged or 200 with data if changed.

Execution Sample

Node.js

const etag = generateETag(resource);
if (req.headers['if-none-match'] === etag) {
  res.status(304).end();
} else {
  res.setHeader('ETag', etag);
  res.status(200).send(resource);
}

Server checks client's ETag and responds with 304 if resource unchanged, else sends resource with new ETag.

Execution Table

Step	Client Request	Server ETag	If-None-Match Header	Condition	Server Response	Client Action
1	GET /resource	"abc123"	None	No If-None-Match header	200 + resource + ETag "abc123"	Cache resource with ETag "abc123"
2	GET /resource	"abc123"	"abc123"	ETag matches	304 Not Modified	Use cached resource
3	GET /resource	"def456"	"abc123"	ETag differs	200 + resource + ETag "def456"	Update cache with new resource and ETag
4	GET /resource	"def456"	"def456"	ETag matches	304 Not Modified	Use cached resource

💡 Requests stop when client uses cached resource or receives updated resource.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
etag	undefined	"abc123"	"abc123"	"def456"	"def456"
req.headers['if-none-match']	undefined	undefined	"abc123"	"abc123"	"def456"
res.statusCode	undefined	200	304	200	304

Key Moments - 3 Insights

Why does the server respond with 304 instead of 200 sometimes?

When the client's If-None-Match header matches the server's current ETag (see execution_table rows 2 and 4), the server knows the client has the latest version and sends 304 to avoid resending data.

What happens if the client sends no If-None-Match header?

The server treats it as a new request and sends the full resource with a 200 status and an ETag header (see execution_table row 1).

How does the client know to update its cache?

When the server responds with 200 and a new ETag different from the cached one (see execution_table row 3), the client replaces its cached resource and ETag.

Visual Quiz - 3 Questions

Look at the execution table, what is the server response at step 2?

A 200 with resource and ETag

B 404 Not Found

C 304 Not Modified

D 500 Internal Server Error

💡 Hint

Check the 'Server Response' column at step 2 in the execution_table.

At which step does the server send a new ETag "def456"?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Server ETag' and 'Server Response' columns in the execution_table.

If the client never sends an If-None-Match header, what will the server response be at first request?

A 200 with resource and ETag

B 304 Not Modified

C 400 Bad Request

D 503 Service Unavailable

💡 Hint

Refer to step 1 in the execution_table where If-None-Match is None.

Concept Snapshot

ETag and Conditional Requests:
- Server generates an ETag for a resource.
- Client sends If-None-Match header with cached ETag.
- If ETag matches, server replies 304 Not Modified (no body).
- If ETag differs or missing, server sends 200 with resource and new ETag.
- Saves bandwidth and speeds up loading by avoiding unnecessary data transfer.

Full Transcript

This visual trace shows how ETag and conditional requests work in Node.js. The client requests a resource. The server creates an ETag string representing the resource version. The client sends this ETag back in the If-None-Match header on subsequent requests. The server compares the client's ETag with the current one. If they match, the server responds with 304 Not Modified, telling the client to use its cached copy. If they differ or the client sends no ETag, the server sends the full resource with a 200 status and the new ETag. This process saves bandwidth and improves performance by avoiding sending unchanged data.

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