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.