If-None-Match and 304 responses in Rest API - Step-by-Step Execution

Concept Flow - If-None-Match and 304 responses

Client sends GET request with If-None-Match header

↓

Server checks resource ETag

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ETag matches

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Send 304 Not Modified

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Client uses cached

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END

The client sends a request with an ETag in If-None-Match. The server compares it to the current resource ETag. If they match, server replies 304 (no new data). Otherwise, server sends the full resource with 200.

Execution Sample

Rest API

GET /resource HTTP/1.1
If-None-Match: "abc123"

Server checks ETag: "abc123"
Response: 304 Not Modified

Client requests a resource with an ETag. Server sees ETag matches current version and replies 304 to save bandwidth.

Execution Table

Step	Client Request	Server ETag	ETag Match?	Server Response	Client Action
1	GET /resource with If-None-Match: "abc123"	"abc123"	Yes	304 Not Modified	Use cached resource
2	GET /resource with If-None-Match: "xyz789"	"abc123"	No	200 OK with resource body	Update cache with new resource
3	GET /resource without If-None-Match	"abc123"	N/A	200 OK with resource body	Cache resource

💡 Requests end after server responds with either 304 or 200 depending on ETag match

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
Client If-None-Match	None	"abc123"	"xyz789"	None
Server ETag	"abc123"	"abc123"	"abc123"	"abc123"
Server Response	None	304 Not Modified	200 OK with resource body	200 OK with resource body
Client Cache	Empty	Cached resource with ETag "abc123"	Updated resource with ETag "abc123"	Cached resource with ETag "abc123"

Key Moments - 3 Insights

Why does the server send a 304 response instead of the full resource?

Because the client's If-None-Match header matches the server's current ETag (see execution_table step 1), meaning the client already has the latest version.

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 200 OK (see execution_table step 3).

Why does the client update its cache after a 200 OK response?

Because the server sent a new resource version (ETag did not match), so the client replaces its cached copy with the fresh data (see execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the server response when the client's If-None-Match matches the server's ETag?

A 304 Not Modified

B 200 OK with resource body

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check execution_table row 1 under 'Server Response'

At which step does the client update its cache with a new resource?

A Step 1

B Step 2

C Step 3

D None of the steps

💡 Hint

Look at execution_table 'Client Action' column for cache updates

If the client sends no If-None-Match header, what response does the server send?

A 400 Bad Request

B 304 Not Modified

C 200 OK with resource body

D 401 Unauthorized

💡 Hint

See execution_table step 3 for requests without If-None-Match

Concept Snapshot

If-None-Match header lets client send ETag to server.
Server compares ETag with current resource version.
If match, server replies 304 Not Modified (no body).
If no match or missing header, server sends 200 OK with resource.
This saves bandwidth by avoiding sending unchanged data.

Full Transcript

This visual trace shows how the If-None-Match header works with 304 responses in REST APIs. The client sends a GET request with an If-None-Match header containing an ETag value. The server compares this ETag with the current resource's ETag. If they match, the server responds with 304 Not Modified, telling the client to use its cached copy. If they don't match or the header is missing, the server sends the full resource with a 200 OK response. The execution table walks through three steps: a matching ETag request, a non-matching ETag request, and a request without the header. The variable tracker shows how the client's If-None-Match, server's ETag, server response, and client cache change over these steps. Key moments clarify why 304 is sent, what happens without the header, and when the client updates its cache. The quiz tests understanding of these steps and responses. This helps beginners see how conditional requests save bandwidth and improve efficiency in web APIs.