ETag for conditional requests in Rest API - Step-by-Step Execution

Concept Flow - ETag for conditional requests

Client sends GET request

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Server checks resource state

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Server generates ETag for resource

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Server sends response with ETag header

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Client stores ETag

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Client sends conditional GET with If-None-Match: ETag

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Server compares ETag with current resource

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ETag matches

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Send 304 Not Modified

The client requests a resource, server sends it with an ETag. Later, client asks conditionally using that ETag. Server replies 304 if unchanged or 200 with new data if changed.

Execution Sample

Rest API

GET /resource HTTP/1.1
Host: example.com

--- Server Response ---
HTTP/1.1 200 OK
ETag: "abc123"
Content: {"data": "value"}

--- Conditional Request ---
GET /resource HTTP/1.1
If-None-Match: "abc123"
Host: example.com

Client first gets resource with ETag, then sends conditional request with If-None-Match header.

Execution Table

Step	Request Type	ETag Sent by Client	Server ETag	ETag Match?	Server Response
1	Initial GET	None	"abc123"	N/A	200 OK with resource and ETag "abc123"
2	Conditional GET	"abc123"	"abc123"	Yes	304 Not Modified, no body
3	Conditional GET	"abc123"	"def456"	No	200 OK with new resource and ETag "def456"

💡 Execution stops after server responds based on ETag match.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
Client ETag	None	"abc123"	"abc123"	"abc123" or updated to "def456" if resource changed
Server Resource State	"value"	"value"	"value"	"new value" if changed
Server ETag	N/A	"abc123"	"abc123"	"def456"

Key Moments - 3 Insights

Why does the server send 304 Not Modified instead of 200 OK?

Because the client's If-None-Match ETag matches the server's current ETag (see execution_table row 2), meaning the resource has not changed.

What happens if the resource changes on the server?

The server generates a new ETag (see execution_table row 3), so the ETag does not match the client's. The server sends 200 OK with the updated resource and new ETag.

Why does the client send the ETag in the If-None-Match header?

To ask the server if the resource has changed since last fetch, enabling efficient caching and saving bandwidth if unchanged.

Visual Quiz - 3 Questions

Look at the execution_table, what response does the server send at step 2?

A 200 OK with new resource

B 304 Not Modified

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check the 'Server Response' column in execution_table row 2.

At which step does the server detect that the resource has changed?

A Step 1

B Step 2

C Step 3

D None of the above

💡 Hint

Look at the 'ETag Match?' column and 'Server Response' in execution_table.

If the client sends a different ETag in If-None-Match at step 2, what changes in the execution_table?

A Server responds 304 Not Modified

B Server responds 200 OK with new resource

C Client ETag becomes None

D Server ETag becomes None

💡 Hint

Refer to execution_table row 3 where ETag does not match.

Concept Snapshot

ETag is a unique identifier for a resource version.
Client stores ETag from server response.
Client sends If-None-Match with ETag to check freshness.
Server compares ETag; if match, sends 304 Not Modified.
If no match, sends 200 OK with new resource and ETag.
This saves bandwidth by avoiding sending unchanged data.

Full Transcript

This visual execution shows how ETag works for conditional requests in REST APIs. First, the client requests a resource. The server responds with the resource and an ETag header, a unique tag representing the resource version. The client saves this ETag. Later, the client sends a conditional GET request with the If-None-Match header containing the saved ETag. The server compares this ETag with the current resource's ETag. If they match, the server replies with 304 Not Modified, meaning the resource has not changed, so the client can use its cached copy. If they differ, the server sends 200 OK with the updated resource and a new ETag. This process helps reduce data transfer and improves efficiency by avoiding sending unchanged data.