Last-Modified and If-Modified-Since in Rest API - Step-by-Step Execution

Concept Flow - Last-Modified and If-Modified-Since

Client sends GET request

↓

Server checks resource last modified date

↓

Does client send If-Modified-Since header?

No→Send full resource with 200 OK

Yes↓

Compare If-Modified-Since date with resource last modified

↓

End

The client sends a GET request. The server checks if the client sent an If-Modified-Since date. If yes, it compares with the resource's last modified date. If the resource is unchanged, server replies 304 Not Modified. Otherwise, it sends the full resource with 200 OK.

Execution Sample

Rest API

GET /image.png HTTP/1.1
Host: example.com
If-Modified-Since: Wed, 21 Oct 2020 07:28:00 GMT

Server response:
HTTP/1.1 304 Not Modified

(no body)

Client requests an image with If-Modified-Since header. Server replies 304 Not Modified if image unchanged since that date.

Execution Table

Step	Client Request	Server Action	Condition	Server Response
1	GET /image.png (no If-Modified-Since)	Check resource last modified date	No If-Modified-Since header	200 OK with full image
2	GET /image.png with If-Modified-Since: 2020-10-21T07:28:00Z	Compare If-Modified-Since with resource last modified	Resource last modified 2020-10-20T10:00:00Z < If-Modified-Since	304 Not Modified, no body
3	GET /image.png with If-Modified-Since: 2020-10-21T07:28:00Z	Compare If-Modified-Since with resource last modified	Resource last modified 2020-10-22T09:00:00Z > If-Modified-Since	200 OK with full image
4	End	No further action	Request handled	Connection closed

💡 Execution stops after server sends response based on resource modification check.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
If-Modified-Since header	None	None	Wed, 21 Oct 2020 07:28:00 GMT	Wed, 21 Oct 2020 07:28:00 GMT	Wed, 21 Oct 2020 07:28:00 GMT
Resource Last-Modified	2020-10-20T10:00:00Z	2020-10-20T10:00:00Z	2020-10-20T10:00:00Z	2020-10-22T09:00:00Z	2020-10-22T09:00:00Z
Server Response	None	200 OK with full image	304 Not Modified	200 OK with full image	200 OK with full image or 304 Not Modified

Key Moments - 3 Insights

Why does the server send a 304 Not Modified response instead of the full resource?

Because the If-Modified-Since date sent by the client is later than the resource's last modified date, meaning the client already has the latest version. See execution_table row 2.

What happens if the client does not send the If-Modified-Since header?

The server sends the full resource with a 200 OK response because it has no information to check if the client has a fresh copy. See execution_table row 1.

If the resource was modified after the If-Modified-Since date, what response does the server send?

The server sends the full resource with a 200 OK response to update the client. See execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution_table row 2. What is the server's response when the resource last modified date is before the If-Modified-Since date?

A 404 Not Found

B 304 Not Modified with no body

C 200 OK with full resource

D 500 Internal Server Error

💡 Hint

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

According to variable_tracker, what is the value of 'If-Modified-Since header' after step 1?

A None

B Wed, 21 Oct 2020 07:28:00 GMT

C 2020-10-20T10:00:00Z

D 2020-10-22T09:00:00Z

💡 Hint

Look at the 'If-Modified-Since header' row and the 'After Step 1' column in variable_tracker.

If the resource last modified date changes to a date after the If-Modified-Since date, how does the server response change?

A It sends 304 Not Modified

B It sends 404 Not Found

C It sends 200 OK with full resource

D It sends 500 Internal Server Error

💡 Hint

Refer to execution_table row 3 where resource last modified is after If-Modified-Since.

Concept Snapshot

Last-Modified and If-Modified-Since headers help cache validation.
Client sends If-Modified-Since with a date.
Server compares with resource's Last-Modified date.
If resource unchanged, server replies 304 Not Modified (no body).
If changed, server replies 200 OK with full resource.
This saves bandwidth and speeds up loading.

Full Transcript

This visual trace shows how the Last-Modified and If-Modified-Since headers work in HTTP caching. The client sends a GET request optionally with an If-Modified-Since header containing a date. The server checks the resource's last modified date. If the client did not send If-Modified-Since, the server sends the full resource with 200 OK. If the client sent If-Modified-Since, the server compares it with the resource's last modified date. If the resource has not changed since that date, the server sends a 304 Not Modified response with no body, telling the client to use its cached copy. If the resource has changed, the server sends the full resource with 200 OK. The execution table shows different scenarios and server responses. The variable tracker shows how the headers and dates change during the process. Key moments clarify common confusions about when 304 is sent and what happens without If-Modified-Since. The quiz tests understanding of these steps. This mechanism helps reduce data transfer and speeds up web browsing by avoiding unnecessary downloads.