If-Modified-Since
Description
The If-Modified-Since header is an HTTP request header used by clients (such
as browsers or APIs) to tell the server to only send a response if the resource
has been modified since a certain date or time. This header is typically used
for caching purposes to avoid unnecessary data transfer. It helps in reducing
the bandwidth by ensuring that a client only fetches new data when the resource
has been updated.
When a client sends an HTTP request with the If-Modified-Since header, the
server checks if the requested resource has been modified after the date and
time specified in the header. If the resource hasn't changed, the server can
respond with a 304 Not Modified status code, indicating that the client can
continue using its cached version of the resource. Otherwise, the server sends
the updated resource.
This header is commonly used in conjunction with caching mechanisms and RESTful APIs to optimize performance, reduce load on servers, and improve the overall user experience.
Syntax
Where:
- <date> is a timestamp representing the last modified date of the resource,
formatted according to the RFC 7231 standard (e.g., "Mon, 23 Jan 2023 15:05:07
GMT").
Example of If-Modified-Since header in a request:
Examples
Checking if a resource has been modified
Let's say a client has a cached version of a resource, and it wants to check if that resource has been modified since its last fetch.
Scenario: A client previously requested the resource /articles/123 and
received the response with a Last-Modified header of Tue, 24 Jan 2023
10:15:00 GMT. The client now wants to check if there have been any changes to
the article since that time.
Client Request:
Server Response (if the resource has not been modified):
In this case, the server checks the timestamp of the requested resource and
finds that it hasn't changed since the time specified in the If-Modified-Since
header. As a result, it responds with a 304 Not Modified status, which means
the client can continue using its cached version.
Fetching an updated resource
Let's consider a scenario where the resource has been modified since the client’s last request. The client will receive the latest version of the resource.
Client Request:
Server Response (if the resource has been modified):
HTTP/1.1 200 OK
Date: Wed, 15 Feb 2025 08:05:00 GMT
Last-Modified: Wed, 15 Feb 2025 08:00:00 GMT
Content-Type: application/json
Content-Length: 120
{
"article_id": 123,
"title": "Updated Article Title",
"content": "This is the updated content of the article."
}
Here, the server responds with a 200 OK status and the new content of the
resource because it was modified after the If-Modified-Since timestamp. The
Last-Modified header in the response indicates the updated timestamp of the
resource.
Missing If-Modified-Since Header
In cases where the client does not include the If-Modified-Since header, the
server will typically return the full content, assuming no previous caching
mechanism.
Client Request (without If-Modified-Since):
Server Response:
HTTP/1.1 200 OK
Date: Wed, 15 Feb 2025 08:10:00 GMT
Last-Modified: Wed, 15 Feb 2025 08:00:00 GMT
Content-Type: application/json
Content-Length: 120
{
"article_id": 123,
"title": "Article Title",
"content": "This is the full content of the article."
}
Since the If-Modified-Since header is absent, the server assumes that the
client does not have a cached version and sends the full content.
Summary
The If-Modified-Since HTTP request header is a key tool for optimizing web
performance by helping clients avoid unnecessary data transfers. It allows
clients to check whether a resource has been modified since a specified date and
time. If the resource has not changed, the server can respond with a 304 Not
Modified status, signaling that the cached version can still be used. If the
resource has been updated, the server sends the full updated content.
This mechanism is especially useful in reducing bandwidth usage and server load,
enhancing the efficiency of web applications, and making use of caching
strategies for better performance. It is a common practice in conjunction with
Last-Modified and ETag headers to implement cache validation in HTTP-based
communication.