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[ADP-134] Last-Modified ​

Overview ​

The Last-Modified header is an important HTTP response header that indicates the date and time at which the origin server believes the resource was last modified.

Purpose ​

The main purposes of the Last-Modified header are:

  1. To enable caching mechanisms in browsers and proxy servers.
  2. To support conditional requests, allowing clients to check if a resource has been modified since their last retrieval.

Guidance ​

  • Servers MUST include the Last-Modified header for cacheable resources whenever possible.
  • The Last-Modified date SHOULD be as accurate as possible.
  • If the last modification time cannot be determined, servers SHOULD NOT send a Last-Modified header.
  • Clients SHOULD use the If-Modified-Since header in conjunction with Last-Modified for efficient caching.
  • For frequently changing resources, consider using the ETag header as an alternative or supplement.
  • For dynamically generated content, use Last-Modified cautiously, ensuring its accuracy.

Usage ​

Server Response ​

When responding to a GET or HEAD request, a server should include the Last-Modified header if it can determine the last modification time of the requested resource.

Example:

http
HTTP/1.1 200 OK
Content-Type: text/html
Last-Modified: Wed, 21 Oct 2023 07:28:00 GMT

Client Request ​

Clients can use the If-Modified-Since header in subsequent requests to check if the resource has been modified since their last retrieval.

Example:

http
GET /example.html HTTP/1.1
Host: www.example.com
If-Modified-Since: Wed, 21 Oct 2023 07:28:00 GMT
  • ETag: Provides a more precise mechanism for validating cached resources.
  • If-Modified-Since: Used in client requests to conditionally retrieve resources.
  • Cache-Control: Specifies directives for caching mechanisms in both requests and responses.

References ​