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[ADP-370] HTTP Caching: Last-Modified ​

Procedures ​

  1. The server includes a Last-Modified header in the response, indicating when the resource was last changed.
  2. The client stores this value and includes it in subsequent requests using the If-Modified-Since header.
  3. The server compares the If-Modified-Since date with the resource's last modification date.
  4. If the resource hasn't been modified since the date in If-Modified-Since, the server responds with a 304 Not Modified status.

Guidance ​

  • SHOULD use Last-Modified in the response header for caching when appropriate.
  • MUST ensure that the Last-Modified date is accurate and updated whenever the resource changes.
  • SHOULD implement proper handling of If-Modified-Since requests on the server-side.
  • MAY use Last-Modified in conjunction with ETag for more robust caching mechanisms, as described in ADP-134.

Implementation Details ​

  • The browser automatically attaches If-Modified-Since in subsequent requests. For non-browser clients (e.g., command-line tools like curl or server-side requests), you may need to manually include the If-Modified-Since header.
  • Servers should implement a reliable method to track and update the Last-Modified date. This could involve database timestamps, file system metadata, or other appropriate mechanisms.
  • When responding to an If-Modified-Since request:
    • If the resource has not been modified, return a 304 Not Modified status with no body.
    • If the resource has been modified, return a 200 OK status with the full resource and an updated Last-Modified header.

Best Practices ​

  1. Use Last-Modified for resources that change infrequently and have a clear last modification time.
  2. Combine Last-Modified with ETag for more precise caching control, as outlined in ADP-134.
  3. Ensure your server's clock is synchronized to avoid issues with time-based caching.
  4. Be cautious with dynamically generated content; ensure the Last-Modified date accurately reflects content changes.
  5. Consider using Cache-Control headers in addition to Last-Modified for more granular cache control, as discussed in ADP-134.

Example ​

Server Response (Initial Request) ​

http
HTTP/1.1 200 OK
Content-Type: application/json
Last-Modified: Wed, 21 Oct 2023 07:28:00 GMT
Cache-Control: public, max-age=3600

{
  "data": "This is the resource content"
}

Subsequent Client Request ​

http
GET /resource HTTP/1.1
If-Modified-Since: Wed, 21 Oct 2023 07:28:00 GMT

Server Response (Not Modified) ​

http
HTTP/1.1 304 Not Modified

Security Considerations ​

  • Ensure that Last-Modified dates do not reveal sensitive information about your system or update patterns.
  • Be aware that clients can manipulate the If-Modified-Since header; always validate it server-side.
  • Consider the implications of caching on sensitive or user-specific data.

References ​