WebCal export : Support for HTTP header `If-Modified-Since`

[relikd]

Is your feature request related to a problem? Please describe.

Hi there, it seems the caldav integration does ignore basic caching mechanisms. The server always responds with:

HTTP/1.1 200 OK
Date: Mon, 18 Oct 2021 21:38:04 GMT

(which is the current date and time)

This makes all attempts to set the If-Modified-Since header impossible.

GET /remote.php/dav/public-calendars/XXX?export HTTP/1.1
If-Modified-Since: Mon, 18 Oct 2021 21:38:04 GMT
...

The calendar will be downloaded regardless if it has any changes or not.
This is especially data-consuming if you want to sync a calendar every 5 minutes.

Describe the solution you'd like

Instead, the calendar app could return the date of the last modification and respect the If-Modified-Since header and return a HTTP 304 status code. That way no unnecessary data is transmitted.

Describe alternatives you've considered
N/A

Additional context
Use Case scenario: A mirrored ical file that is downloaded every 5 minutes to a different server. It is crucial to reflects recent changes as soon as possible and yet don't download the ical 288 times a day if there are no changes.

[joshtrichards]

You can accomplish this in other ways with CalDav. See:

• https://sabre.io/dav/ics-export-plugin/
• https://sabre.io/dav/building-a-caldav-client/
• https://manual.davx5.com/technical_information.html#synchronization

[relikd]

I dont see how your links help. Can you comment what exactly I am supposed to find in the documentation that will prevent unnecessary data transfer between client and server? There is also no etag which I could check against.