e-tag identifier for a specific version of a resource

For scraping and fetching data from the htwkalender ical service its interesting to implement the e-tag id. This id should change if there is something new in the requested resource. In our case the requested feeds should contain this attribute.

Calculation should be done in a new table with changelogs that contain logs of changed module ids that could then compared with the feed module ids and a response could be send.

The Easiest Version is to hash the content of the feed, as suggested in the docs:
https://developer.mozilla.org/docs/Web/HTTP/Headers/ETag