filesystem we'll use inotify.
*dav this is TBD.
For JMAP, we can use its native websocket ability.
This might be a good fit for filesystem: https://github.com/notify-rs/notify
vdirsyncer-py discussion on the topic: https://github.com/pimutils/vdirsyncer/issues/24
More discussion on Push notifications for CalDav: https://github.com/nextcloud/server/issues/22379
It might make sense for the API to provide a parameter for specifying an
interval: Duration, which would be used for storages which do not support any "push" mechanism.