Multi-way sync and being service agnostic

[robflate]

Apologies for posting as an issue but there's no Discussion section in the repo.

I am attempting to sync;

Calendars: Google, Apple & CalDAV (Davis)
Address Book: Google, Apple & CardDAV (Davis)

CalDav and CardDav are the sources of truth. They are served from Davis which is a front end for sabre/dav. It's like Baïkal. Davis and Vdirsyncer run on the same machine.

My end goal is to be service agnostic for my contacts and calendars. I will always have the CalDav/CardDav sources of truth and if I ever move away from Apple or Google or start using an additional service, it will be easy for me to setup any new service in Vdirsyncer to quickly get my calendars and contacts synced to the new service.

I have got Vdirsyncer setup and have the basics working. I have authorised Google and can discover Google, Apple and Davis collections but what is the suggested workflow for achieving a Multi-way sync? If anyone can provide a config example, I'd appreciate it. Thanks.