Spike: Existing Open-Source Tools
Summary
There are credible community examples, but they split into two categories:
- point solutions for a narrow provider pair
- broader sync engines that may already cover Outlook and Apple
Candidate tools
MShekow/outlook-calendar-sync
Use when:
- you want a Power Automate package for Outlook-to-Outlook
Pros:
- directly relevant
- concrete algorithm
- quick proof of concept
Cons:
- Outlook-focused
- Power Automate operational limits still apply
phw198/OutlookGoogleCalendarSync
Use when:
- you want a mature example of calendar mapping, privacy controls, and bidirectional sync behavior
Pros:
- long-lived project
- proven sync concepts
- privacy and filtering features
Cons:
- Google-focused rather than Apple-focused
- desktop-tool operational model
ridafkih/keeper.sh
Use when:
- you want the closest open-source example of a self-hostable multi-provider sync engine
Pros:
- explicitly targets Google, Outlook, Office 365, iCloud, CalDAV, and ICS
- self-hostable
- close to the architecture Marvin likely needs
Cons:
- still requires evaluation of its exact mirror semantics
- may do more than you need
Recommendation
Evaluate keeper.sh first as the likely base to adapt.
Keep MShekow/outlook-calendar-sync as an algorithm reference and fallback proof of concept for two Microsoft calendars.
Treat OutlookGoogleCalendarSync as a design reference, not the main base.