Exchange Calendar to CalDAV sync

This script synchronizes the default calendar of a Microsoft Exchange account with a calendar on a CalDAV server. Currently, only one-way synchronization from Exchange to CalDAV is supported. The synchronization is restricted to events – no tasks – but includes event series.

The script uses the prodigious exchangelib to connect to Exchange's EWS interface and fetch incremental changes from the calendar folder. It then uses the iCal representation in the mimedata field, provided by Exchange, of each changed calendar entry to add or update the entry in the CalDAV calendar. python-caldav is used for interfacing with the CalDAV server.

Before storing the entry, a few changes are made to the data, using the icalendar library:

A new custom property X-EWSSYNC-ITEMID with the EWS ItemId is added to query items for propageting item deletion from the Exchange server
The TRANSP property is set to TRANSPARENT when the event's status is "FREE" according to the 'X-MICROSOFT-CDO-BUSYSTATUS property
The STATUS property is set to TENTATIVE when the event's status is "TENTATIVE" according to the 'X-MICROSOFT-CDO-BUSYSTATUS property

Setup & Usage

Clone this repository

Optional: create a Python virtual environment and activate it

python3 -m virtualenv -p python3 venv && . venv/bin/activate

Install the dependencies
```
pip install -r requirements.txt
```
Copy and customize the config file with the credentials of your Exchange and CalDAV accounts
```
cp config.example.toml config.toml && $EDITOR config.toml
```
Run ews_calendar_sync.py regularly to synchronize changes in the EWS calendar to the CalDAV calendar. Typically, you would do this using a cron job (crontab, etc.) or a systemd Timer Unit. Make sure to use the Python interpreter from the virtualenv (if you use one) and run the process in the working directory where the config.toml file is located.

Alternatively, using the Docker container:

Download the config file template from this repository, save it as config.toml and customize it with the credentials of your Exchange and CalDAV accounts

Run ews_calendar_sync via docker:

docker run -it --rm --name ews-caldav-sync -v ./config.toml:/app/config.toml -v ./syncstate.txt:/app/syncstate.txt ghcr.io/mhthies/ews-caldav-sync:latest

On the first execution, the script will synchronize all contents of the Exchange Calendar to the CalDAV calendar. This may take a while (currently, there is no parallelization implemented). On every subsequent execution, only changes (new events, updated events, deleted events) since the last execution are propagated to the CalDAV server. This is achieved by using the EWS SyncFolderitems service and storing the sync_state token from the Exchange server in the statefile as defined in the config file.

Known Issues

Issues when syncing to a Nextcloud calendar:

Nextcloud (since version 22) forbids recreating/updating a calendar item with the id of a previously deleted item. ("400 Bad Request: Deleted calendar object with uid already exists in this calendar collection.") Updates/recreations of previously deleted events in the Exchange calendar – which often happen due to meeting invitation updates – will result in an exception of the ews_calendar_sync script and the update being ignored in the CalDAV calendar. This issue is caused by Nextcloud's new calendar recycle bin feature and is already reported to Nextcloud: nextcloud/server#30096
Nextcloud sets ContentType to text/html in emtpy responses. The caldav library logs this as a warning to the console ("unexpected content type from server: text/html; charset=UTF-8. Please raise an issue[…]"). Reported to caldav library: python-caldav/caldav#142 Fixed with caldav version 0.8.1.

General issues:

Events with with a slash in their iCal UID cause errors when being uploaded to the CalDAV server, due to unescaped slashes in the CalDAV object path. Exchange does not create such UIDs itself, but they may be imported with existing iCal datasets into Outlook/Exchange Reported to caldav library: python-caldav/caldav#143 Mostly fixed with caldav version 0.8.1.