Calendar event data and Contacts data are stored in a PostgresQL database, with some support files in the file system.
When backing up Calendar or Contacts server files, make sure to back up the /Library/Server/Calendar and Contacts/Data/ folder.
Backing up using Time Machine saves all the necessary data for later restoration.
Database files
Calendar Server uses database files for various purposes. It uses PostgresQL database to store calendar data and to store proxy relationships. To troubleshoot or resolve problems, an administrator needs to use PostgresQL database queries. The explanation of postgres database manipulation is beyond the scope of this topic.
To access the database, use the postgres, psql, and pg_ctl command-line tools.
File system files
By default, the root data store location is /Library/Server/Calendar and Contacts/.
The Calendar and Contacts folder contains two folders, Data and Config:
The Data folder contains the sqlite databases for proxies, an xml list of resources and locations in the calendar system, and the Document folder that contains event attachments.
The Config folder holds the service configuration files.
To access the files, you need root access to the /Library/Server/Calendar and Contacts/ folder and its subfolders.