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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
EteSync is a cloud backup and synchronization program that is secure, end-to-end encrypted, and respects your privacy (e.g. contacts and calendars).
DecSync - Using file synchronization software like Syncthing, DecSync can synchronize RSS, contacts, and calendars without a server.
Nextcloud is a client-server application that allows you to create and use file hosting services.
Tip: Use Cloud backups - Exporting your calendar and/or contacts on a regular basis and storing them up on a separate storage device or uploading them to cloud storage is a good idea (ideally after encrypting them).