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nut
The Network UPS Tools repository. UPS management protocol Informational RFC 9271 published by IETF at https://www.rfc-editor.org/info/rfc9271 Please star NUT on GitHub, this helps with sponsorships!
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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.
Others have answered the hardware part of your question. I have not seen much mention about the software to control graceful shutdowns. For this I suggest you look at the ”Network UPS Tool” (NUT for short) project (https://networkupstools.org/). It is mostly manufacturer agnostic with clients for a vast array of hardware components.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.