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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.
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plaintextaccounting
The plaintextaccounting.org website, a portal to Ledger, hledger, beancount and co. Also the PTA wiki.
The hledger project could be a helpful resource (source code).
They use plain text files instead of a database which is massively better! Indeed, I would say the main purpose of these "Database Management Systems" in this domain is vendor lock-in.
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.