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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.
There are in-memory databases such as H2 which you can use for testing that is just a library you import. However, syntax can vary between databases. So it's only really appropriate if you're also using something like Hibernate which abstracts away a lot of the differences.
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