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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.
Use Anki. It is a flashcard program, and I believe it has some cool geography sets that also includes flags and capitals. Try this one to start out with: https://ankiweb.net/shared/info/2109889812.
NOTE:
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Hence, a higher number means a more popular project.