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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.
If you're looking for something slightly less low-level, The Graph is a server architecture/protocol for mutating low-level blockchain data into structured GraphQL endpoints, which could be another way to approach it: https://thegraph.com/
Reading from blockchain. Using blockchain for api read is called an Oracle eg https://chain.link/ but you can also set up your own.
NOTE:
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Hence, a higher number means a more popular project.