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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 software wallets for phones- think of those like the wallet you carry with you to keep cash on you, and your hardware wallet is your bank. Good practice not to store any more than a couple hundred dollars worth on a software wallet. I recommend BlueWallet due to its support of Lightning and ability to integrate with your node should you ever decide to go that far: https://bluewallet.io
Mastering Bitcoin is also offered free on Github where it's somewhat maintained: https://github.com/bitcoinbook/bitcoinbook
NOTE:
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Hence, a higher number means a more popular project.