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Alpaca-API
The Alpaca API is a developer interface for trading operations and market data reception through the Alpaca platform.
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finnhub-python
Finnhub Python API Client. Finnhub API provides institutional-grade financial data to investors, fintech startups and investment firms. We support real-time stock price, global fundamentals, global ETFs holdings and alternative data. https://finnhub.io/docs/api
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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.
He said he uses Finnhub right there :). I haven't tried Finnhub, but I use Alpaca which lets you do 200 calls per minute, also unlimited and free. I haven't tried trading on their live market yet, but their paper market is pretty nice. This is of course not financial advice :).
That depends on how you integrate with it. The Finnhub Python API has example code, but it boils down to http calls, so it should work in any language provided you use the right calls.