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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.
They have client side and server side libraries. I've used it on a project recently where I used the python library with flask on the server and the js library on the frontend with Vue. This means you do not need to build an API between your client and server for database specific things, which is awesome. I hate the boilerplate REST api code that I've built 100 times. The server app can push code into the database, and the client can subscribe to changes and get updates in real time with a few lines of code.
Most recently, I have been leaning towards Firebase for apps that need a lot of real time data, for the sake of simplicity.