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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.
I love the Grape gem for building APIs - might be worth considering if you are building something new: https://github.com/ruby-grape/grape
While doing basic REST APIs are sometimes a great solution and super easy (like Grape mentioned in another comment) and much faster - if this is a larger project you might want to check out https://graphql-ruby.org/ ... it makes typical asks of clients like "but I want to know all the schools that user is in and, for each school, it's email, phone and fb" much easier. It also makes fine grained security pretty simple. It's also pretty standard in tons of projects in many different languages.
Second most important stop: api.rubyonrails.org
Jobs or Tasks - https://github.com/mperham/sidekiq
Performance/Monitoring - https://appsignal.com/
Authentication - https://github.com/nsarno/knock