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InfluxDB
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As for the original question, OP you should check out https://github.com/ueberauth/guardian if you want to roll your own auth. If not rolling you own you can just use a standard Oauth provider and validate their tokens (surprise they are JWTs) in you backend using their public keys.
For a standard Open API styled json api, I've used open-api-spec and it worked pretty well. It gives you Swagger out of the box and some nice validation tools.
The Elixir ecosystem loves GraphQL so, if you're interested in going that direction, there's good tooling and plenty of support with absinthe.