reverse proxy, with either rails or phoenix in front.. (in your case it sounds like you would keep rails in front, given a slower tempo)
phoenix in front:
https://blog.fourk.io/replace-your-production-api-with-elixir-today-4426a8903642
rails in front:
https://littlelines.com/blog/2016/09/27/using-phoenix-with-a-legagy-rails-app
depending on your setup/use case you can also run with two URIs on the client - (eg if you do header auth or something)
