If you're looking for a front-end library for autocompletion, Twitter's typeahead.js is a nice one: https://github.com/twitter/typeahead.js
If you want one that works seamlessly with your React setup, React Autosuggest is pretty neat: https://github.com/moroshko/react-autosuggest
Have you considered using FoundationDB as a storage layer to match that feature?
Disclaimer: I have no idea how to build a distributed search engine.
Having said that, exploring a FoundationDB integration is definitely an interesting idea. However, quite a lot of use cases can be served perfectly well with a simple master+slave set-up, so my primary focus is on that until there is enough demand for horizontal scalability. For e.g. Typesense is not a great fit for things like log data that typically need large amounts of storage.