Ask HN: Best way to open up an internal API between your two servers

We are a small startup of 5 people. Between people coming in and out and poor technical decisions, we ended up with two backend: one in Ruby and one in Go. Both are serving public data via a federated GraphQL API. Most of business domain logics are encapsulated in one server. But we have some shared logic that needs to be implemented in both. Instead of duplicating logic, I want to write these shared 'services' in the new backend (Go) and expose an internal API for Ruby to consume. What's the low lift way for a startup to implement something like this? Is it just a simple REST API consumed via JSON with an API key in the header?