How to Build an Open-Domain Question Answering System?

[Updated on 2020-11-12: add an example on closed-book factual QA using OpenAI API (beta).

A model that can answer any question with regard to factual knowledge can lead to many useful and practical applications, such as working as a chatbot or an AI assistant🤖. In this post, we will review several common approaches for building such an open-domain question answering system.