What Are the Fundamentals of Microservices Design?

This is probably one of the most frequently asked microservices interview questions. Here is what you need to keep in mind while answering to it:

Define a scope.
Combine loose coupling with high cohesion.
Create a unique service which will act as an identifying source, much like a unique key in a database table.
Creating the correct API and taking special care during integration.
Restrict access to data and limit it to the required level.
Maintain a smooth flow between requests and response.
Automate most processes to reduce time complexity.
Keep the number of tables to a minimum level to reduce space complexity.
Monitor the architecture constantly and fix any flaw when detected.
Data stores should be separated for each microservice.
For each microservice, there should be an isolated build.
Deploy microservices into containers.
Servers should be treated as stateless.

You can also follow this article explaining 9 Fundamentals to a Successful Microservice Design.