I am currently working on a project where we use microservice architecture. I am also somewhat new to this architecture and have had a few concerns. I understand the concept of microservices in general and also how we can have one database per service. This brings me to a point where I get confused on how to pull data from different databases for a particular user.

Scenario Assuming I have a Users and a Posts service with their schema like this

User

const schema = {
  name: String
  id: String
  ...
}

Post

const schema = {
  text: String
  user: Id // reference of the user who made this post.
}

Now on the UI, I want to load a set of posts and the associated users who made the post, how do I get a Post alongside the User who made the respective Post. I am using MongoDB, how do I populate data that are stored in other databases? I am also using Kafka handle async operations, how do I leverage Kafka for this usecase? Or is there a much better way of doing this? The final response of a Post could be something like this.

{
 text: 'Some random message',
 user: {
  name: 'John Doe',
  id: 1234
 }
}

Also, I know I could make a call to the User service to get the User, then make a call to the Post service to get the Post and merge both objects together, but is there a much better option than this basically? I am thinking in cases where I want to do multiple lookups for a user, e.g to get a User and their associated Posts, Messages, etc, how can I handle scenarios like this, are their any techniques I could leverage for situations like this?

Thank you in advance!