Monolithic architectures, its very easy to query different entities, Querying data across multiple tables is straightforward.

▪ Microservices architectures uses polyglot persistence, has different databases, need strategy to manage queries.

▪ What if the client requests are visit more than one internal microservices ?

▪ E-commerce application we have product, basket, discount, ordering microservices that needs to interact each other to perform customer use cases.

▪ Integrations are querying each services data for aggregation or perform logics.

https://medium.com/@john_freeman/querying-data-across-microservices-8d7a4667668a