By microservice architecture here I mean an app split in independent parts, connected via MQ, database, internal RPC or independent APIs. Usually deployed as containerized microapps onto a horizontally scaled cluster.
Main problems with ORM
Different technology stack
Most likely you have a mixed tech stack, using different tech/framework/language for different services. Some parts are JVM apps, but others are easier to write using Golang or Python. In some cases you use an external 3rd party app, customized for your needs.
Lifecycle and backward compatibility
You probably don’t want to restart whole cluster, every single service, just to upgrade some part of the db layer. So you have to deal with different versions of Data Models, that should live together in same shared environment (remember «caching»)
Subset of data
Another important moment that each microservice operates own subset of data, and does different types of operations against the data. It’s even common to split app into microservices that do only following:
- put data into db from a 3rd party storage
- process/transform stored data
- display data for end user