Here's an abstract question with real world implications.
I have two microservices; let's call them the CreditCardsService and the SubscriptionsService.
I also have a SPA that is supposed to use the SubscriptionsService so that customers can subscribe. To do that, the SubscriptionsService has an endpoint where you can POST a subscription model to create a subscription, and in that model is a creditCardId that points to a credit card that should pay for the subscription. There are certain business rules that say whether or not you can use said credit card for the subscription (expiration is more than 12 months away, it's a VISA, etc). These specific business rules are tied to the SubscriptionsService
The problem is that the team working on the SPA want a /CreditCards endpoint in the SubscriptonsService that returns all valid credit cards of the user that can be used in the subscriptions model. They don't want to implement the same business validation rules in the SPA that are in the SubscriptionsService itself.
To me this seems to go against the SOLID principles that are central to microservice design; specifically separation of concerns. I also ask myself, what precedent is this going to set? Are we going to have to add a /CreditCards endpoint to the OrdersService or any other service that might use creditCardId as a property of it's model?
So the main question is this: What is the best way to design this? Should the business validation logic be duplicated between the frontend and the backend? Should this new endpoint be added to the SubscriptionsService? Should we try to simplify the business logic?
In microservices and DDD the subscriptions service should have a credit cards endpoint if that is data that is relevant to the bounded context of subscriptions.
The creditcards endpoint might serve a slightly different model of data than you would find in the credit cards service itself, because in the context of subscriptions a credit card might look or behave differently. The subscriptions service would have a creditcards table or backing store, probably, to support storing its own schema of creditcards and refer to some source of truth to keep that data in good shape (for example messages about card events on a bus, or some other mechanism).
This enables 3 things, firstly the subscriptions service wont be completely knocked out if cards goes down for a while, it can refer to its own table and work anyway. Secondly your domain code will be more focused as it will only have to deal with the properties of credit cards that really matter to solving the current problem. Finally if your cards store can even have extra domain specific properties that are computed and materialized on store.
Obligatory Fowler link : Bounded Context Pattern