[HelloNurse]

Are you advocating sending and processing notifications about customer changes so that the order component can maintain redundant stale copies of customer data? Why would one do that instead of an appropriate and selective query when e.g. a new order is being entered?

[hvidgaard]

For a MSA system, yes it should maintain just enough knowledge about customers to work, not everything. For instance it does not need to know the customers name, just the id. Systems that query the orders can query the customer component for additional data.

The denormalization and distribution of redundant data is required for it to scale. If you make the order component query the customer component, you haven't solved the problem from the other way around, and suddenly you have a hard coupling where a transient failure in one component automatically fails the other.

It might not be a tradeoff you're willing to make, but then you probably do not need the scaling - at least not along that vector.