So let's say we have microservices that uses an event broker to communicate each other. To secure sovereignty of data, each microservices has denormalized documents. So whenever the data is changed, from the service changed the data, 'DataAHasChanged' event gets fired. Next, all the microservices that have subscribed this event will change document they have to maintain consistency of data A. (A here is not foreign key, but it's actual data, since it's denormalized)
This seems really not good to me if services have multiple documents that have data A. And if data A is changing often. I would just send API call to other services using data A's ID as a foreign key.
Real world use case would be: User creates 'contract requests' and it has multiple vendor information. Vendors information will be changed often. So if there are 2000 contract requests. It means whenever vendor changes their information. We should go through every contract requests and change the denormalized document.
Is eventual consistency still the best practice in this case? or should I just use synchronous call to just read data from vendor service?
Thank you.
I would revisit the microservices decoupling and would ask a question - who is the source of truth for each type of data? You'll probably arrive to one service owning documents and that service will be responsible for updating those documents as well.
Even with a dedicated service owning documents, you still have to answer what are the consistency guarantees you need. Usually you start with SLA's - how available your service should be? How the data is stored? Often the underlaying data storage will dictate those.
Also, I would like to note that even with synchronous calls your system will be eventually consistent - since it takes time to execute all those calls, it will be a period when the system as a whole might see non-latest data.
If you really need true strong consistency, you may will have to pick right storage for that. I would go with a strongly consistent option assuming my performance and availability goals are met. And the reason for strong consistency - it is much easier to reason about; hence the system gets simpler.