DDD - How to ensure aggregate consistency when integrating with third party systems?

This question is really two-fold with emphasis on the most important question.

Aggregates define the consistency and transactional boundaries of a system.

Say your company provides some service through an app and you are about to add a new feature through an integration with a third party. The third party has its own API and you need to create the user in their system when they choose to use the new feature. Now, you're not a monster, so you allow users to toggle off the feature if they had previously started using it. This means you must also be able to delete the user in the third party API.

• If we were to design an Aggregate for such a feature (lets say it is a subscription), where should we put the interaction with the third party API?

• What if the third party we integrate with allow the users to remove themselves from their system bypassing our app completely? Now our aggregate would be out of sync.

Generally, an aggregate enforces consistency not on a system level, but over some data. The concept was introduced in the DDD blue book, and at a time where most solutions relied on traditional large-scale SQL db's, and it intended to remove responsibility for consistency from the storage layer, and into business logic. It did so because that's what consistency is: business logic. If you have to decrease stock when you sell something, this is a consistency rule, and one which should live in application code, with other business logic instead of being delegated to a stored procedure.

With that in mind, any aggregate you'd have wouldn't (indeed couldn't) enforce consistency with the outside world (it's not data you own and hold internally). However, what you could do is introduce two (or one combined) process managers:

• One process manager that would enforce the policy of "when a user asks to get deleted, delete them from the external system". This would observe internal requests and call out to the 3rd party service to delete a user when the user requested to be deleted
• One process manager that would enforce the policy of "when a user gets deleted from the third party service, mark them as deleted in our system too" which would do pretty much what the policy says

You may or may not have a subscription aggregate. What would determine that is if you need to enforce some consistency around data you own and are responsible for.