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Discussion on: To Domain Driven Design

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mjremijan profile image
Michael Remijan

Hi Kevin, in this blog, you stated a "product" (or feature) of the platform is a Bounded Context in the DDD world. After you have the "product", you show how to "slice the product into modules". Would you consider the "module" you describe as an Aggregate/Entity inside a Bounded Context? It seems like that is what they would be based on their names...Buyer, Book, Stock, etc.

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kmruiz profile image
Kevin Mas Ruiz Author

Hi Michael!

Some of the modules can be aggregate roots, and others can be just query models, depending on the business logic.

What I've found is that usually you would have only once an aggregate. For example, the aggregate root Book, which handles the consistency of book info, will be in a single product. The reason is to guarantee consistency of business invariants. Other books will be some variant of read models.

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Michael Remijan

Ok that gets to another question. When you say "Other books will be some variant of read models", how would get that book data to the other "products"? I like your body analogy when it comes to coupling Microservices with direct calls. Even with small applications I can see a "death star" pattern emerging quickly. So would use a Domain Event to push aggregate book data to other products with the caveat they must remain read-only? Or would the other products use the same database as the aggregate book data? Or (like in most situations) is it a combination based on the situation. What have you found to be most successful based on past experience?

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kmruiz profile image
Kevin Mas Ruiz Author

You can use both of them, I would suggest usually sharing snapshots of data. For example, sending the snapshot through SQS, Kinesis or even better Kafka so you can replay the state.

This way you give data ownership to the writer, guaranteeing data consistency, and you let the readers consume the information they need. However, this kind of infrastructure is quite expensive and teams can have a hard time learning about them.

As always, consider the cheapest solution. For example, right now, in a project, we are using just materialized views in a Oracle database as it's easier, trading off availability for simplicity (which most of the times makes sense).

The most successful approach to CQRS that I've seen is using Kafka to share snapshots, partitioned by the aggregate id, and a topic per type of domain entity. This allowed teams to consume from Kafka without fear of breaking other services, as they just need another consumer group. New services can just plug in to the topic, consume the whole state, and generate a view in whatever fits better for them.

Thanks!