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Implementing Domain-Driven Design by Vaughn Vernon

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Rule: Model True Invariants in Consistency Boundaries

When trying to discover the Aggregates in a Bounded Context (2), we must understand the model’s true invariants. Only with that knowledge can we determine which objects should be clustered into a given Aggregate.

An invariant is a business rule that must always be consistent. There are different kinds of consistency. One is transactional consistency, which is considered immediate and atomic. There is also eventual consistency. When discussing invariants, we are referring to transactional consistency. We might have the invariant

c = a + b

Therefore, when a is 2 and b is 3, c must be 5. According to that rule and conditions, if c is anything but 5, a system invariant is violated. To ensure ...

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