Which ones are necessary? The simple answer is: those that must be consistent with others, even if domain experts don’t specify them as rules. For example, Product has name and description attributes. We can’t imagine name and description being inconsistent, modeled in separate Aggregates. When you change the name, you probably also change the description. If you change one and not the other, it’s probably because you are fixing a spelling error or making the description more fitting to the name. Even though domain experts will probably not think of this as an explicit business rule, it is an
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