On Page 449, there is a mentioning of invariants. First of all, I have seen this term invariants in many articles but not clear what it is. Does it mean business validations?

When I think about an Auction and Bids, I think Auction can be an aggregate. This is because an auction can't exist without a bid. If anyone wants to access the bid, it has to go through the auction.

Please clarify.

An invariant is relationship between two things that must always fall within a certain range. For example, a soccer match must have two teams else it's not a soccer match.

In the examples of auctions and bids, you can have an auction that has no bids. When an auction starts, there are no bids placed. So in most auction domains, I don't think an auction have at least 1 bid would be an invariant.

Invariants aren't necessarily business rules, they can be facts about he problem domain as with the soccer match example I gave.

Thanks for posting your question.

Hope that helps

A very good explanation. I also read Udi's explanation on a similar analogy.

Yes, the auction can exist without any bids, but the auction must have an associated listing, otherwise it's not an auction. So, shouldn't the auction be th e aggregate root for that reason?

Or, with the question/answer aggregate, does the answer really need to be aggregated with the question? If a question exists, but has not been answered yet, wouldn't the question still be in a consistent state?