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bobthedeveloper
bobthedeveloper

Reputation: 3783

Validation using other models

What are the options to validate an event which uses another model?

Shopping cart example:

When adding a cart item to the shopping cart, there should be a check if the item isn't sold out yet.

Upvotes: 0

Views: 114

Answers (2)

martinezdelariva
martinezdelariva

Reputation: 5049

What are the options to validate an event which uses another model?

A domain event is an important happening from the point of view of the business. It's something that happen in the past, so it can't be changed. In OO, it's commonly represented as a Value Object, that's say, an immutable object where the interesting part are their attributes.

Commonly, those Domain Events are yield from an operation in a Aggregate Root (DDD jargon). The client of the Aggregate Root is the Application Service (aka use case). The Application Service receive a Command object and base on that, executes operation in the Aggregate Root.

The validation could consist in primitive validation, object validation and/or composed object validation. Then the object in charge to perform this validation should be the Aggregate Root itself and/or some objects with the specific goal in validation.

When adding a cart item to the shopping cart, there should be a check if the item isn't sold out yet

Following your example the objects woulb be:

  • Command: AddItemToShoppingCartCommand. Holds information about the item to be added, and shopping cart identifier for example.
  • Application Service: AddItemToShoppingCartService.
  • Aggregate Root: ShoppingCartInventory. I've used deliberately Inventory in the name to be explicit that this Aggregate Root satisfy the invariant "...if the item isn't sold out yet."

Note: In my opinion, the invariant that checks the inventory in the Aggregate Root, makes the Aggregate too big. My advice is relax this invariant and embrace eventual consistency if this "sold out" it's not happen normally.

Upvotes: 1

tomliversidge
tomliversidge

Reputation: 2369

You would usually validate a command rather than an event as an event should be something that cannot be changed

In answer to the question, typically it depends on what the business cost of the process is. In your example for instance, what is the cost to the business of ordering an item that is sold out? Probably very little - an email saying the item is out of stock, with an estimate of how long it will take.

In this type of scenario you could use an eventually consistent read model over the data, where you'd query the read model / cache for stock levels, but accept that some orders might go through for things that are out of stock.

If you have tighter constraints then you'll have to enforce them, ideally by remodelling your aggregates, or having transactional and/or blocking on the ordering process.

Upvotes: 4

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