Command Validation
In Extending Greg Young’s Simplest Possible Thing I thought up a couple of functional requirements. The first functional requirements were:
(I’ve added Github issue numbers so you can easily check the changes on Github)
Positive numbers, please
Let’s start with validating a RemoveItemsFromInventory command, against the simple rule that Number should be greater than 0. This is how a RemoveItemsFromInventory command is issued from the HomeController:
[HttpPost]
public ActionResult Remove(Guid id, int number, int version)
{
_bus.Send(new RemoveItemsFromInventory(id, number, version));
return RedirectToAction("Index");
}
No validation there yet. I choose to add server side validation, using this validation method on the HomeController:
private void ValidateForRemoval(int numberToRemove)
{
if(numberToRemove <= 0 )
ModelState.AddModelError("Number", "Should be greater than 0.");
}
And then add a call to ValidateForRemoval(...) to the Remove(...) action:
[HttpPost]
public ActionResult Remove(Guid id, int number, int version)
{
ValidateForRemoval(number);
if(!ModelState.IsValid)
{
ViewData.Model = _readmodel.GetInventoryItemDetails(id);
return View();
}
_bus.Send(new RemoveItemsFromInventory(id, number, version));
return RedirectToAction("Index");
}
The validation method for a CheckInItemsToInventory looks pretty similar.
We don’t have that many items in stock
How would we validate a RemoveItemsFromInventory command if we don’t have access to the domain object? Well, we can use the read model for validation, let’s add it to the validation method:
private void ValidateForRemoval(InventoryItemDetailsDto item, int numberToRemove)
{
// ... other validation
if(numberToRemove > item.CurrentCount)
ModelState.AddModelError("Number",
"You cannot check out more items than currently are in stock.");
}
And pass the read model InventoryItemDetailsDto to the validation method:
[HttpPost]
public ActionResult Remove(Guid id, int number, int version)
{
var model = _readmodel.GetInventoryItemDetails(id);
ValidateForRemoval(model, number);
// etc.
}
Because the read model is eventually consistent, it’s possible that we’re validating against stale data here. From a user perspective however, this is still useful validation and it will prevent many invalid commands being passed to the domain objects.
It is simple to imagine a scenario however, where this command validation alone isn’t enough. Imagine we have one item in stock. Two users both want to remove one item and their RemoveItemsFromInventory command is validated against the read model with CurrentCount == 1. Both commands will validate, but clearly only one should succeed. Validation alone is not sufficient here.
The functional requirement “As a user I should not be able to remove more items from inventory than are currently in stock.” should be translated into an invariant protected by a domain entity. That will be the subject of the next post.