CQRS Documents by Greg Young
Cart Created
Added 2 Socks Item 137
Added 4 Shirts Item 354
Shipping Information Added
Figure 15 Transactional View of Order
This can be applied to any type of object. By replaying through the events the object can be returned to the last known state. It is mathematically equivalent to store the end of the equation or the equation that represents it. There is a structural representation of the object, but it exists only by replaying previous transactions to return the structure to its last known state, data is not persisted in a structure but as a series of transactions. One very interesting possibility here is that unlike when storing current state in a structural way there is no coupling between the representation of current state in the domain and in storage, the representation of current state in the domain can vary without thought of the persistence mechanism. It is vitally important to note the language in Figure 3. All of the verbs are in the past tense. These are Domain Events. Consider what would happen if the language were in the imperative tense, “Add 2 socks item 137”, “Create Cart”. What if there were behaviors associated with adding and item (such as reserving it from an inventory system via a webservice call), should these behaviors be when reconstituting an object? What if logic has changed so that this item could no longer be added given the context? This is one of many examples where dual contexts between Commands and Events are required, there is a contextual difference between returning to a given state and attempting to transition to a new one.
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