Strongly Typed

We believe that domain language, being the most important part of the application, must be strongly typed.

Value objects

Value Object is a measure or description of something. Examples of value objects are things like numbers, dates, monies and strings. Usually, they are small objects which are used quite widely. Their identity is based on their state rather than on their object identity. This way, you can have multiple copies of the same conceptual value object.

Martin Fowler also emphasizes an immutable nature of the Value Objects:

A general heuristic is that value objects should be entirely immutable. If you want to change a value object you should replace the object with a new one and not be allowed to update the values of the value object itself - updatable value objects lead to aliasing problems.

Commands and Events

Unlike some of CQRS/ES frameworks which treat commands and events as Json objects, Spine promotes commands and events to be first class citizens in the application. That gives a lot of benefits in describing the business logic. Not having to convert back-and-forth with Json gives some performance advantage at the same time.

With commands and events strongly typed we get an important advantage — the model can be extended without breaking binary compatibility with client applications.

Identifiers

Identifiers must be typed too. The framework does allow to have IDs of type String, Long, or Integer. But we recommend to have typed identifiers. So, if you have the Order class for one of your aggregates, there should be an OrderId.

There are two benefits of this:

  1. Strong typing eliminates errors related to passing wrong value by mistake. Suppose you have a command like this:

     CreateOrder cmd = CreateOrder.newBuilder()
                                  .setId(orderId)
                                  .setCustomerId(customerId)
                                  .setCostCenter(costCenterId)
                                  .setDepartmentId(departmntId)
                                  .setNotes(notes)
                                  .build();
    

    It’s easy to pass a wrong ID if it is a string. With typed identifiers, your IDE (and compiler) would point the error.

  2. Identifiers can be extended as business grows Imagine, you have customer IDs that are auto-generated and are long. Then you decide to integrate with another system in which customer IDs are based on email addresses, and with another system in which customer IDs are phone numbers. Having CustomerId class in the first place would make the integration much easier.

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