Chapter 3 - The aggregate mixes technical and business aspects

-

continues from Chapter 2 - The Aggregate does not fit the story telling

......

Let me introduce villain number 2.

The aggregate mixes technical and business aspects.

For some reason, consciously or not, we try to bring together, in the aggregate, two different models:

  1. the model to guarantee the invariants
  2. our mental model of the domain concept

Although this may seem natural, often, these two models do not coincide. Strong consistency constraints do not always reside within the same business concept. It may happen that there are consistency constraints in the intermediate area between two or more business concepts.

For some people, this difference between the domain concept and the consistency boundary is crystal clear. I was positively impressed by the technique used by my friend Mauro Servienti to model the aggregates. He takes a reverse approach, first grouping data that share consistency constraints, bringing together information that changes together or whose change is dependent. Only later, after the groups have been identified, he tries to assign them a meaningful name.

However, this approach is not used by everyone. On the contrary, too often I have seen the aggregate modeling exercise reduced to looking for the names, the entities of our domain. Going beyond this naive approach, gaining a deeper understanding of the meaning of the aggregate, often requires time and experience.

Despite some initial difficulties, we managed to identify our first aggregate: the course.


It will be able to handle some types of commands and publish some types of events.
If we choose to use a model based on entities and value objects, we could represent the aggregate course in this way:

Please note that,  whenever the aggregate has to manage a student subscription or unsubscription command, then it makes use of its entire state to make the relative decisions. When instead it has to manage a command for updating its own capacity, it uses only part of the state.

Indeed, the invariant relative to the capacity update concerns only the current value of the capacity. Nothing else. There is no constraint related to the current number of subscriptions.


The same if we decide to use event sourcing. All events are important for handling the subscription and unsubscription commands. While just some events are important for handling the capacity update command.

The story continues to the next chapter
Chapter 4 - The larger the Aggregate, the greater the contention

... or watch the full story on youtube

Comments

Post a Comment

Popular posts from this blog

Chapter 1 - I am here to kill the aggregate

-
Image
The aggregate has always been, for me, one of the most controversial and weakest elements of Domain Driven Design. During my consultancy activity, I have been able to verify that the full understanding of the aggregate concept remains difficult for many. It is easy to make mistakes in the modeling phase, which will be very complex to fix in more advanced stages of development. As a result, some developers feel that the whole architecture is excessively difficult and complicated. I want to show you another world, where there is no aggregate. I am here to kill the aggregate. For those unfamiliar with it, let me introduce it to you. The Aggregate Its definition is, in my opinion, is quite vague: An aggregate is a cluster of associated objects  that we treat as a unit for the purpose of data changes. From blue book - Domain-Driven Design: Tackling Complexity in the Heart of Software - Eric Evans If you are confused by this definition, let's try to read what the blue book recommends for...
Read more

Chapter 2 - The Aggregate does not fit the storytelling

-
Image
continues from   Chapter 1 - I am here to kill the aggregate ...... One of the most powerful elements of DDD is that it has broken down the barriers between technicians and domain experts, thanks to tools capable of exploiting the immense potential of storytelling, first of all Event Storming. Storytelling is at the heart of human communication. Storytelling does not require any special competence or skill. It helps break down the barriers between technicians and business experts. In my experience, the usage of storytelling ensures an exceptional fluidity in the domain discovery process, but it hits a snag when we try to introduce the aggregate into the story. Indeed, by definition, the aggregate is the boundary of consistency. The aggregate is the component responsible for ensuring strong consistency constraints within an ecosystem that is, by nature, eventually consistent. But, at the same time, it tries to represent the business entity acting as the guardian of a set of bus...
Read more

Chapter 10 - The Aggregate is dead

-
Image
continues from  Chapter 9 - An event is just a fact, pure ...... The aggregate is finally dead, together with its negative aspects. This new approach, which focuses on the decision block, has several benefits: Perfect match with storytelling Focus on behavior instead of model Less contention Less complexity  Simpler to refactor But be careful: this does not mean having to organize the system in order to have hundreds of completely unrelated command handlers. Absolutely not! Clustering commands into logical groups and delegating their execution to the same model is and remains a correct programming approach. The difference lies in the fact that before, by the very definition of aggregate, these models necessarily had to be isolated from each other. Killing the aggregate means admitting the possibility of contamination between these models without renouncing consistency. A less dogmatic approach, which in any case must not disregard good programming practices, but which, thanks ...
Read more