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Software Architecture – Explicit Constraints, Processes, Specification Pattern, and more

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Know what Joe and Waldo have in common? We can’t find either, as this week, it’s just Allen and Michael to continue the dive into Domain Driven Design.

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Explicit Constraints

  • “[Constraints] often emerge implicitly, and expressing them explicitly can greatly improve a design.”

What is a constraint?

  • A constraint can be as simple as a boundary check, i.e. <, >, <=, >=
  • Instead of _just_ using the math, instead, put that logic into a specific method. Or a class.
    • A fundamental concept in MODEL-DRIVEN-DESIGN is that it’s easy to “see” relationships in the model
    • By giving it a name, we can now discuss it as necessary
    • By giving it an intention-revealing name, others will understand it’s purpose without caring about its implementation
  • If a constraint obscures the objects responsibility, it may be time to factor that constraint out into a separate class or set of classes with relationships
  • This can be done using the Strategy Design Pattern – example in the book uses the overbooking policy from Shipping Cargo p. 17

How to know that your class is distorted by constraints

  • “Evaluating a constraint requires data that does not otherwise fit the object’s definition.”
  • “Related rules appear in multiple objects, forcing duplication or inheritance between objects that are not otherwise a family.”
  • “A lot of design and requirements conversation revolves around the constraints, but in the implementation, they are hidden away in procedural code.”

Processes as Domain Objects

  • So, procedural code in our model right? Well no.
  • An example is a service
    • A service allows us to express a process explicitly and encapsulate any complex logic within it.

But, what if it can be done in multiple ways?

  • Make the algorithm, or some key part of it, an object of its own
  • Each “process” object represents a different *strategy*

Q: How to tell when a process should be explicit?

  • Do domain experts talk about this process?
    • If so, make it explicit
  • No, it’s just part of the overall mechanism.
    • If so, keeping it hidden is OK. Maybe even preferred.

Specification Pattern

Before we get into what the pattern is, let’s talk about how things typically are done … You have a class with complicated validation / business rules and you start baking all that into methods in your class – it quickly becomes hard to reason. OK, that didn’t work so well, let’s move that logic out into a layer abstracted away – now your domain object has lost it’s meaning and it’s more or less turned into a property bag and your application layer has gotten very complicated.

  • “Business rules often do not fit the responsibility of any of the obvious ENTITIES or VALUE OBJECTS”
    • Moving those business rules out of the domain object is not the answer because now the object model is no longer expressive
  • “A specification states a constraint on the state of another object, which may or may not be present.”
  • “… a specification can test _any_ object to see if it satisfies the specified criteria.”
  • Specifications are VALUE OBJECTS
  • Specifications keep the rules in the domain layer.
  • Why? As business rules become increasingly complicated in an object, the clarity of the role of the object can get lost in all the conditions
  • Nice example of the specification pattern: https://www.codeproject.com/Articles/670115/Specification-pattern-in-Csharp

Another use for factories

  • Use a factory to create/configure a specification using information from multiple sources
    • This way, only the factory has those or knows of those dependencies, keeping your domain objects clean

Applying and Implementing

  • To validate that an object is ready to fulfill some need
  • To select objects from a collection
  • Specify creating a new object for a specific need

Another way to use repositories

  • Because repositories are the mechanism for querying access to domain objects, specifications are a great fit

What is a specification not?

  • It is not a filter for preexisting objects. That’s querying.
  • It is not a test for an existing object. That’s validation.

So what is it?

  • A whole new object, or set objects, are created or configured to satisfy a specification.

Intention-Revealing Interfaces

  • “If a developer must consider the implementation of a component in order to use it, the value of encapsulation is lost”
    • Clean Code anyone?!
  • “The names of classes and methods are great opportunities for improving communication between developers…”
  • Kent Beck wrote of the Intention-Revealing Selector (1997)
  • “Name classes and operations to describe their effect and purpose, without reference to the means by which they do what they promise.”
  • “Write a test for a behavior before creating it, to force your thinking into client developer mode.”
    • Michael loves this idea. Never before considered that TDD makes you think about your code as some other developer (i.e. user) might.

In the public interfaces of the domain …

  • State relationships and rules, but not how they are enforced
  • Describe events and actions, but not how they are carried out
  • Formulate the equation, but not the numerical method to solve it
  • Pose the question, but don’t present the means by which the answer shall be found

When writing unit tests …

  • Write your tests they way you would _like_ to use the class under test
  • Write your tests to explore the interface design of your class under test
  • _Then_ make it compile.

Side-Effect-Free Functions

  • There are two types of operations:
  • Side effect – “… in computer science, it means any effect on the state of the system.”
  • Functions – operations that produce a result without side effects
  • “Perform all queries and calculations in methods that cause no observable side effects. (Meyer 1988).”
  • Value objects (immutable) and functions are safer to use and easier to test
  • Put as much logic as you can into functions.
  • When using commands, keep them segregated, keep them simple, and do not let them return domain information
  • Control side-effects by keeping complex logic in value objects whenever possible
  • Maintaining invariants in Entities is much more difficult to replacing VALUE OBJECTS

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