Standard Types Expressed as Values

Definition

• Standard Types are descriptive objects that indicate the types of things.

• The industry lacks a standard name for this concept.

  • Also referred to as a Lookup or Type Code.

Examples

• Phone Number Type: Home, Mobile, Work, Other

• Currency Type: USD, CAD, CNY, EUR, GBP, etc

• Medication Administration Type: IV, Oral, Topical

Why not use a string? Strings allows the model to be put into an invalid state.

Standardization

Depending on the level of standardization, options for maintaining these types can include:

1. Application Level

2. Shared Corporate Database

3. National or International Standards

Implementation

Separate Bounded Context

• For the sake of maintenance, it is common for Standard Types to reside in a separate Context from the models that consume them.

• The would be modeled as Entities with a persistent lifecycle.

• Attributes would include Id, Name, Description, and more.

Enum

Best to be biased towards an Enum for Standard Types

• An Enum is very simple way to support a Standard Type

• Provides a well-defined finite number of Values.

• Lightweight

• Side-Effect-Free behavior (not an Entity)

• What about a description?

  • This is a User Interface Concern.

  • User Interface will often need to support localization. Localization is not an appropriate model concern.

• What about the name?

  • Often the Name of the Standard Type is the best value to use.

  • What about when names change?

    • Use the Parallel Change Pattern

Aggregate

Think twice before you run with this.

• Implement one instances of an Aggregate per Type.

• Think twice before you run with this.

  • Standard Types should generally NOT be maintained inside the Bounded Context that consumes them

  • Widely used Standard Types should normally be maintained in a separate Context with carefully planned updates to consumers.

• Instead, expose standard type aggregates as immutable consumer contexts.