The Foundation layer, according to the OutSystems Architecture Canvas, is designed to contain the core, non-functional building blocks of an application portfolio. Its primary data elements are the Core Entities, which represent the fundamental business concepts (e.g., Customer, Product). A critical best practice is to expose these entities as read-only (Expose Read Only = Yes) to enforce the use of public wrapper actions (CRUD actions) that contain business logic and validation, thus ensuring data integrity and encapsulation.

A. CRUD actions for Entities: These are server actions, which are logic elements, not data elements. While they are correctly placed in a Foundation module, they are not the data element itself.

C. Non-core Entities: Non-core entities are specific to a business process or a single application and should be placed in Core or End-User layers to avoid bloating the Foundation layer.

D. Entities to save UI states: These entities are directly tied to the user interface and belong in the End-User layer to prevent incorrect dependencies from a stable foundation to a volatile UI layer.

1. OutSystems - Official Documentation

"The 4 Layer Canvas": Under the "Foundation Layer" section

it states: "They usually implement the data model by defining the Core Entities... and provide all the necessary core services to handle data

like Create

Read

Update

and Delete actions (CRUD wrappers)." This distinguishes entities as the data model and actions as services.

2. OutSystems - Official Documentation

"Designing the architecture of your OutSystems applications"

Section: "Isolate core services in Foundation modules": This guide explicitly states

"A Foundation module should define a data model with Core Entities." It further recommends: "To enforce this

set the Entities' Public property to No

or expose them as read-only by setting the Expose Read Only property to Yes." This directly supports the correct answer.

3. OutSystems - Official Documentation

"Architecture Dashboard"

Section: "Code analysis patterns"

Finding: "Incorrectly Placed Entity (IE)": This documentation explains the violation that occurs when an entity is not in the correct layer. It reinforces that core business concepts belong in Foundation modules

while entities specific to application logic or UI belong in Core or End-User modules

respectively.