In the context of OutSystems architecture, the term "OAPI" is likely used to refer to the internal Application Programming Interface of an OutSystems module, specifically a Service Module. The core component of this interface, used to expose reusable, non-breaking business logic and functionality to other modules within the same environment, is the Service Action. This provides a strongly-typed, compile-time-checked contract between modules. This is distinct from a REST API, which is a technology used to expose functionality over web standards (HTTP) primarily for external systems or decoupled front-ends.

B. REST API: A REST API is a specific type of web service interface for exposing functionality, not a component inside the fundamental, internal API of a Service Module. A REST API method typically calls a Server Action to execute its logic.

1. OutSystems Official Documentation

"Service Actions": "A Service Action is a public Server Action in a Service Module. It encapsulates a piece of business or integration logic and can be reused across different modules... Service Actions are the key element for building a service-oriented architecture." This source establishes the Service Action as the key element of a Service Module's reusable API.

2. OutSystems Official Documentation

"Designing the architecture of your applications"

The 4 Layer Canvas: In the section describing Core Services

it states: "They offer reusable business services... exposing a well-defined and non-breaking public API of Service Actions." This reinforces that the API of a Core Service module is composed of Service Actions.

3. OutSystems Official Documentation

"Expose REST APIs": "Expose REST APIs to make your application's data and functionality available to other systems." This document defines REST APIs as a mechanism for external exposure

distinguishing them from the internal

module-to-module communication facilitated by Service Actions.