The best practice for designing System APIs within an API-led connectivity framework is to make them reusable assets. A System API should expose the capabilities and data of an underlying system of record (e.g., a Customer entity from a CRM) in a canonical, standardized format. By allowing a System API to return a complete data model, even if not all fields are currently needed by a specific consumer, it decouples the API from the immediate requirements of a single project. This design promotes broader reusability across the organization for future projects that may require different data attributes, thus preventing API proliferation and rework.

Why Incorrect Options are Wrong

A) Handle customizations for the end-user application at the Process API level rather than the Experience API level

This is incorrect. The primary purpose of an Experience API is to tailor data and functionality for a specific end-user channel or context. Pushing this logic to the Process API layer would make it less reusable.

C) Always use a tiered approach by creating exactly one API for each of the 3 layers (Experience, Process and System APIs)

This is an overly rigid and false constraint. The number of APIs at each layer depends entirely on the complexity of the business process, the number of systems, and the number of consuming channels.

D) Use a Process API to orchestrate calls to multiple System APIs, but NOT to other Process APIs

This is an incorrect limitation. While Process APIs do orchestrate System APIs, they can and often should also call other Process APIs to compose more complex business capabilities from reusable process components.

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1. MuleSoft Documentation, "API-led connectivity": This document outlines the roles of each layer. It states that System APIs "expose the data in a canonical format," supporting the idea that they should be complete and not tied to a single consumer's needs. It also defines Experience APIs as being for "a specific audience," which contradicts option A.

Reference Section: "The layers of API-led connectivity"

2. MuleSoft Whitepaper, "API-led Connectivity: The next step in the evolution of SOA": This paper emphasizes the reusability of APIs created through this methodology. On page 5, it describes System APIs as providing a "consistent and easily accessible interface to the underlying systems," which implies a stable, canonical contract independent of specific consumer requirements (supporting option B). It also discusses composition, which allows Process APIs to call other Process APIs (contradicting option D).

Reference Section: "Unlocking and Insulating" (Page 5), "Composing and Recomposing" (Page 6).

3. MuleSoft Catalyst Knowledge Hub, "Designing APIs": This resource details best practices for API design. It reinforces that System APIs should be designed for broad reuse by exposing a consistent view of a backend system. The design should be driven by the source system's domain model, not the specific, immediate needs of a consuming application.

Reference Section: "System API Design Principles".