In today's digital landscape, Application Programming Interfaces (APIs) play a crucial role in enabling communication and integration between various software systems. However, designing an API that is both functional and user-friendly requires careful planning and consideration. This article explores the best practices for API design, focusing on the importance of early stakeholder engagement, collaborative design processes, model-driven development, and the use of appropriate API design tools.

1. The first step in API design is to establish clear goals. The design phase should primarily focus on capturing and validating the domain data model and the state-lifecycle of business resources. The quality of the data model has a significant impact on the usability, evolvability, and security of an API. By conducting collaborative design workshops and leveraging modeling tools, stakeholders from different domains can interact and contribute to the same domain data model, ensuring that it remains the source of truth and maximizing collaboration.
2. To ensure the effectiveness of an API, it is crucial to align it with the business domain it serves. From the business information, events, and processes managed by a System-of-Record business service, one or more canonical business resource APIs should be abstracted. These APIs represent the nouns of the system and provide a context for interaction with the business capability. When consistently modeled and cataloged, these APIs become the backbone of a federated, self-service data platform.
3. Microservices architectures and the REST architectural style promote decoupling, self-service, and reusability. To achieve this, it is essential to design APIs that are composable. Composability refers to the ability of client systems to compose data via self-service integration without a blocking dependency on external teams. A good REST model finds a balance between granularity and cohesive units of business data, allowing API clients to access core information about a specific business object quickly.
4. Engaging stakeholders with expertise in the business domain and API design standards is crucial for successful API design. By incorporating the input of various stakeholders such as business owners, domain architects, enterprise architects, and developers, the design phase can benefit from diverse perspectives. Early and continuous stakeholder engagement helps validate and shape the API model, ensuring it meets the needs of all involved parties.
5. API design is an iterative process that requires collaboration among stakeholders. During the planning phase, representatives from stakeholder groups should be identified and engaged. Collaborative design workshops, such as Event Storming, provide a forum for mutual discovery, validation, and agreement on models, boundaries, and business events. These workshops allow stakeholders to share their perspectives and ensure a shared understanding of the domain.
6. To capture and validate the aggregates and entities that emerge from design workshops, collaborative data modeling tools should be used. These tools enable key stakeholders to validate and enrich the model, ensuring its accuracy and relevance. Throughout the process, iterations should be made, and significant model changes should be circulated to stakeholders for feedback. Additionally, model validation and refinement should consider REST modeling guidance and API design standards.
7. Once the model is relatively stable, an API specification should be generated for technical review and validation. The API specification is based on the REST model and follows relevant standards and policy-conformant rules. This specification serves as a template for the implementation of the API. During this stage, it is important to validate the API specification and refine it based on stakeholder feedback, utilizing linting tools that provide immediate feedback on API quality.
8. Model-driven and API-first development approaches prioritize APIs as the primary means of interacting with a business capability. By building APIs from a validated domain model and API specification document, development teams can accelerate the development process using code generators. These generators automatically create a consistent implementation for supported languages, streamlining the development process. Additionally, mock servers and API tests can be generated from the API specification document, ensuring robustness and quality.
9. In some cases, organizations may need to work with legacy or proprietary APIs that lack a shareable data model. To address this challenge, it is important to retrospectively capture the API data model using model-driven design tooling. Elements of the API and OpenAPI document can be iteratively refined to align with API and document rules. For a clean and conformant API, consider implementing a tactical "anti-corruption-layer" integration solution.
10. Effective API design requires the use of appropriate tools throughout the process. Here are some recommended tools for different stages of API design:

     Design Workshop Tools

• Miro or Mural (digital whiteboard platforms)
• Event Storming templates

     Domain Data Modeling & Model Driven Design Tools

• Jargon Domain Data Modeling Platform
• Stoplight Studio Enterprise
• Visual Paradigm
• Mendix Low-Code Platform
• Hackolade Studio
• Sparx EA + OpenAPI plugin

   API Specification Technical Validation Tools

• Spectral (OpenAPI linter)
• Visual Studio Code with Spectral extension
• Stoplight Studio (API specification validation)
• CI/CD pipeline tooling for automated OpenAPI linting

   Code Generation Tools

• OpenAPI Design & Documentation Tools (Swagger)
• Various OpenAPI generators

   Test Generation Tools

• Postman API Platform
• Thunder Client (Extension for VS Code)
• Karate
• Pact
• Katalon Quality Management
• Insomnia API Dev Platform
• SOAPUI
• REST-Assured + Tcases

Effective API design requires a holistic approach that incorporates early stakeholder engagement, iterative design, and model-driven development. By aligning the API with the business domain, designing for composability, and continuously validating the API specification, organizations can create APIs that are reusable, stable, and coherent. The use of appropriate API design tools further enhances the design process.