Today’s post is really short and concise. I am only going to mention the most important things to have in mind when designing the interface. As always in CPSA posts, I am inspired by the book Software Architecture Foundations.
API vs SPI
API – Application Programming Interface – Provided Interface – it exposes everything what is needed to use the component.
Example: A library’s API might include methods for performing calculations, retrieving data, or handling user input.
SPI – Service Provided Interface – it’s API intended to be implemented or extended by another component.
Example: A logging framework might define an SPI that developers implement to create custom logging behavior (e.g., writing logs to a database)
What Makes a Good Interface?
An interface includes everything necessary for clear and effective interaction with the component:
- Methods/Functions: Clearly defined signatures with well-documented parameters, return types, and exceptions.
- Contracts: Pre-conditions, post-conditions, and invariants that describe the expected behavior.
- Data Types: Input/output formats and expected data structures.
- Error Handling: Defined strategies for handling errors, such as exceptions or return codes.
- Versioning: Mechanisms for backward compatibility and deprecation warnings.
- Documentation: Comprehensive descriptions of usage patterns, limitations, and examples.
Desired Characteristics:
- Easy to learn: Minimize complexity for new users.
- Easy to extend: Support future requirements without breaking compatibility.
- Hard to misuse: Provide clear contracts and guidelines to prevent unintended use.
- Complete: Offer all the necessary functionalities from the user’s or building block’s perspective.
Key Aspects of Interface Design:
- Syntax: Use meaningful and self-explanatory method/function names.
- Semantics: Clearly define the responsibilities of each interface element.
- Protocol: Document the expected interactions, such as call order, preconditions, and postconditions.
- Physical/Technical Transport: In distributed systems, ensure that network or transport characteristics are accounted for.
- Quality Characteristics: Address performance, load, throughput, and reliability requirements.
Best practices
Clarity and Simplicity:
- Use self-descriptive names that reflect the domain model.
- Design interfaces to be easy to learn and use, minimizing the learning curve for developers.
Separation of Concerns:
- Treat internal and external interfaces differently, exposing only what’s necessary to external consumers.
- Follow the information hiding principle, ensuring implementation details remain private.
Backward Compatibility:
- Plan for versioning from the start. Use semantic versioning to avoid breaking existing clients.
- Provide default methods or overloads for extending interfaces without disruption.
Consistency Across the System:
- Use similar naming conventions and concepts for all interfaces in a system to reduce cognitive load on users.
Error Handling and Robustness:
- Provide explicit error states and handling mechanisms, such as exceptions or error codes.
- Prevent failures by using sanity checks or validation within the interface.
Document Extensively:
- Include examples, sample code, and unit tests as part of the documentation.
- When possible, provide automated tools or extensions that demonstrate correct usage.
Test Interfaces for Usability:
- Use interfaces yourself during development to ensure they “feel right” for consumers.
- Focus on minimizing surprises and side effects.
Practical Checklist for Designing Interfaces
- Is the interface easy to learn and use?
- Does it clearly separate external and internal responsibilities?
- Have you documented usage patterns, quality characteristics, and edge cases?
- Does the interface support future extensibility through versioning or modular design?
- Are error states explicit and easily recoverable?
Summary
By following these guidelines, you’ll create interfaces that are robust, user-friendly, and maintainable. Proper interface design is a cornerstone of modular and scalable systems.