[[Software Architecture & API Contract Design]]
📌 Brief Summary
Software Architecture and API Contract Design refers to the formal definition of boundaries, data structures, and behavioral expectations between decoupled system components. In the context of TypeScript, this involves utilizing the type system to enforce structural integrity, ensuring that both producers and consumers adhere to a shared, verifiable schema that minimizes runtime errors and integration friction.

📖 Core Content
* **Structural Typing & Interface Enforcement**: At the heart of API contract design in TypeScript is the principle of structural typing (duck typing). Unlike nominal typing, the contract is satisfied if the shape of the provided object matches the interface definition. Effective architecture leverages `interface` and `type` aliases to define the "shape" of data, creating a compile-time guarantee that all required properties and method signatures are present across service boundaries.
* **Contractual Robustness via Discriminated Unions**: To design resilient APIs, architects use Discriminated Unions (Tagged Unions). By including a literal type property (e.g., `type: 'success' | 'error'`), the developer creates an exhaustive contract. This allows the TypeScript compiler to perform control-flow analysis, forcing consumers to handle all possible states of an API response, thereby preventing "undefined" errors in downstream logic.
* **Schema Validation & Type Guards**: A critical gap exists between compile-time types and runtime data (e.g., JSON from a REST API). Professional architecture integrates runtime validation libraries like `Zod` or `io-ts`. These tools bridge the gap by generating TypeScript types directly from runtime schemas, ensuring that the "Contract" is not just a developer's intention but a strictly enforced reality during execution.
* **Versioning and Compatibility (The Open/Closed Principle)**: API contracts must be designed for evolution. Using optional properties (`?`) and avoiding the removal of existing members allows for backward compatibility. Advanced designs utilize `Readonly<T>` to ensure that once a contract is fulfilled, the data remains immutable throughout its lifecycle in the application architecture, preventing side-effect-driven contract violations.
* **Abstraction via Generics**: To create reusable and scalable architectures, API contracts often employ Generics (`<T>`). This allows for the definition of standardized response envelopes (e.g., `ApiResponse<T>`) where the metadata (status, timestamp) is fixed by the architecture, but the payload remains flexible, maintaining type safety across diverse data entities.

🔗 Knowledge Connections
* Related Topics: [[Structural Typing]], [[Discriminated Unions]], [[Runtime Type Validation]]
* Projects/Contexts: [[Microservices Communication]], [[Full-stack Type Safety (End-to-end Type Safety)]]
* Contradictions/Notes: While `interface` is preferred for declaration merging and performance in large scales, `type` aliases are necessary for complex intersections and unions; the choice depends on whether the contract needs to be extensible or strictly defined.

Last updated: 2026-04-17