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concept#Architecture#Integration#Software Engineering

API Specification

Formal description of interfaces, data models, and contracts to enable interoperability and automation.

An API specification formally defines a system's interfaces: endpoints, data models, authentication, error codes, and contracts.
Established
Medium

Classification

  • Medium
  • Technical
  • Architectural
  • Intermediate

Technical context

API gateways (e.g., Kong, AWS API Gateway)CI/CD pipelines (e.g., GitHub Actions, GitLab CI)Documentation and mock tools (e.g., Redoc, Prism)

Principles & goals

Contract-first design: define contracts before implementation.Explicit versioning and backwards compatibility.Schema-driven validation and automation.
Build
Enterprise, Domain, Team

Use cases & scenarios

Compromises

  • Lack of alignment leads to incompatible implementations.
  • Excessive rigidity prevents necessary evolution.
  • Untested specifications give false security (e.g., unverified contracts).
  • Prefer contract-first approach and verify implementations.
  • Run automated linting and contract tests in CI.
  • Establish clear versioning strategy and deprecation policies.

I/O & resources

  • Functional use cases and API requirements
  • Data models and JSON schemas
  • Security and compliance requirements
  • OpenAPI/AsyncAPI documents
  • Generated SDKs and client stub code
  • Automated tests and mock services

Description

An API specification formally defines a system's interfaces: endpoints, data models, authentication, error codes, and contracts. It acts as a verifiable agreement between providers and consumers and enables automated documentation, testing, and code generation. Specifications guide design decisions, versioning, security assessments, and streamline integration workflows across the development lifecycle.

✔

  • Increased interoperability between systems.
  • Automated documentation, testing, and code generation.
  • Improved governance and traceability of changes.

✖

  • Requires discipline in versioning and compatibility.
  • Complex data models can make specifications hard to understand.
  • Tools and generators are not equally mature across languages.

  • Specification coverage

    Share of endpoints and models covered by the canonical specification.

  • Contract test pass rate

    Percentage of successful contract tests in CI pipelines.

  • API change lead time

    Time from proposing a specification change to production use.

Stripe API

Well-documented REST API with clear specifications and sample code for integrators.

GitHub REST API

Public API with versioning, authentication, and extensive reference documentation.

OpenAPI-driven microservices

Internal microservices using contracts as single source of truth and generating clients from them.

1

Capture use cases and define domain models.

2

Author and validate the specification in OpenAPI.

3

Integrate automated tests, mocks, and SDK generation into CI.

⚠️ Technical debt & bottlenecks

  • Outdated schemas without migration path
  • Insufficient test coverage for contract changes
  • Manually maintained documentation not generated automatically
Versioning conflictsData model complexityGovernance overhead
  • Ignoring the specification and allowing divergent team implementations.
  • Failing to document critical security requirements in the specification.
  • Not enforcing versioning and introducing breaking changes uncoordinated.
  • Assumptions in spec that are not backed by tests.
  • Specifying internal implementation details too early.
  • Lack of synchronization between spec and production API.
API design and REST/HTTP principlesOpenAPI/JSON Schema expertiseAutomation and CI/CD knowledge
Interoperability between systemsAutomation of testing and generationSecurity and access control
  • • Backwards compatibility must be ensured
  • • Availability of suitable toolchain (linting, generators)
  • • Alignment with legal and privacy requirements