API-First Enterprise Architecture: Modern Design Patterns and Implementation Playbook

API‑first aligns business capabilities to well‑designed interfaces that scale across teams and systems. This tutorial provides a practical blueprint: domain design, API contracts, gateways, events, security, governance, and a staged approach to migrate from point‑to‑point integrations to a resilient platform.

Principles

- Domains before endpoints: capabilities mapped to domain boundaries, not systems - Contracts as products: versioned, documented, and tested like code - Backward compatibility: additive evolution; deprecate with timelines and telemetry - Resilience and idempotency by default - Observability and product‑grade developer experience (DX)

Domain and boundary design

- Use domain‑driven design (DDD) to map core domains: Matters, Documents, Parties, Billing, Knowledge, Identity - Distinguish internal vs. external (client) APIs; privacy levels per domain - Anti‑corruption layers to isolate legacy systems

API contract patterns

- Resource modeling: nouns, relationships, and pagination; avoid over‑verbing - Versioning: URI or header versioning; additive schema changes; sunset policies - Idempotency: keys for POST; conditional updates with ETags; retries safe by design - Error model: problem+json with machine‑readable codes; correlation IDs - Metadata: consistent pagination, filtering, sorting conventions

Security and privacy

- OAuth2/OIDC, mTLS for service‑to‑service; fine‑grained scopes - ABAC: matter‑scoped claims; dynamic policy checks at gateway and service - Data minimization: field‑level filtering; redaction for logs; PII tokenization where appropriate

Gateway and edge policies

- API gateway for auth, rate limiting, request/response validation, schema enforcement - Spike arrest, token buckets; priority lanes for critical traffic - Request/response transformation for backward compatibility

Events and async integration

- Event‑driven patterns for decoupling: publish domain events (e.g., DocumentClassified, ClauseDeviationScored) - Exactly‑once semantics via idempotent consumers + dedupe windows - Outbox pattern to avoid dual‑write problems

Observability and quality

- Structured logs, distributed tracing, metrics (latency p95, error rates, saturation) - Contract tests in CI; consumer‑driven contracts to prevent breaking changes - SLOs per API; error budgets guide release velocity

Developer experience (DX)

- Single developer portal: discovery, docs, try‑it, API keys, guides - Scaffolding: templates for new services; linting for style consistency - Golden paths for common patterns (auth, pagination, error model)

Migration from legacy

- Inventory current integrations; map to domains and canonical models - Strangle pattern: front legacy with new APIs; incrementally move capabilities - Build adapters for high‑value consumers; retire brittle point‑to‑point links

Reference diagram (textual)

[Diagram] Left: Client/Partner → API Gateway (auth, rate limit, schema) → Domain Services (Matters, Docs, Billing, Knowledge) → Event Bus (domain events) → Consumers (CLM, Search, Analytics) → Observability (logs/traces) and Governance (catalog, policies)

How BASAD helps: BASAD builds API‑first platforms: domain definition, contract design, gateway policies, eventing, and DX. We implement idempotent, secure APIs with clear SLOs and contract tests, and run staged migrations from legacy with measurable risk reduction and time‑to‑value.