How do I build APIs across dozens of microservices?

Building APIs across dozens of microservices is mostly a coordination and governance problem, not just a coding problem. The goal is to make services easy to build independently while keeping the overall API surface consistent, secure, and maintainable.

1. Start with an API strategy

Decide what role APIs play in your system:

• Internal service-to-service APIs
• APIs for frontend/BFF layers
• Public/external APIs
• Partner APIs

Each one may need different standards for versioning, authentication, and stability.

2. Use clear service boundaries

Before defining endpoints, make sure each microservice owns a well-defined domain.

Good service boundaries:

• One service = one business capability
• It owns its data
• It exposes only the operations other systems need

If boundaries are blurry, your APIs will become tangled and hard to evolve.

3. Standardize API design

Create organization-wide standards so every service doesn’t invent its own style.

Common standards:

• REST or gRPC, with clear guidance on when to use each
• Naming conventions for resources
• Error response format
• Pagination/filtering/sorting patterns
• Date/time and ID formats
• Authentication and authorization patterns
• Correlation/request IDs for tracing

A simple example standard for REST:

• GET /customers/{id}
• POST /customers
• PATCH /customers/{id}
• GET /customers?limit=50&cursor=...

4. Define API contracts early

Use contract-first development where possible.

Tools/patterns:

• OpenAPI for REST
• Protobuf for gRPC
• AsyncAPI for event-driven APIs

Benefits:

• Easier code generation
• Better documentation
• Contract testing
• Fewer integration surprises

5. Version intentionally

At scale, API changes are inevitable.

Rules to follow:

• Avoid breaking changes whenever possible
• Prefer additive changes
• Version only when needed
• Deprecate with a clear timeline
• Support old and new versions during migration

Example approaches:

• URI versioning: /v1/...
• Header-based versioning
• Separate protobuf package versions for gRPC

6. Put governance in place

You need a lightweight central platform or architecture group to enforce consistency without slowing teams down.

Governance should cover:

• API linting rules
• Security requirements
• Naming conventions
• Deprecation policy
• Ownership and documentation
• Review process for shared contracts

This is where an API style guide helps a lot.

7. Use an API gateway where it fits

An API gateway is useful for:

• Authentication/authorization
• Rate limiting
• Request routing
• Aggregation
• Observability
• Public API management

But don’t use the gateway as a dumping ground for business logic. Keep business rules in services or BFFs.

8. Design for observability

With dozens of services, debugging becomes hard unless observability is built in.

Every API should support:

• Structured logs
• Distributed tracing
• Metrics
• Correlation IDs
• Health/readiness endpoints

Track:

• Latency
• Error rates
• Throughput
• Dependency failures

9. Automate testing across services

Use multiple layers of testing:

• Unit tests in each service
• Contract tests between providers and consumers
• Integration tests for critical flows
• End-to-end tests for user journeys

Contract testing is especially important to prevent one service from breaking another.

10. Manage dependencies carefully

In microservices, too many synchronous dependencies can create fragility.

Best practices:

• Keep call chains short
• Avoid chatty APIs
• Prefer async events for non-immediate workflows
• Use caching where appropriate
• Implement retries with backoff, timeouts, and circuit breakers

11. Generate and share tooling

Make it easy for teams to do the right thing:

• API templates/scaffolds
• Shared libraries for auth, logging, error handling
• Shared schema validation
• CI checks for API standards
• Docs generated from contracts
• SDK generation for consumers

12. Treat documentation as a product

For dozens of services, undocumented APIs become invisible.

Each API should have:

• Purpose
• Ownership/contact
• Auth requirements
• Sample requests/responses
• Error codes
• Rate limits
• SLA/SLO if applicable
• Deprecation status

A searchable internal developer portal can help a lot.

13. Organize around consumer needs

Sometimes the cleanest microservice API is not what the frontend wants.

Consider:

• Backend-for-Frontend (BFF) services
• API composition layers
• GraphQL for certain read-heavy or UI-driven use cases

This avoids forcing the frontend to orchestrate many backend calls.

14. Watch for common failure modes

Common problems in large microservice API ecosystems:

• Inconsistent endpoint patterns
• Breaking changes without deprecation
• Shared databases or hidden coupling
• Overuse of synchronous calls
• No ownership or unclear service contracts
• Too much logic in the gateway
• No versioning or compatibility testing

15. A practical operating model

A simple approach that works well:

1. Set standards: API style guide, security, naming, versioning
2. Define contracts: OpenAPI/Proto/AsyncAPI
3. Automate checks: linting, testing, compatibility validation
4. Provide platform tooling: templates, auth libs, observability
5. Use review gates for new APIs and breaking changes
6. Monitor and evolve with deprecation and migration plans

Recommended stack pattern

A common enterprise setup might look like:

• REST for external/public APIs
• gRPC for internal synchronous service calls
• Events for asynchronous workflows
• API gateway for edge concerns
• Contract testing and schema registry
• Central documentation portal
• Distributed tracing everywhere

If you want, I can also give you:

1. a reference architecture for microservice APIs,
2. an API style guide template, or
3. a practical rollout plan for a team of 20+ services.