How do I build APIs across dozens of microservices?

Building APIs across dozens of microservices is less about “making endpoints” and more about setting up a consistent platform, clear contracts, and strong governance so teams can move independently without creating chaos.

1. Start with an API strategy

Decide what kind of APIs you’re building:

• External APIs for customers/partners
• Internal service APIs for service-to-service communication
• BFFs / aggregators for web/mobile clients
• Event-driven interfaces for async integration

A common pattern is:

• Public edge APIs: stable, curated, versioned
• Internal service APIs: optimized for service needs, not exposed externally
• Events for cross-domain changes instead of excessive synchronous calls

2. Define API ownership by domain

Each microservice should own:

• Its data
• Its business logic
• Its API contract

Avoid a central team building every endpoint. Instead:

• Use domain boundaries
• Assign a product/engineering owner per service
• Make APIs part of the service’s lifecycle, not an afterthought

3. Standardize API design

Create a company-wide API style guide covering:

• Naming conventions
• Resource modeling
• Pagination/filtering/sorting
• Error format
• Auth patterns
• Versioning strategy
• Idempotency rules
• Correlation/request IDs
• Rate limiting behavior

Example standards:

• Use nouns, not verbs: /orders, /orders/{id}
• Use consistent errors:

  {
    "error": {
      "code": "ORDER_NOT_FOUND",
      "message": "Order does not exist",
      "traceId": "abc123"
    }
  }
  

4. Prefer contract-first development

For many services, especially at scale, define the contract before implementation.

Options:

• OpenAPI for REST
• AsyncAPI for events
• gRPC/proto for internal high-performance APIs

Benefits:

• Clear interface before code
• Easier code generation
• Better compatibility checks
• Easier documentation and testing

5. Build a shared platform/tooling layer

Do not reinvent every service’s API plumbing.

Provide shared libraries or platform capabilities for:

• Authentication/authorization
• Logging/tracing/metrics
• Validation
• Error handling
• Serialization
• Client generation
• Retry/circuit breaker policies
• Rate limiting
• Pagination helpers

But keep shared code light. Don’t create a “god library” that all services are forced to upgrade together.

6. Use an API gateway at the edge

An API gateway helps with:

• Authentication
• TLS termination
• Request routing
• Rate limiting
• WAF policies
• API keys/OAuth
• Observability
• Sometimes response aggregation

Good for:

• Public/external APIs
• Centralized policies

Be careful:

• Don’t cram business logic into the gateway
• Don’t make the gateway a bottleneck or monolith

7. Separate internal and external API concerns

Internal service APIs can be simpler and more performance-focused.

External APIs should often:

• Be more stable
• Hide internal service structure
• Aggregate across multiple services
• Be designed around consumer needs

A common pattern is:

• Microservices expose internal APIs
• A BFF or API gateway layer exposes consumer-facing APIs

8. Manage versioning and compatibility carefully

With dozens of services, breaking changes become expensive.

Use these principles:

• Prefer backward-compatible changes
• Add fields rather than changing semantics
• Deprecate before removing
• Avoid tight coupling between services
• Track API lifecycle states: alpha, beta, stable, deprecated

For REST:

• Prefer additive evolution
• Use versioning only when necessary, not by default
• If you version, be consistent: /v1/...

For events:

• Treat events as immutable
• Add fields, don’t rename/remove casually
• Use schema registry or contract validation if possible

9. Automate testing at multiple layers

You need more than unit tests.

Recommended layers:

• Unit tests for business logic
• Contract tests between consumers and providers
• Integration tests for database and dependencies
• End-to-end tests for critical journeys
• Schema compatibility checks for APIs/events

For microservices, contract testing is especially important to prevent breaking consumers.

10. Make observability mandatory

Every service API should include:

• Structured logs
• Distributed tracing
• Metrics
• Correlation IDs
• Error and latency dashboards

Track:

• Request volume
• Latency percentiles
• Error rates
• Timeout/retry rates
• Dependency health

If you can’t observe a service API well, you can’t operate it well.

11. Design for resilience

Microservice APIs fail often enough that resilience matters.

Implement:

• Timeouts
• Retries with backoff
• Circuit breakers
• Bulkheads
• Fallbacks where appropriate
• Idempotency keys for safe retries

Rule of thumb:

• Retry only when the operation is safe to repeat
• Never let retries amplify outages

12. Control sprawl with governance

With dozens of services, you need light governance:

• API review process
• Automated linting against standards
• Security reviews for public APIs
• Deprecation policy
• Ownership registry
• Documentation requirements

Use automation instead of manual policing:

• OpenAPI linting
• CI checks for breaking changes
• Required metadata: owner, SLA, lifecycle, contact, docs

13. Document everything in a service catalog

Maintain a catalog with:

• Service name
• Owner/team
• Purpose
• API spec link
• Dependencies
• Data classification
• SLOs
• On-call/contact
• Deprecation status

This becomes crucial at scale.

14. Keep service boundaries sane

A lot of API pain comes from bad service boundaries.

Good boundaries:

• Align with business domains
• Minimize cross-service synchronous chatter
• Avoid “chatty” APIs
• Reduce shared databases
• Prefer eventual consistency where suitable

If one request requires 15 services synchronously, the architecture likely needs rethinking.

15. A practical operating model

A good setup for dozens of microservices looks like this:

• Platform team provides:
  • API gateway
  • Auth
  • Observability
  • Contract tooling
  • API guidelines
  • CI templates
• Domain teams own:
  • Service implementation
  • Service API contracts
  • Backward compatibility
  • Domain-specific docs/tests

Recommended stack patterns

Depending on your environment:

REST-heavy

• OpenAPI
• API gateway
• Contract tests
• Shared linting rules

Internal high-performance services

• gRPC
• Proto contracts
• Code generation
• Strong schema compatibility checks

Event-driven architecture

• Kafka/PubSub/SNS/SQS/etc.
• AsyncAPI or schema registry
• Immutable event contracts
• Consumer-driven compatibility checks

Biggest mistakes to avoid

• No API standards
• No ownership
• Breaking changes without automation
• Putting business logic in the gateway
• Overusing synchronous service calls
• Sharing databases across services
• Ignoring observability
• Letting every team invent its own error formats and auth rules

If you want a simple default approach

For most organizations, a strong baseline is:

• REST for external APIs
• gRPC or REST for internal service APIs
• Events for cross-domain integration
• OpenAPI + contract tests + API gateway + shared observability
• Per-domain ownership with a service catalog

If you want, I can also give you:

1. a reference architecture for this setup,
2. a team operating model, or
3. a practical API standards template you can adopt across services.