Building APIs at scale? Learn about our Software Development services.

Read also: Software Project Estimation: Methods and Best Practices

A well-designed API is a force multiplier — it enables teams to build independently, partners to integrate seamlessly, and systems to evolve without breaking consumers. A poorly designed API becomes a permanent source of friction, workarounds, and breaking changes. This checklist covers the decisions and standards that separate production-grade APIs from prototypes.

Resource Design

The foundation of a REST API is its resource model — how you represent business entities as URLs.

Naming conventions

  • Use plural nouns for collections: /users, /orders, /products
  • Use the resource ID for individual items: /users/123, /orders/456
  • Use nested resources for clear parent-child relationships: /users/123/orders
  • Keep nesting to a maximum of 2 levels — deeper nesting indicates a design problem
  • Use kebab-case for multi-word resources: /order-items, not /orderItems or /order_items
  • Avoid verbs in URLs — the HTTP method is the verb: POST /orders not POST /create-order

HTTP methods

  • GET — retrieve a resource (idempotent, cacheable, no side effects)
  • POST — create a new resource (not idempotent)
  • PUT — replace a resource entirely (idempotent)
  • PATCH — update specific fields of a resource (idempotent)
  • DELETE — remove a resource (idempotent)

Response codes

  • 200 OK — successful GET, PUT, PATCH, or DELETE
  • 201 Created — successful POST (include Location header with the new resource URL)
  • 204 No Content — successful DELETE with no response body
  • 400 Bad Request — malformed request syntax or invalid parameters
  • 401 Unauthorized — missing or invalid authentication
  • 403 Forbidden — authenticated but not authorized for this resource
  • 404 Not Found — resource does not exist
  • 409 Conflict — request conflicts with current state (e.g., duplicate creation)
  • 422 Unprocessable Entity — valid syntax but semantic validation failed
  • 429 Too Many Requests — rate limit exceeded
  • 500 Internal Server Error — unexpected server failure (never by design)

Pagination, Filtering, and Sorting

Collection endpoints must handle large datasets gracefully.

Pagination

  • Use cursor-based pagination for large or real-time datasets: ?cursor=abc123&limit=20
  • Use offset-based pagination for simple use cases: ?page=3&per_page=20
  • Return pagination metadata in the response: total count, next/previous links, current page
  • Set a reasonable default page size (20-50) and a maximum (100-200)
  • Include Link headers for HATEOAS-compliant pagination

Filtering

  • Support field-level filtering: ?status=active&role=admin
  • Use clear operators for ranges: ?created_after=2026-01-01&created_before=2026-03-01
  • Support multiple values: ?status=active,pending or ?status[]=active&status[]=pending
  • Document all available filters per endpoint

Sorting

  • Support sorting with clear syntax: ?sort=created_at (ascending), ?sort=-created_at (descending)
  • Support multi-field sorting: ?sort=-priority,created_at
  • Define a default sort order for every collection endpoint
  • Index database columns used for sorting

Versioning

Strategy selection

  • Choose one versioning strategy and apply it consistently across all endpoints
  • URL path versioning (/v1/users) — recommended for most cases
  • Document the current version and all supported versions
  • Set a deprecation policy: minimum 12 months support after new version release

Version lifecycle

  • Announce new versions at least 3 months before old versions are deprecated
  • Provide a migration guide for each version upgrade
  • Track usage of deprecated versions — do not remove them while clients still use them
  • Return deprecation warnings in response headers for old versions

Authentication and Security

Authentication

  • Use OAuth 2.0 with JWT tokens for user-facing APIs
  • Use API keys for server-to-server communication
  • Transmit credentials only over HTTPS — never in URL parameters
  • Set appropriate token expiration (15-60 minutes for access tokens, 7-30 days for refresh tokens)
  • Implement token refresh without requiring re-authentication

Authorization

  • Implement role-based or attribute-based access control
  • Validate permissions at the resource level — not just the endpoint level
  • Return 403 Forbidden (not 404) when the resource exists but the user lacks access — unless hiding resource existence is a security requirement
  • Log all authorization failures for security monitoring

Input validation

  • Validate all input parameters — type, length, format, allowed values
  • Sanitize inputs to prevent injection attacks (SQL, NoSQL, XSS)
  • Reject unknown fields in request bodies (strict parsing)
  • Set maximum request body size limits

Rate limiting

  • Implement per-user and per-IP rate limits
  • Return 429 Too Many Requests with Retry-After header
  • Use different limits for different endpoint categories (read vs write, public vs authenticated)
  • Monitor rate limit hits to detect abuse patterns

Security headers

  • Strict-Transport-Security — enforce HTTPS
  • X-Content-Type-Options: nosniff — prevent MIME sniffing
  • X-Frame-Options: DENY — prevent clickjacking
  • CORS headers configured for specific allowed origins — never * in production

Error Handling

Standard error format

  • Use a consistent error response structure across all endpoints:
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "The request body contains invalid fields.",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address."
      }
    ],
    "request_id": "req_abc123"
  }
}
  • Include a machine-readable error code for programmatic handling
  • Include a human-readable message for developer debugging
  • Include a request ID for support and log correlation
  • Never expose stack traces, file paths, or database errors in production responses

Documentation

OpenAPI specification

  • Maintain an OpenAPI 3.x specification file as the single source of truth
  • Document every endpoint: path, method, parameters, request body, responses
  • Include example requests and responses for every endpoint
  • Document error responses — not just the happy path
  • Generate interactive documentation (Swagger UI, Redoc) from the spec

Developer experience

  • Provide a getting-started guide with authentication setup and first API call
  • Include code examples in at least 3 languages (curl, Python, JavaScript)
  • Maintain a changelog documenting all API changes by version
  • Provide a sandbox environment for testing without affecting production data

Testing

Unit tests

  • Test each endpoint for valid input — correct response code, response structure, side effects
  • Test each endpoint for invalid input — proper error codes and messages
  • Test authentication and authorization — unauthenticated, unauthorized, and authorized requests
  • Test pagination boundaries — first page, last page, empty page, invalid page

Integration tests

  • Test end-to-end request flows — create, read, update, delete sequences
  • Test error propagation from downstream services
  • Test rate limiting behavior under load
  • Test with realistic data volumes — not just single records

Contract tests

  • Validate API responses against the OpenAPI specification
  • Run contract tests in CI/CD to catch breaking changes before deployment
  • Test backward compatibility when releasing new versions

Performance tests

  • Benchmark response times for critical endpoints under expected load
  • Test with concurrent requests to identify bottlenecks
  • Verify pagination performance with large datasets (100K+ records)
  • Set response time SLAs: p50 < 100ms, p95 < 500ms, p99 < 1,000ms

Performance Optimization

Caching

  • Set Cache-Control headers for GET endpoints: max-age, no-cache, private
  • Use ETag headers for conditional requests — reduce bandwidth for unchanged resources
  • Implement server-side caching for expensive queries (Redis, Memcached)
  • Cache at the CDN level for public, read-heavy endpoints

Payload optimization

  • Support field selection: ?fields=id,name,email to reduce response size
  • Use compression (gzip, Brotli) for response bodies
  • Avoid deeply nested response structures — flatten where possible
  • Return IDs for related resources instead of full objects (with expand option for when full data is needed)

Database optimization

  • Index all fields used in filters and sorting
  • Use database connection pooling
  • Implement query timeouts to prevent runaway queries
  • Monitor slow queries and optimize them before they become bottlenecks

How ARDURA Consulting Supports API Development

Designing and building production-grade APIs requires senior backend engineers with experience in distributed systems, security, and performance optimization. ARDURA Consulting provides the talent:

  • 500+ senior specialists including backend architects, API designers, and security engineers experienced in building APIs that serve millions of requests — available within 2 weeks
  • 40% cost savings compared to permanent hiring, allowing you to staff API projects with senior expertise without long-term headcount commitments
  • 99% client retention — engineers who understand your domain deeply enough to design APIs that evolve gracefully with your business
  • 211+ completed projects including platform API design, microservices communication layers, and public API programs for SaaS products

Whether you need an architect to design your API strategy or senior developers to implement and test it, ARDURA Consulting provides the expertise to build APIs that scale, secure, and last.