Design Task: API, Validation, and Error Handling for a Top-K Frequency Service

Context

Design a production-grade service that, given an array of elements, returns the top-k most frequent values. Focus specifically on API design, input validation, and error handling, with clear operational guardrails.

Requirements

1. Define the API surface:
   • Request/response schema, including versioning and content types.
   • Limits: payload size, bounds on k, element type domains and sizes.
   • Idempotency semantics and request deduplication.
   • Authentication and authorization.
   • Rate limits and quotas.
   • Timeouts and service-level objectives.
   • Error taxonomy: classify client vs. server errors and include structured error responses.
2. Describe validation layers and schema enforcement:
   • Client-side validation expectations.
   • Gateway/WAF validation and protections.
   • Service-level validation and enforcement.
   • How partial-failure behavior should work (e.g., invalid elements in the array).
3. Observability and safety:
   • Metrics, logs, and distributed traces.
   • Safeguards for malformed or adversarial payloads (e.g., large, deeply nested, high-cardinality inputs).
4. Testing and production readiness:
   • How you would test the system pre-deployment (unit, integration, fuzz, property-based).
   • How you would monitor, alert, and run this safely in production.

Assume synchronous processing for modest requests, with an option to propose an asynchronous path for larger payloads if needed.