backend-designlisted

# Backend Design Skill This skill guides the design of reliable backend systems, APIs, and data models with clear trade-offs, strong correctness guarantees, and operational readiness. ## Design Thinking - **Scope**: Identify core use cases, data flows, and integration points. - **Constraints**: Latency, throughput, availability, compliance, and deployment limits. - **Data**: Define entities, relationships, invariants, and lifecycle. - **Reliability**: Error handling, retries, idempotency, and consistency model. ## Architecture Guidelines - **API Design**: Explicit contracts, versioning, validation, and error shape. - **Storage**: Choose stores based on access patterns and durability needs. - **Performance**: Indexing, caching, batching, and async processing. - **Security**: AuthN/AuthZ, least privilege, secrets handling, auditability. - **Observability**: Logging, metrics, tracing, and alerting signals. - **Scalability**: Stateless services where possible, clear scaling boundaries. ## Output Requirements Provide a clear backend plan or implementation that includes: - Data model and key invariants - API endpoints or service interfaces - Failure modes and recovery strategy - Performance and scaling considerations - Security and observability notes ## Response Checklist ```markdown Backend Design Checklist - [ ] Confirm requirements and constraints - [ ] Define data model and invariants - [ ] Specify API contracts and error shape - [ ] Address failure modes and consis