backend-engineering

## Overview Backend services are the load-bearing walls of software systems. They must be reliable under partial failure, observable in production, and secure against malicious input. This skill applies the engineering discipline required for production-grade services. ## When to Use - Before implementing a new service or significant backend feature - When a service has reliability, performance, or security issues - As part of the `/review` workflow for backend PRs ## Process ### Step 1: Define the contract Document: endpoints/methods, request/response schemas, error codes, authentication requirements, rate limits. No implementation before the contract is defined. ### Step 2: Design for failure Every outbound call must have: timeout, retry with backoff, circuit breaker, fallback. Services that don't handle dependency failures will fail when their dependencies fail. ### Step 3: Input validation at the boundary Validate all incoming data before processing: types, required fields, length limits, format constraints. Reject invalid input — don't try to fix it. ### Step 4: Idempotency for mutations All write operations should be idempotent where possible. Use idempotency keys for payment and other sensitive operations. ### Step 5: Database access - No N+1 queries - Transactions for operations that must be atomic - Read replicas for read-heavy workloads - Connection pooling configured ### Step 6: Logging and observability Implement structured logging, metrics (four golden signals), and distributed tracing. See `observability-and-monitoring`. ### Step 7: Security - Authenticate every request - Authorize every resource access - Validate all inputs - No secrets in logs or responses See `security-and-hardening`. ### Step 8: Graceful shutdown Handle: SIGTERM, drain in-flight requests, close database connections, flush logs. Services that don't handle graceful shutdown cause errors during deployments. ## Verification Requirements - [ ] Contract documented before implementation - [ ] All outbound calls have timeout and retry - [ ] Input validation at the service boundary - [ ] No N+1 queries - [ ] Authentication and authorization on every request/resource - [ ] Graceful shutdown implemented - [ ] Four golden signals instrumented