Software Backend Engineering

Use this skill for backend service implementation and review: API boundaries, auth, data access, jobs, caching, observability, and production hardening. If the main question is platform selection, system topology, or API-contract design without implementation, hand off early.

Defaults

When this skill is active, prefer these defaults unless the repo or user says otherwise:

• validate at the boundary and keep types explicit
• use PostgreSQL plus pooling for relational workloads
• use structured logs, OpenTelemetry, explicit timeouts, and rate limits
• make mutations idempotent and background work retry-safe
• use RFC 9457 Problem Details for machine-readable errors

Quick Reference

When to Use This Skill

• building or reviewing REST, GraphQL, tRPC, Connect, or gRPC services
• implementing auth, validation, rate limits, caching, queues, or webhook handling
• modelling schemas and running safe migrations
• hardening service behavior for retries, timeouts, and observability
• scaffolding or refactoring a backend with production defaults

Route Elsewhere

• frontend-only work -> [software-frontend](../software-frontend/SKILL.md)
• infrastructure provisioning and cluster design -> [ops-devops-platform](../ops-devops-platform/SKILL.md)
• API contract design without implementation -> [dev-api-design](../dev-api-design/SKILL.md)
• BaaS platform selection (data/auth layer) -> [software-baas-platforms](../software-baas-platforms/SKILL.md)
• PaaS hosting selection (compute layer: Vercel, Fly.io, Railway, Render, Cloudflare Workers, Deno Deploy) -> [software-paas-hosting](../software-paas-hosting/SKILL.md)
• SQL tuning and indexing deep dives -> [data-sql-optimization](../data-sql-optimization/SKILL.md)
• security reviews and threat modelling -> [software-security-appsec](../software-security-appsec/SKILL.md)
• broader system architecture -> [software-architecture-design](../software-architecture-design/SKILL.md)

Workflow

1. Confirm the real constraint: latency, team skill, runtime, compliance, data model, or delivery speed.
2. Choose the transport and framework based on that constraint, not on trend-chasing.
3. Define the boundary:

- request and response contracts - auth and authorization rules - error model - idempotency and rate limiting

1. Define the data path:

- schema and migrations - transaction boundaries - pooling and query budgets - cache and invalidation rules

1. Define the async path:

- queue semantics - retry ownership - deduplication and DLQ

1. Add operability before calling it complete:

- timeouts and cancellation - health checks - structured logs and traces - deploy and rollback expectations

ASCII Flow

Backend task
  -> Define endpoint, job, service, or data boundary
  -> Confirm runtime, framework, persistence, and integration contracts
  -> Design request validation, auth, errors, and idempotency
  -> Implement bounded slice with tests and observability
  -> Check performance, security, and rollout risk
  -> Verify behavior and document follow-up handoffs

Technology Selection

Pick based on the strongest operational constraint:

• TypeScript-heavy team -> Fastify, Hono, or NestJS plus Prisma or Drizzle
• audited SQL and predictable concurrency -> Go with sqlc/pgx
• Python ecosystem or ML adjacency -> FastAPI plus SQLAlchemy
• enterprise .NET stack -> ASP.NET Core plus EF Core or explicit SQL access
• memory safety and explicitness -> Rust with Axum plus SQLx
• edge or serverless first -> lightweight stateless handlers with hard CPU and timeout budgets

Use [software-baas-platforms](../software-baas-platforms/SKILL.md) first when the real requirement is "ship auth, storage, and realtime quickly with less custom service code."

Backend Non-Negotiables

Performance and Reliability Triage

When a service is slow or unstable, debug in this order:

Do not add caching before you understand the real bottleneck.

Operational Playbooks

• use [references/operational-playbook.md](references/operational-playbook.md) for full service design and review checklists
• use [qa-resilience](../qa-resilience/SKILL.md) when retries, deadlines, breakers, or degraded-mode behavior are the main question
• use [dev-api-design](../dev-api-design/SKILL.md) when the contract itself is the main artifact

Production Readiness Checklist

Before marking a service production-ready:

• All mutating endpoints have idempotency keys or safe-retry semantics
• Every outbound call has an explicit timeout (no framework-default infinite waits)
• Health endpoint distinguishes liveness from readiness
• Correlation IDs propagated from inbound request to all downstream calls and logs
• DLQ policy defined for every queue consumer (what happens to poison messages)
• New query plans verified (EXPLAIN ANALYZE) before merge to main
• Rollback procedure documented and smoke-test list exists

Known Traps

• Introducing asynchronous jobs to hide a broken synchronous path instead of fixing the contract, timeout budget, or workload shape.
• Shipping retries without deadlines, jitter, and idempotency keys, then multiplying load during incidents.
• Changing API or webhook behavior without a compatibility window, replay plan, or structured error-versioning posture.
• Adding caches before proving whether the real bottleneck is query shape, pooling, lock contention, or outbound dependency latency.
• Treating background consumers as “fire and forget” even though poison-message handling, replay semantics, and observability are undefined.

Common Anti-Patterns

• Letting framework defaults define the service contract, error model, and cancellation semantics.
• Using one generic repository abstraction for every query, including hot paths that need explicit SQL, batching, or shape control.
• Mixing request handling, domain logic, external side effects, and persistence concerns in one controller or handler.
• Relying on eventual retries to clean up non-idempotent side effects.
• Calling a backend “production ready” before timeouts, readiness checks, trace correlation, and rollback smoke tests exist.

Navigation

Core references

• [references/backend-best-practices.md](references/backend-best-practices.md)
• [references/edge-deployment-guide.md](references/edge-deployment-guide.md)
• [references/infrastructure-economics.md](references/infrastructure-economics.md)
• [references/database-patterns.md](references/database-patterns.md)
• [references/message-queues-background-jobs.md](references/message-queues-background-jobs.md)
• [references/rpc-and-transport-patterns.md](references/rpc-and-transport-patterns.md)
• [references/go-best-practices.md](references/go-best-practices.md)
• [references/rust-best-practices.md](references/rust-best-practices.md)
• [references/python-best-practices.md](references/python-best-practices.md)
• [references/nodejs-best-practices.md](references/nodejs-best-practices.md)
• [references/csharp-best-practices.md](references/csharp-best-practices.md)
• [data/sources.json](data/sources.json)

Shared review utilities

• [../software-clean-code-standard/assets/checklists/backend-api-review-checklist.md](../software-clean-code-standard/assets/checklists/backend-api-review-checklist.md)
• [../software-clean-code-standard/assets/checklists/secure-code-review-checklist.md](../software-clean-code-standard/assets/checklists/secure-code-review-checklist.md)
• [../software-clean-code-standard/references/auth-utilities.md](../software-clean-code-standard/references/auth-utilities.md)
• [../software-clean-code-standard/references/error-handling.md](../software-clean-code-standard/references/error-handling.md)
• [../software-clean-code-standard/references/config-validation.md](../software-clean-code-standard/references/config-validation.md)
• [../software-clean-code-standard/references/resilience-utilities.md](../software-clean-code-standard/references/resilience-utilities.md)
• [../software-clean-code-standard/references/logging-utilities.md](../software-clean-code-standard/references/logging-utilities.md)
• [../software-clean-code-standard/references/testing-utilities.md](../software-clean-code-standard/references/testing-utilities.md)
• [../software-clean-code-standard/references/observability-utilities.md](../software-clean-code-standard/references/observability-utilities.md)

Templates

• [assets/nodejs/template-nodejs-prisma-postgres.md](assets/nodejs/template-nodejs-prisma-postgres.md)
• [assets/nodejs/template-nodejs-fastify-drizzle-postgres.md](assets/nodejs/template-nodejs-fastify-drizzle-postgres.md)
• [assets/go/template-go-fiber-gorm.md](assets/go/template-go-fiber-gorm.md)
• [assets/go/template-go-chi-sqlc-pgx.md](assets/go/template-go-chi-sqlc-pgx.md)
• [assets/rust/template-rust-axum-seaorm.md](assets/rust/template-rust-axum-seaorm.md)
• [assets/rust/template-rust-axum-sqlx.md](assets/rust/template-rust-axum-sqlx.md)
• [assets/python/template-python-fastapi-sqlalchemy.md](assets/python/template-python-fastapi-sqlalchemy.md)
• [assets/csharp/template-csharp-aspnet-efcore.md](assets/csharp/template-csharp-aspnet-efcore.md)

Related Skills

Gate before invoking any foundation below: Each foundation has a When to Apply / When to Skip section. If your task matches a skip-condition, route to the foundation it names instead — don't pull in primitives the task doesn't need.

• [software-architecture-design](../software-architecture-design/SKILL.md)
• [software-security-appsec](../software-security-appsec/SKILL.md)
• [ops-devops-platform](../ops-devops-platform/SKILL.md)
• [qa-resilience](../qa-resilience/SKILL.md)
• [qa-testing-strategy](../qa-testing-strategy/SKILL.md)
• [foundations-queueing-theory](../foundations-queueing-theory/SKILL.md) — Little's Law, M/M/c, and Kingman's formula for queue sizing in message-queue and rate-limiter design
• [foundations-distributed-systems](../foundations-distributed-systems/SKILL.md) — CAP, consistency models, quorum sizing, and idempotency contracts for service mesh and RPC patterns
• [foundations-reliability-theory](../foundations-reliability-theory/SKILL.md) — MTBF/MTTR, availability composition, and error-budget math for SLO-driven backend design
• [software-code-review](../software-code-review/SKILL.md)
• [dev-api-design](../dev-api-design/SKILL.md)
• [data-sql-optimization](../data-sql-optimization/SKILL.md)

Fact-Checking

• Known bugs, regressions, framework/compiler/runtime footguns, and version-specific crash or workaround guidance must be verified against current primary web sources before being treated as current fact.
• Verify current runtime versions, support windows, framework capabilities, and cloud-platform constraints before final answers.
• Prefer official docs and release or support policy pages for version-sensitive recommendations.
• If web access is unavailable, mark version or support guidance as unverified.

Learnings Loop

Before applying this skill on a non-trivial task, read learnings.consolidated.md in this directory (and learnings.md if present).

After applying it, if you encountered a pattern worth remembering, a mistake worth preventing, or a domain fact that surprised you, append one dated bullet to learnings.md via agents-skills-feedback-loop/scripts/append_learning.py. Do not modify SKILL.md itself.