nestjs-backend-patterns

name	nestjs-backend-patterns
description	Stack-adaptive Node.js / TypeScript backend patterns — recon-driven dispatch across NestJS / Express / Fastify with cross-cutting concerns (TypeORM / Prisma / Drizzle ORMs, class-validator / Zod validation, multitenancy via AsyncLocalStorage + PostgreSQL RLS, pino + OpenTelemetry observability, jest + supertest + testcontainers-node testing, NestJS DI scopes discipline, TypeScript strict mode). Loaded by `nestjs-backend-engineer` agent only; closed to main-context auto-invocation. Use when this capability is needed.
metadata	{"author":"CATWILLgh"}

NestJS / Node.js backend patterns — stack-adaptive entry

Preloaded into the nestjs-backend-engineer sub-agent. Provides a dispatch table from project recon to per-stack pattern files, plus universal principles applied across all stacks.

How to use

1. Recon first. Run the script recon.js — node ~/.claude/skills/mainframe/skills/nestjs-backend-patterns/recon.js [project_root] — for deterministic parse of package.json + tsconfig + lockfile. Manual fallback per recon.md when the script is unavailable.
2. Apply universal principles (below) — they hold regardless of stack.
3. Dispatch by recon outcome — read the relevant supporting file(s) from the table below. Do NOT pre-read files irrelevant to the recon outcome (token discipline).
4. For endpoint-specific situational concerns (idempotency, pagination, rate limiting, health probes, config-from-env) — consult api-conventions.md when the concern is in scope.
5. Test per testing.md — the 4-scenario contract for every endpoint is non-negotiable.

Dispatch table

Recon outcome	Read this
framework: nestjs	nestjs.md
framework: express	express.md
framework: fastify	fastify.md
framework: niche-name	fastify.md as closest async-first analogue + flag mismatch
orm: typeorm	typeorm.md
orm: prisma	prisma.md
orm: drizzle	drizzle.md
validation: class-validator OR zod	validation.md
multitenancy: rls OR app-filter	multitenancy.md
observability: pino+otel OR console	observability.md
Any TS strictness question	typescript.md
Any testing task	testing.md

Universal principles (apply across stacks)

These hold regardless of framework / ORM / validation choice. Cross-reference the umbrella CLAUDE.md rules (CQS, debug residue, marker bans, etc.) — they apply here too, not duplicated.

The server is canonical — authority, state, computed values

Validation of inbound request data at the trust boundary is mandatory; that rule lives in the umbrella CLAUDE.md Engineering practices ("Trust framework and type-system guarantees": "data at system boundaries… is untrusted and must be validated"). Apply it. This bullet adds the authority half beyond schema validation:

• Authorization on every protected endpoint, server-checked against actual tenant + role from JWT. Per OWASP Authorization Cheat Sheet: "Access control checks must be performed server-side… client-side checks may be permissible for improving the user experience, they should never be the decisive factor". Guards / decorators alone are not enough for high-stakes operations — re-check ownership in the service layer.
• Business state transitions controlled by the server. Status flow (e.g. draft → submitted → approved) is a whitelist defined and enforced in the service layer. Reject unauthorised transitions there, not at the controller.
• Computed and derived values come from the server. Totals, percentages, aggregate counts, computed prices, derived statuses — recompute server-side. Never accept these as input fields even if the client computed them.
• Related-resource IDs are ownership-verified server-side. When a request body contains machineId, jobId, etc., load the row server-side and check ownership against the JWT tenant before any operation. The client cannot be trusted to send only IDs it owns.
• Client-side validation is a UX accelerator only. Per MDN: "Never trust data passed to your server from the client. Even if your form is validating correctly… a malicious user can still alter the network request". Reproduce all schema and business checks server-side regardless of what the client form did.

Layer split

controller (HTTP orchestration) → service (business logic) → repository (data access) → schemas / DTOs (validation boundaries) → utils (pure helpers). Names map to framework: NestJS Controller/Service/@InjectRepository, Express/Fastify router/service/repo. Business logic NEVER lives in HTTP handlers.

Tenant identity is JWT-sourced

orgId / tenantId comes from the JWT claim, set on every protected request. Endpoints that accept it from the request body are a privilege-escalation pattern — reject at the schema level. Fallback req.body.organization_id ?? 0 is forbidden. See multitenancy.md for AsyncLocalStorage propagation pattern.

Audit trail on state-changing operations

Every CRUD + status-transition on a business entity emits an audit event with orgId, actorUserId, action, entityType, entityId, newValues (omit secrets). Append-only store, never updated. Use a structured auditLog.record(...) helper, not ad-hoc log lines.

Structured logging + tracing

pino logger per module; bind request-scoped context once per request (requestId, userId, orgId). OpenTelemetry auto-instrumentation for the framework + ORM. Logs carry traceId via pino-opentelemetry-transport so backend joins logs + traces. Never log raw request bodies — whitelist fields. See observability.md.

Typed exception handling

Throw framework exceptions (BadRequestException, NotFoundException, ConflictException in NestJS; httpErrors.Conflict in Fastify; or custom domain errors caught by error filter). Never catch (e) and swallow. Domain errors → mapped to HTTP at the framework boundary, never raw ORM errors leaked.

Eager loading discipline

The N+1 query problem is the prime backend regression. List endpoints MUST pre-load relationships used by the response. TypeORM: relations: { ... } (avoid eager: true on entity); Prisma: include (avoid deep nested chains); Drizzle: with: { ... } relations API. See per-ORM files.

Response envelope unification

Paginated list responses: one consistent envelope shape across the API surface, e.g. { items: [...], total: N, page: N, perPage: N, hasMore: bool }. NOT a different shape per resource. Backward-compat aliases acceptable only during documented migration.

HTTP status codes

POST create → 201 Created. POST state-transition → 200 OK. DELETE → 204 No Content. PUT / PATCH → 200 OK with body. 409 Conflict for unique violations (idempotency, race losers). 422 Unprocessable Entity for business-rule violation (semantically valid input, but rule says no).

Bulk endpoints have hard limits

Any endpoint that accepts an array of IDs / objects MUST cap input size — if (items.length > N) throw new BadRequestException("too many items"). Typical N: 50-100. Unbounded bulk endpoints are DoS vectors.

TypeScript strict mode is non-negotiable

strict: true is the floor. noUncheckedIndexedAccess is the high-value addition. See typescript.md. any / as cast without runtime validation / @ts-ignore are banned per umbrella CLAUDE.md.

Out of scope

• Data pipelines (ETL, transform large datasets) — separate data-engineer role.
• Full ML serving stacks — separate ml-engineer role.
• Infrastructure ownership (Kubernetes operators, full IaC) — separate devops-engineer role. Backend engineer reads Dockerfile / CI config, doesn't own them.
• React / Vue / Next.js frontend — separate frontend-engineer role (future).

Sources

Per-supporting-file authoritative URLs are at the bottom of each file. Umbrella enterprise pattern references that informed this skill:

• State of JS 2024 / Stack Overflow Developer Survey 2024 — framework adoption data informing the NestJS / Express / Fastify Top 3 coverage decision.
• TypeORM 0.3 docs — https://typeorm.io/
• Prisma docs — https://www.prisma.io/docs/
• Drizzle ORM docs — https://orm.drizzle.team/
• NestJS docs — https://docs.nestjs.com/
• Node.js AsyncLocalStorage — https://nodejs.org/api/async_context.html
• PostgreSQL Row Security — https://www.postgresql.org/docs/current/ddl-rowsecurity.html
• pino + OpenTelemetry JS — https://getpino.io/ + https://opentelemetry.io/docs/languages/js/
• OWASP Input Validation + Authorization Cheat Sheets — https://cheatsheetseries.owasp.org/
• TypeScript tsconfig — https://www.typescriptlang.org/tsconfig/#strict

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