Backend Design

This skill creates a backend design brief through structured conversation. It is the server-side counterpart to design-brief. Skip any question the codebase already answers — read first, then ask only what is unresolved.

Example prompts

• "Write a backend brief for the notifications service"
• "Plan the backend for the checkout flow"
• "I need to think through the data model and auth before building this API"
• "Backend brief: a job queue that processes uploaded videos"

Process

1. Ask the user for a one-paragraph description of what they want to build, who/what calls it, and any constraints they already have in mind (latency, region, compliance, expected scale).

2. Explore the existing codebase to learn what is already decided. Scan specifically for:

   Fastify-specific (check first, since this is the primary stack)

   • fastify in package.json and the server entry (often server.ts, app.ts, src/server.ts)
   • Plugin tree: every fastify.register(...) call and any files using fastify-plugin (fp(...)) — these define the encapsulation boundaries
   • Route definitions: fastify.get/post/put/delete, fastify.route({...}), route schemas (schema: { body, querystring, params, response })
   • Schema strategy: raw JSON Schema, @sinclair/typebox, fastify-type-provider-zod, or @fastify/type-provider-typebox
   • Hooks in use: onRequest, preParsing, preValidation, preHandler, preSerialization, onSend, onResponse, onError
   • Decorators: fastify.decorate(...), fastify.decorateRequest(...), fastify.decorateReply(...) — these reveal cross-cutting concerns (auth context, db handle, current user)
   • Common ecosystem plugins already wired up: @fastify/cors, @fastify/helmet, @fastify/rate-limit, @fastify/jwt, @fastify/cookie, @fastify/session, @fastify/multipart, @fastify/static, @fastify/swagger, @fastify/under-pressure, @fastify/auth, @fastify/oauth2, @fastify/websocket
   • Logger config: Pino is the default — check logger: { ... } options, transport, redaction config, log level per environment
   • Error handling: setErrorHandler, setNotFoundHandler, custom error classes
   • Server lifecycle: fastify.ready(), fastify.listen(...), graceful shutdown (closeWithGrace, @fastify/graceful-shutdown)

   Other backends (scan if Fastify isn't present, or to identify hybrid setups)

   • Express, NestJS, Koa, Hapi, FastAPI, Django, Rails, Go (chi/gin/echo), tRPC routers, GraphQL servers (Apollo, Mercurius — note Mercurius is Fastify-native)

   ORM / schema

   • Prisma (schema.prisma), Drizzle (schema.ts, drizzle.config.ts), Kysely, TypeORM, MikroORM, raw SQL with postgres/pg
   • Migrations directory and how migrations are run (Prisma Migrate, Drizzle Kit, custom)

   Datastores

   • Postgres (most common with Fastify), MySQL, SQLite, MongoDB, Redis (@fastify/redis or ioredis directly), DynamoDB
   • Connection pool config — Fastify benefits a lot from sized pools tied to plugin lifetime

   Async / jobs

   • BullMQ (often paired with Fastify via Redis), Inngest, Trigger.dev, Temporal, raw cron, webhook receivers

   Caching

   • Redis, in-memory (@fastify/caching, lru-cache), HTTP cache headers, edge cache

   Deployment / runtime

   • Dockerfile, docker-compose.yml, fly.toml, railway.toml, render.yaml, Kubernetes manifests
   • Serverless adapters: @fastify/aws-lambda, @fastify/serverless — note Fastify is happier as a long-running process than as a per-request lambda

   Observability

   • Pino transports (pino-pretty in dev, pino-loki/pino-datadog in prod), @fastify/under-pressure for load shedding, OpenTelemetry instrumentation (@opentelemetry/instrumentation-fastify), Sentry (@sentry/node with Fastify integration), healthcheck routes

   Config / secrets

   • @fastify/env with JSON Schema validation, dotenv, secret managers, feature flag clients
   • If a frontend brief or IA exists at .design/<slug>/DESIGN_BRIEF.md or INFORMATION_ARCHITECTURE.md, read it. The data model and routes must serve those flows.
   • Treat what exists as the starting vocabulary. Extend, don't replace.

3. Interview the user on each unresolved area below. Ask one question at a time. For each, propose a recommended answer and explain the tradeoff so the user can push back. Skip any area the codebase scan answered definitively.

   Data model

   • What are the core entities and how do they relate?
   • Which fields are required vs. optional? Which are derived?
   • What are the natural unique constraints and indexes?
   • What is the expected row count per entity in 1 month, 1 year?
   • Soft delete vs. hard delete? Audit trail needed?

   Auth model

   • Who calls this? (end users, internal services, third-party integrations)
   • Authentication: sessions, JWT, API keys, OAuth, mTLS?
   • Authorization: role-based, attribute-based, resource-scoped, multi-tenant isolation?
   • What is the blast radius if a credential leaks?

   Fastify shape (skip if not Fastify)

   • Plugin boundaries: which concerns are isolated plugins (auth, db, business domain) vs. flat in the root scope?
   • Schema strategy: raw JSON Schema, TypeBox, or Zod via fastify-type-provider-zod? Pick one and stick to it — mixing is painful.
   • Where does validation happen: route schema (preferred — Fastify compiles it), preValidation hook, or downstream service?
   • Decorators planned for cross-cutting state: request.user, request.tenantId, fastify.db, fastify.cache?
   • Error handling: one global setErrorHandler mapping to RFC 7807 problem details, or per-plugin error handlers?
   • Logger: keep Pino default, or wire a transport (loki, datadog, otel)? What gets redacted?

   Scale & latency targets

   • p50 / p95 / p99 latency budget for the hot path?
   • Requests per second at launch, in 6 months, at the ceiling we'd celebrate?
   • Read-heavy, write-heavy, or balanced?
   • Geographic distribution of callers?

   Consistency requirements

   • Strong consistency, read-your-writes, or eventual consistency acceptable?
   • Where are transactions required? Where can we tolerate retries / idempotency keys instead?
   • Any operations that must be exactly-once vs. at-least-once?

   Deployment target

   • Serverless, long-running container, edge runtime, or VM?
   • One region or multi-region? Cold-start tolerance?
   • How are migrations applied? Zero-downtime requirements?
   • Blue/green, canary, or rolling deploys?

   Observability

   • What signals tell us this is healthy? (latency, error rate, queue depth, business metrics)
   • What gets logged at info vs. error? Any PII redaction needed?
   • Tracing across services? Sampling rate?
   • Alerts: who gets paged, on what threshold?

4. Once you have a complete picture, write the brief using the template below.

File Output

Save the brief to .design/<feature-slug>/BACKEND_DESIGN.md, using the same <feature-slug> as any existing frontend brief in .design/. If no .design/ folder exists yet, create one and pick a slug derived from the feature name (e.g., notifications-service, checkout-api, video-processor).

If a DESIGN_BRIEF.md already exists in the chosen subfolder, cross-reference it: the data model and API should serve the flows and components named there.

Example:

.design/
└── checkout-api/
    ├── DESIGN_BRIEF.md       (if frontend brief exists)
    └── BACKEND_DESIGN.md     ← this skill produces this

Brief Template

# Backend Design: [Service / Feature Name]

## Problem

What this backend exists to do, framed by the calling context (which frontends, jobs, or external systems depend on it and why). Not implementation detail — purpose.

## Solution Sketch

The shape of the solution in plain terms: what kind of system this is (CRUD API, event processor, job runner, gateway), and the one or two architectural choices that define it.

## Callers & Consumers

| Caller            | Pattern              | Auth method | Notes |
| ----------------- | -------------------- | ----------- | ----- |
| [frontend / svc]  | [request/response, webhook, subscribe] | [session, JWT, API key] | [rate, criticality] |

## Data Model

### Entities

For each entity: fields, types, required/optional, indexes, relationships.

EntityName ├── id [pk, type] ├── field_a [type, required, indexed] ├── field_b [type, optional] └── relations [foreign keys / joins]

### Invariants

Business rules the schema must enforce (uniqueness, referential integrity, state machines, allowed transitions). Note which are enforced at the DB layer vs. application layer and why.

### Growth & Retention

Expected size per entity at 1 month, 1 year. Retention policy (keep forever, archive after N, hard delete after N).

## API Surface

| Method | Path | Purpose | Auth | Idempotent? |
| ------ | ---- | ------- | ---- | ----------- |
| GET    | /... | ...     | ...  | yes         |
| POST   | /... | ...     | ...  | no (use idempotency key) |

For non-trivial endpoints, sketch request/response shape inline.

## Fastify Architecture (if applicable)

- **Plugin tree**: how the app is decomposed into plugins and which use `fastify-plugin` to escape encapsulation.
- **Schema/validation**: chosen strategy (JSON Schema / TypeBox / Zod) and where it lives.
- **Decorators**: cross-cutting state attached to `fastify`, `request`, `reply`.
- **Hooks**: which lifecycle hooks are used and for what (auth in `preHandler`, audit in `onResponse`, etc.).
- **Error handling**: shape of the error response, where `setErrorHandler` lives.
- **Logger**: Pino config, transports per environment, redaction list.

## Auth Model

- **Authentication**: [mechanism — sessions/JWT/API keys/OAuth/mTLS, where credentials live, rotation policy]
- **Authorization**: [model — RBAC/ABAC/resource-scoped, who can do what]
- **Tenancy**: [single-tenant, multi-tenant with row-level isolation, multi-tenant with separate DBs]
- **Threat model notes**: [what we worry about — credential leak, replay, IDOR, enumeration]

## Scale & Latency Targets

| Metric           | Target            | Notes |
| ---------------- | ----------------- | ----- |
| p50 latency      | [ms]              | [hot path / cold path] |
| p95 latency      | [ms]              |       |
| p99 latency      | [ms]              |       |
| RPS at launch    | [n]               |       |
| RPS ceiling      | [n]               |       |
| Read:write ratio | [e.g. 90:10]      |       |

## Consistency Model

What guarantees we promise and where. Call out:
- Operations that need ACID transactions
- Operations safe under eventual consistency
- Idempotency strategy for retried writes (idempotency keys, dedup tables, natural keys)
- Exactly-once vs. at-least-once expectations for async work

## Deployment

- **Runtime**: [serverless / long-running container / edge / VM]
- **Region(s)**: [one / multi, primary, failover]
- **Migrations**: [tool, when applied, zero-downtime requirements, backfill strategy]
- **Rollout**: [blue/green, canary %, rolling, manual]
- **Rollback**: [how, how fast]

## Observability

- **Logs**: [what gets logged, levels, format, PII handling]
- **Metrics**: [the handful that matter — latency, error rate, queue depth, business KPIs]
- **Traces**: [tooling, sampling rate, propagation across services]
- **Alerts**: [signal → threshold → who gets paged]
- **Healthcheck**: [endpoint, what it actually checks]

## Failure Modes

The top 3-5 ways this can go wrong and the chosen response: retry, fail loud, degrade gracefully, queue for later.

| Failure                  | Response                        |
| ------------------------ | ------------------------------- |
| [downstream times out]   | [retry with backoff, then ...]  |
| [DB unavailable]         | [...]                           |

## Out of Scope

Explicit non-goals. Things this brief does not cover so the build stays bounded.

## Open Questions

Anything still unresolved that needs a decision before or during the build.