Fastapi Service Scaffold

FastAPI Service Scaffold

When to use

Invoke when starting a new FastAPI service, standardizing an existing service baseline, modernizing a service that lacks validated settings, structured logging, health probes, or layered error handling, or preparing a FastAPI service for container deployment.

Do not use for: auth, session, or OWASP review work (use fastapi-auth-and-security-review), observability vendor wiring — OpenTelemetry, prometheus-client, SLO/alerts (use fastapi-observability-readiness), Celery/RQ/arq or Kafka task and event integration (use fastapi-async-and-task-integration), or performance and resilience gating — pool sizing, circuit breakers, load-test gates (use fastapi-performance-and-resilience).

Inputs

Required:

• Service name and the business capability it serves.
• Approved backend-architecture.md, or explicit confirmation that backend architecture is intentionally deferred.

Optional:

• Approved architecture/security decisions on auth provider, session model, and secret handling.
• Data layer named by architecture (SQLAlchemy 2.x + Alembic default) — registered as a seam only, not implemented here.
• ASGI server choice if silent (Uvicorn standalone for dev; Gunicorn + uvicorn.workers.UvicornWorker for prod).
• Target Python version (3.11+ default), dependency manager (uv default), and monorepo vs single-service repo placement.

Operating rules

• Never generate tutorial-grade scaffolding. Assume multiple environments, structured logging, container deployment, ASGI-lifespan shutdown, and operational ownership.
• Consume backend-architecture.md; do not invent decisions. Domain boundaries, data layer, and contracts belong to backend-architecture.md; auth provider and secret handling belong to architecture/security. If either is silent on a decision this scaffold needs, pause and raise an ADR candidate rather than guessing.
• Owns the DI and principal-provider baseline only — the shell, not the flow. Token verification, session, and protected-route logic belong to fastapi-auth-and-security-review. Scaffold the seam (a typed Depends); do not implement the flow.
• Secure by default: settings are validated at boot and fail fast; no secrets in source, committed config, the image, or .env; the container runs as a non-root user from a digest-pinned base image.
• Settings are validated, not read ad hoc: a single typed Pydantic Settings (BaseSettings) instance built at startup. Missing or malformed required variables abort boot with a clear message — never a runtime None.
• Error handling is layered in a specific order: sys.excepthook and the asyncio loop exception handler (log fatal, flush, exit non-zero for unhandled) → FastAPI/Starlette exception handlers (structured body, no traceback or internals in non-dev) → ASGI-lifespan graceful shutdown (stop accepting, drain in-flight, close registered resources within a bounded timeout) wired to uvicorn signal handling. All four are required.
• Observability is a seam, not an implementation: a structlog structured logger bound to a contextvars request context (request id correlated) is mandatory in the baseline; OpenTelemetry, metrics, and alerting are explicitly deferred to fastapi-observability-readiness behind a no-op tracer/metrics interface.
• Health probes are mandatory: a liveness endpoint (process is up) and a readiness endpoint (registered dependency checks pass) are separate and wired before any feature route.
• A scaffold that does not run is not done. Run typecheck, lint, the test run, and a boot smoke check before declaring completion; fix and re-run on failure.
• Confirm the target directory before writing files. Recommend services/<service-name>/ in a monorepo or repo root for a single-service repo. Refuse to write into a plugin or skill repository without explicit user override.

Output contract

The generated service shell MUST conform to:

• deployment-standards — env-agnostic image; runtime config injected, not baked; one artifact per environment; non-root container.
• observability-standards — structured logging with request-id correlation and an environment tag; tracer/metrics seam present.
• security-standards — no secrets in source, committed config, or image; settings fail fast on missing required secrets.
• naming-conventions — service, module, and file naming.

Upstream contract: backend-architecture.md is the source of truth for domain boundaries, data layer, and contracts; architecture/security is the source of truth for auth provider, session model, and secret handling. If either is silent on a decision this scaffold needs, pause and raise an ADR candidate rather than guessing.

Progressive references

• Read references/fastapi-scaffold-playbook.md when implementing any owned area or checking the anti-pattern list.
• Read references/fastapi-scaffold-quality-rubric.md before declaring the scaffold complete.
• Use assets/fastapi-service-scaffold.template.md as the directory-layout, settings, and lifespan reference.

Process

1. Gather context: service name, business capability, target directory, target Python version, dependency manager, and the auth provider decision from architecture/security. Load backend-architecture.md and extract domain boundaries, data layer, and contracts. If a needed decision is missing, raise an ADR candidate before proceeding. Confirm the target directory is not a plugin or skill repository.
2. Generate the project layout: pyproject.toml (deps pinned exactly with a hashed lockfile — no unbounded specifiers, requires-python set), mypy/ruff config, .gitignore, and the package structure (domain layout per backend-architecture.md). Reference assets/fastapi-service-scaffold.template.md for the canonical layout.
3. Generate validated settings: a Pydantic Settings(BaseSettings) in app/config.py parsing the environment at import, instantiated once; abort with a clear message on any missing or malformed required variable. Create .env.example documenting every variable with placeholder values only.
4. Generate the structured logging and request-context seam: a structlog logger in app/observability/logging.py; a contextvars request context in app/observability/context.py; ASGI middleware that assigns or propagates a request id and binds a bound logger. Add a no-op tracer/metrics interface in app/observability/telemetry.py with explicit TODO comments naming fastapi-observability-readiness as the owner.
5. Generate the layered error handling: sys.excepthook and loop.set_exception_handler (log fatal, flush, exit non-zero) in app/server/process_handlers.py; FastAPI/Starlette exception handlers returning a structured body with no traceback or internals in non-dev; an ASGI-lifespan (lifespan context manager) that on shutdown stops accepting, drains in-flight, and closes registered resource hooks within a bounded timeout.
6. Generate health probes: a liveness endpoint (/healthz) returning 200 when the process is up, and a readiness endpoint (/readyz) iterating registered dependency-check callables and returning 503 until all pass. Wire both before any feature router.
7. Generate the DI and principal-provider baseline: FastAPI Depends providers in app/container.py. Register the principal/auth-context provider as a typed dependency shell. Add explicit TODO comments naming fastapi-auth-and-security-review as the owner of token verification, session, and protected-route logic, and the data-layer client (SQLAlchemy session) as a seam owned by the data implementation.
8. Generate container packaging: a multi-stage Dockerfile (build stage, slim runtime stage) running as a non-root user from a digest-pinned base image, a .dockerignore, and a documented HEALTHCHECK hitting /healthz. No secrets baked; runtime config arrives via environment.
9. Generate local-run documentation in the service README: how to run each environment, every required environment variable (mirrored from .env.example), the runtime-config contract, and the explicit table of seams downstream archetypes fill (auth, observability vendor, task/event, performance gates, data layer).
10. Build verification (mandatory): run mypy, ruff check, the test command (pytest), and a boot smoke check (start the app, assert httpx GET /healthz returns 200, shut down cleanly via lifespan). Fix and re-run on failure. Then validate against the standards in the Output contract; revise until all pass or explicitly document any unresolved gap in the README.

Outputs

Required:

• Python/FastAPI service package with pyproject.toml (pinned deps + hashed lockfile, requires-python) and mypy/ruff config.
• Validated Pydantic Settings with fail-fast boot and .env.example.
• structlog logger + contextvars request context + request-id correlation; no-op tracer/metrics seam.
• Layered error handling: process/asyncio handlers, FastAPI/Starlette exception handlers, ASGI-lifespan graceful shutdown (all wired).
• Separate liveness (/healthz) and readiness (/readyz) probes.
• Depends DI providers with a typed principal-provider shell (auth flow explicitly deferred).
• Multi-stage non-root Dockerfile from a digest-pinned base, .dockerignore, documented HEALTHCHECK.
• Service README listing all seams downstream archetypes fill.

Output rules:

• Generated files are functional and the app boots, not placeholder-heavy.
• No secrets in source, committed config, .env.example, or the image.
• The principal provider is a seam, not a flow — token verification, session, and route guards are explicitly deferred to fastapi-auth-and-security-review.
• The same image runs in every environment; configuration arrives at runtime via environment variables.

Quality checks

• mypy and ruff check report zero errors.
• The test command runs and the boot smoke check (GET /healthz → 200, clean lifespan shutdown) passes, or a skip is documented with reason.
• pyproject.toml pins every dependency with a hashed lockfile and sets requires-python.
• Settings are validated at boot and abort on a missing required variable — verified by removing one required var and observing a clear non-zero exit.
• No secrets appear in source, committed config, .env.example, or the built image.
• All four error layers are wired: sys.excepthook, asyncio loop handler, FastAPI/Starlette exception handlers, ASGI-lifespan graceful shutdown.
• Exception handlers return no traceback or internal details in non-dev environments.
• /healthz and /readyz are separate; /readyz returns 503 until registered dependency checks pass.
• The structlog logger binds a request id from the contextvars context; the tracer/metrics seam is a no-op with a TODO naming fastapi-observability-readiness.
• The Depends principal provider is a typed shell tied to an architecture/security decision; token/session logic is explicitly deferred with TODO comments.
• The Dockerfile is multi-stage, runs as non-root, uses a digest-pinned base, and bakes no secrets.
• The service README lists all seams downstream archetypes are expected to fill.

References

• Upstream: architecture/backend-architecture, architecture/security.
• Related FastAPI archetype skills (extend this baseline): fastapi-auth-and-security-review, fastapi-observability-readiness, fastapi-async-and-task-integration, fastapi-performance-and-resilience.
• Standards: deployment-standards, observability-standards, security-standards, naming-conventions.