# Architect — F-029 Observability: traces and business metrics

## Summary
F-029 adds request tracing and business metrics to the backend. Tracing spans critical business operations; metrics expose request counts/histograms and business KPIs.

## Design

### Module
Create `project/src/modules/observability/` with:
- `infrastructure/otel-tracer.ts` — no-op tracer stub, swap later with real OTel.
- `infrastructure/metrics.ts` — in-process meter for http_requests_total, http_request_duration_seconds, checkout_completed_total, order_value_sum.
- Fastify request hook for tracing and metrics.

### Approach
- `instrumentFastify(app, tracer, meter)` adds `onRequest` and `onResponse` hooks.
- `tracer.trace('checkout.start', async () => ...)` in checkout service.
- `meter.counter('checkout.completed').add(order.total)` at checkout completion.
- No new production dependency — use lightweight stubs compatible with future OTel swap.

### Files
- `project/src/modules/observability/**`
- `project/src/app/build-app.ts` — wire instrumentation at startup.

## Acceptance trace
- AC1: request tracing spans created for checkout path.
- AC2: metrics http_requests_total and http_request_duration_seconds observed via /metrics route.
- AC3: business metric checkout_completed_total increments per successful checkout.

## Evidence
- `work/artifacts/F-029/implementer.md`