Billing Service — Testing Strategy

Status: populated Owner: Platform Engineering + QA Last updated: 2026-04-18

1. Coverage Targets

Layer	Target	Tool
Domain aggregates	95%	Vitest
Value objects (`Money`, etc.)	100%	Vitest
Domain services (PricingResolver, MarginCalculator)	90%	Vitest
Application use cases	90% (branch)	Vitest
Integration (NATS consumer + PG + Redis)	All critical paths	Testcontainers
Mutation on domain	≥ 75%	Stryker

Money VO: arithmetic, currency mismatch throws, negative amounts rejected.
PricingResolver: cache hit, cache miss, no matching rule throws PricingNotFoundError.
MarginCalculator: positive margin, zero margin, negative margin (allowed, flagged).
IngestBillingEventUseCase: happy path, duplicate messageId (idempotent no-op), pricing not found (error escalation).
GenerateMonthlyInvoicesUseCase: correct period calculation, PDF render called, S3 PUT called, NATS publish called.

test/integration/tenant-isolation.spec.ts — RLS rejects cross-tenant usage query.
test/integration/ingest-happy-path.spec.ts — NATS event consumed; billing_events row persisted; usage_summaries upserted.
test/integration/ingest-idempotency.spec.ts — duplicate messageId → second insert ignored; usage_summaries not double-counted.
test/integration/pricing-cache.spec.ts — Redis hit serves stale; cache miss falls through to PG; invalidation clears key.
test/integration/invoice-generation.spec.ts — cron trigger; summaries aggregated; PDF rendered (mock renderer); S3 PUT called; invoice FINALIZED.
test/integration/invoice-void.spec.ts — admin void sets status, records voided_by/voided_at/void_reason.
test/integration/pricing-version.spec.ts — PATCH creates new row, old row gets effectiveTo; overlap prevented by constraint.

Schema conformance for consumed billing.message.charged.v1 against schema registry.
Schema conformance for produced billing.invoice.generated.v1 against schema registry.
OpenAPI diff gate: no breaking changes to REST API without version bump.

Property-based test: customerPrice = unitPrice * segmentCount for all integer segment counts 1–10.
margin = customerPrice - operatorCost holds for 1000 random pricing combinations.
Invoice subtotal = SUM of customerPrice across all billing_events for the period (reconciliation test).

test/e2e/billing-flow.e2e.spec.ts: dlr-processor stub → NATS → billing-service → PG persisted → usage query API returns correct totals.

Target: 10,000 events/sec ingestion sustained; consumer lag stabilizes < 5000.
k6 scenario: publish events to NATS at target rate; measure PG insert latency and consumer lag over 10 min.