Multi-Tenant Billing Platform

Problem

The client’s homegrown billing logic lived inside controllers, made flat-rate assumptions, and broke every time sales negotiated a custom plan. Finance spent days each month reconciling invoices by hand.

Architecture

I extracted billing into a dedicated domain layer with an append-only usage ledger. Metered events stream into Redis, roll up nightly into PostgreSQL, and feed a deterministic invoice builder.

// Every metered event is immutable; invoices are pure projections over the ledger.
UsageEvent::record($tenant, 'api.request', quantity: 1, occurredAt: now());

• Multi-tenancy: single database, row-level scoping with a global tenant guard.
• Idempotency: Stripe webhooks deduplicated by event id before mutating state.
• Auditability: invoices are reproducible from the ledger at any point in time.

Impact

• Invoice disputes down 70% quarter-over-quarter.
• Onboarding a custom plan went from a code deploy to a config row.
• Sustained 12,000+ active tenants with p95 invoice render under 400ms.