CarDealerTool.
A dealership,
run from one screen.

Next.js App Router, one codebase serving both the dealer application and every tenant storefront. Auth is NextAuth v5 with JWT sessions; billing runs through Stripe subscriptions with a webhook. There is no dedicated job queue — nine periodic tasks run as secret-protected HTTP routes called by cron: BHPH due dates, automated customer communications, credit bureau reporting, feed sync, accounting sync, expiring account suspension. Long imports persist per-row state in the database rather than holding it in memory, so a restart can't lose an import halfway through. Files are on local disk, not object storage — a deliberate stage decision, and a mandatory migration at the second node.

MySQL through Prisma, 80 models. The core is Tenant → Vehicle, Customer, Deal. Every internally financed deal carries a BhphAccount one-to-one, and that account holds the credit terms frozen at origination — principal, APR, number of instalments — kept separate from the deal, so editing the transaction later never rewrites credit history. Under the account sit two distinct tables: the schedule (what is owed and when) and the actual payments (what was collected, by what method). Amounts are Decimal, not float — in a system that computes interest, floating point isn't an option.

One database, row-level isolation: 60 of the 80 models carry a tenantId, the JWT session carries tenant and role, and every query is explicitly scoped. Users belong to tenants many-to-many — a single-dealership user gets it straight in the session, a multi-dealership user picks one and the session reloads with the role from that membership. Subdomains resolve in a proxy that reads the Host and rewrites internally: shorelineautocenter.cardealertool.com/inventory becomes /dealers/shorelineautocenter/inventory internally, and the internal path redirects back to the clean one. The host parser is a pure function reused by sitemap, robots and the 404 page, so the four can't drift apart. A custom domain is a field on the tenant; every public URL — canonical, sitemap, robots — is built through one function, so it switches over the moment the field is set. The application side is complete; TLS certificate automation for arbitrary domains is next.

The Metro 2 file generator for credit bureau reporting. Fixed-width, written from scratch against the CDIA specification: 426-character records, header, one segment per account, trailer, padded with spaces and zeros under strict conventions. It wasn't hard as programming — it was hard as specification. Every field has a mandated position, length and format; a single deviation invalidates the whole file, and bureau feedback is slow and opaque. The real work was deciding what to populate realistically for a dealer who finances his own customers, and what to leave empty. What's shipped covers the ordinary case; a bureau-certified submission needs additional fields — a known stage, not an assumption that it's finished.