How we built the DEV3 Shipdrop: order routing, stock holds and an append-only ledger

DEV3 Shipdrop is a multi-tenant dropshipping platform: merchants import from a shared supplier network and the platform routes paid orders to suppliers, collects money and pays everyone.

Stack: Next.js 16, React 19, TypeScript, Tailwind 4 and MongoDB, with a shared DEV3 core for auth, billing and tenancy.

Decisions that mattered

• Stores are tenants. A Mongoose plugin scopes every query by an immutable organisation id and fails closed, and a unit test greps for violations.
• The routing planner is pure: it takes a basket and candidate sources and returns a plan (priority, cheapest or fastest). 24 routing vectors run in unit tests.
• A stock reservation writes a journal row first, then updates the count and the hold in a single update, so a crash is recoverable.
• Every supplier order state change is a conditional update, so two racing calls produce one outcome.
• Merchant postings carry a unique key per entry, which makes retries safe.

Limits: the item page lists a few rare race windows in refunds and rerouting. It is a web app with no mobile apps, and there is no public live demo.

What we would do next: close those documented edge cases.

How would you test racing state transitions? Feedback welcome.