#architecture

A checkout handler that calls the ledger, the warehouse, the email service and analytics in a row is as slow as all of them added together and as fragile as all of them multiplied. And every team that cares about payments makes it a little worse.

One product-page endpoint, the same code, ten times more traffic, then ten times more again. Each step breaks something different, and the last one breaks something extra capacity can't fix.

Give each tenant its own schema and you get real isolation. You also get two new problems: knowing where each tenant lives, and running one migration correctly five hundred times instead of once.

One app, many customers, one WHERE clause keeping them apart. Forget it once and nothing crashes: tenant A just sees tenant B's invoices. How to make isolation something the system enforces, not something engineers remember.

The interviewer asks how much traffic you're expecting. Round to powers of ten, turn per-day into per-second, and end every number on the design decision it forces: one node, a cache, or a shard.