Every SaaS I build is multi-tenant on day one. Retrofitting tenancy later is how startups end up as a data-leak headline.

The trap is seductive. Early on you have one customer, so you skip the tenant boundary to move faster. Then you have twenty, and some query forgot to filter by tenant, and customer A is looking at customer B's invoices. That bug traces straight back to an architecture you chose under deadline.

I put the tenant context in from the first migration. Every row knows its tenant. Every query is scoped through a shared tenancy layer instead of each developer remembering to add a WHERE clause. The platform enforces isolation by default, so it doesn't ride on everyone staying disciplined forever.

I've built this across a suite of SaaS products, from garage management to clinics, and the tenancy layer is shared, so one fix to the isolation logic covers all of them at once.

For a founder, that's the line between a product you can scale and a lawsuit waiting for its trigger.

Building SaaS and unsure how to draw the tenant boundary cleanly? Ask me. I've drawn it more than a dozen times.