Tenancy, roles, and billing are the three decisions in a SaaS product that are cheap to get right early and expensive to change later. They deserve real attention before the first customer signs up.
Choosing an isolation model deliberately
Shared schema with tenant IDs, separate schemas, or fully separate databases each trade off differently between operational simplicity and isolation guarantees. The right choice depends on your compliance needs and expected scale, not on whichever is fastest to prototype.
Roles and billing as first-class concerns
Permission models and pricing structures both tend to get bolted on after the fact, and both are painful to retrofit. Deciding early how roles compose, and how usage maps to billing, avoids a lot of awkward migrations once real customers and real invoices exist.
None of these decisions need to be perfect on day one. They just need to be made on purpose, with an understanding of what changing them later will cost.
