One of the earliest SaaS architecture decisions is tenant data strategy. A shared database can reduce cost and simplify early operations. A dedicated database can improve isolation and enterprise flexibility. Neither choice is universally correct.
A shared database works well when tenants are similar, data volume is manageable, and cost efficiency matters. The risk is that tenant isolation must be enforced carefully in every query, API, background job, and reporting path.
A dedicated database model can improve isolation, backup strategy, and customer-specific operations. It can also increase cost and operational complexity. Provisioning, migrations, monitoring, and support workflows must be automated early.
Many platforms use hybrid patterns. Smaller tenants share infrastructure while large enterprise tenants get stronger isolation. The right choice depends on compliance, pricing, expected tenant size, customization needs, and operational maturity.
The key is to design tenant identity, authorization, migrations, and observability from the start. Retrofitting tenant isolation later is much harder than making a deliberate decision early.