Every architectural constraint in a SaaS platform descends from how tenants are isolated. This is the decision to make deliberately, because reversing it later is close to a rewrite.
Shared everything is cheapest and hardest to isolate
One application and one database with a tenant identifier on every row. Lowest cost per tenant, simplest operations, and the model where a single missing filter in a query leaks one customer’s data to another. It suits high-volume, low-value tenants.
Shared application with isolated data sits in the middle
One application tier, a separate schema or database per tenant. Isolation is enforced by connection routing rather than by query discipline, per-tenant restore becomes possible, and operations get harder as tenant count grows into the thousands.
Fully isolated stacks are the enterprise answer
A complete deployment per tenant gives the strongest isolation, per-tenant customisation and regional placement. Cost per tenant is high and every operation multiplies, so this is normally reserved for a small number of large contracts.
Action step
Write down which model you are on and what would have to change to move up a tier. That is your migration cost.
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