The compliance foundation of SaaS multi-tenancy: no isolation, no compliance

Multi-tenancy makes SaaS efficient but puts different customers' data in one system. When isolation fails, the mild case is data bleeding across tenants; the severe case is a cross-customer breach and regulatory liability.

The key is a clear isolation strategy for data, permissions, and keys: shared database partitioned by tenant, or a database/key per tenant; and whether the 'edges' — logs, backups, caches — are isolated too.

The pragmatic approach is to fix the isolation model at the architecture stage, technically block and test cross-tenant access, and reserve stronger isolation options for regulated or highly sensitive customers.