Why isolation matters

When you start a SaaS product you quickly discover that the database is the biggest source of technical debt. Every piece of business logic depends on how you separate one customer's data from another's. A poor choice can make it hard to add features, can expose data unintentionally, and can turn routine scaling work into a nightmare.

In this article I will walk through two common isolation patterns, show where each shines, and give you a checklist you can use when you sit down with your team.

The two basic patterns

1. Shared schema with a tenant column

The simplest way to support many customers is to keep a single set of tables and add a tenant_id column to every row that needs isolation. Queries always filter on that column, and the application layer is responsible for never mixing tenant data.

Pros

Easier to set up initially.…