Multi-tenancy is one of those things that sounds straightforward until you actually build it. Every user belongs to a workspace. Data is isolated per workspace. Roles control what each user can do inside that workspace.

Get it wrong and you end up with data leaking between customers, permission bugs, or an architecture that falls apart when you try to add team invites six months later.

I built this from scratch while working on a SaaS product. Here is everything I learned — the schema design, the session pattern, the data isolation approach, and the role system — with real code you can use.

What multi-tenancy actually means

In a single-tenant app, one installation serves one customer. In a multi-tenant app, one installation serves many customers simultaneously, with each customer's data completely isolated from every other customer.

In practice for a B2B SaaS this means:

User signs up → a workspace (Organization) is created for them
That user is the Owner of their workspace
They can invite teammates who become Members
Every piece of data (jobs, invoices, whatever your app does) belongs to an Organization
A user in Organization A can never see data from Organization B