Multi-tenancy means one running application serves many customers while keeping their data apart. Here is what it means, the apartment-building analogy that makes it click, the three models, and how tenants stay separated, with links to go deep on each.
Row-Level Security is one layer, not the whole answer. Real tenant isolation is defense in depth across the data, the API, and the keys, so that no single mistake exposes another customer. Here are the layers and where each one earns its place.
Row-Level Security moves tenant isolation out of every developer's memory and into the database itself. Here is the exact Postgres pattern for a pooled multi-tenant app, plus the four footguns that leak data in production if you miss them.
The question is not which one wins, it is where you draw the isolation boundary. Here is how single and multi-tenant differ on cost, isolation, compliance, and ops, and why most SaaS that scales ends up running both at once.
Silo, pool, or bridge is one of the few architecture choices you cannot cheaply reverse. Here are the three models in plain terms, how they map to a real database, and a decision framework so you pick the one your customers and your margins actually need.
Both are excellent databases and for most SaaS workloads either one is fine. The honest answer: pick Postgres for greenfield, keep what you run well, and switch only for a reason you can name. Here are the few differences that actually decide it.
