What is a Multi-Tenancy?

In a multi-tenant system, tenants share the same underlying infrastructure, application code, database servers, compute, which makes the system cheaper to run and easier to update than provisioning a fully separate stack per customer. Isolation is enforced in software rather than by physical separation: every query, cache key, and background job is scoped by a tenant identifier, so tenant A's data is never visible to tenant B even though both live in the same database and application process. The alternative, single-tenancy, gives each customer a dedicated instance, simpler to reason about for isolation but far more expensive to operate and update at scale. Multi-tenant systems have to guard against a specific class of bug: any code path that forgets to filter by tenant ID risks leaking data across customers, which is why tenant scoping is usually enforced at the data-access layer itself rather than left to be remembered in every individual route or query.