GLOSSARY
Multi-tenant architecture
Multi-tenant architecture is a software design where a single instance of an application and its database serves many customers at once, with each customer's data kept separate through logic inside the application rather than physically separate infrastructure per customer.
Why it matters
It is the reason SaaS pricing can be low per customer: one set of servers, one deployment, one team maintaining it, serving thousands of accounts instead of one dedicated stack per account. The cost of adding the hundredth customer is a fraction of standing up the first, which is the entire economic case for building a platform as SaaS rather than bespoke per client. It also means a single codebase receives every customer's bug fixes and improvements at once, rather than the same fix being deployed separately across dozens of isolated instances. That efficiency is also what allows white label licensing to work economically at all, since one platform genuinely can serve many licensed partners at once.
What goes wrong when it's handled badly
The most damaging failure mode is a scoping bug: a query that is not correctly filtered to the customer making it can expose one customer's data to another. This is treated as the single highest-severity class of bug in a multi-tenant system, tested for deliberately rather than assumed to work because the code looks correct. A regulator or contract that requires physical separation instead of logical scoping is the situation where single-tenant architecture is used instead. A concrete version of this failure is an internal support tool built quickly that queries records by ID without checking which tenant owns that ID, so an agent, or an attacker who guesses a valid ID, can pull up another customer's data by changing one number in a URL. Automated tests that specifically try to access one seeded test account's data from another are a standard, deliberate defence against exactly this class of bug, run on every release rather than trusted to manual review alone.