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Advanced Web Machinery

Multi-tenant architecture offers significant advantages even for single-customer systems. Key benefits include parallelizable integration testing through tenant isolation, production monitoring that mimics real user behavior without affecting actual clients, safe demo environments for sales and developers, and flexibility for future business evolution. The complexity overhead can be minimized using database-level features like PostgreSQL's row-level security, which centralizes tenant filtering rather than requiring it in every query.

Why I prefer multi-tenant systems

I think they are talking about application level tenancy, and not necessarily DB level tenancy. You can have a single application serving all your customers and isolate the data in the DB.
This adds complexity, but you get all the benefits mentioned in the article while still isolating the customer’s data!
That being said, I would interested in reading more about the row level security in PSQL.

is storing customers’ data in the same db with rbac applied gdpr compliant?