Nice, but very simplistic (on purpose, it seems), write-up on the topic. For a much more comprehensive and excellent coverage of designing & implementing large-scale systems, see https://github.com/donnemartin/system-design-primer. Also, I want to mention that an important - and relevant to scaling - aspect of multi-tenancy is very often (as is in Alex's post) not addressed. Most of the large-scale software-intensive systems are SaaS, hence the multi-tenancy importance and relevance.