One of the biggest killer-arguments in favour of NoSQL databases is that they are supposed to natively scale horizontally, unlike traditional relational databases. Personally, I believe that you still have to be an excellent architect, in order to actually take advantage of this “native feature” of some NoSQL databases. You can still design a system that goes terribly wrong. On the other hand, it’s nice to read a success story of a medium-scale application (25 photos, 90 likes per second) that achieves horizontal scaling using sharding techniques on a Postgres SQL database. After all, it is possible. Read the full success story here:
http://instagram-engineering.tumblr.com/post/10853187575/sharding-ids-at-instagram