How exactly do you think eBay, GMail, Facebook etc work ? They aren&#x27;t relying on relational database joins.<p>If you want to write a truly scalable application you structure everything such that you do joins in your application layer.<p><a href="http:&#x2F;&#x2F;highscalability.com&#x2F;ebay-architecture" rel="nofollow">http:&#x2F;&#x2F;highscalability.com&#x2F;ebay-architecture</a><p>And in the case of MongoDB you avoid joins since it is a document database. You embed data instead.