[VLM]

There can only be one DB much like the LOTR can only have one ring. Why? Thats the only area the linked article falls down on. Its a pretty good article other than that.

So you properly normalized your entire system, customer billing transaction records all the way up to article tags. Then article tags gets too huge. So next version looks at RDBMS and Redis, and the next version after that only looks at Redis. Customer billing transactions remains on a "real" DB and the tag cloud lives on redis. And the problem with that is... what exactly?

Its obsolete thinking. I can't have two databases because we're a poor startup and the only databases that exist are DB2 and Oracle and everyone knows they're super expensive so super expensive times two is unaffordable. Dude, its almost 2014 not 1980, Postgres/mysql/redis its all free.

[raycmorgan]

Thank you for your comment. I agree that there is nothing wrong with using a plurality of systems when needed. Your example of moving from one to another is great! Start with a simple system, and once you find bottlenecks, optimize with specialized stores.