So... this NoSQL thing

Question

I've been looking at MongoDB and I'm fascinated. It appears (although I have to be suspicious) that in exchange for organizing my database in a slightly different way, I get as much performance as I have CPUs and RAM for free? It seems elegant, and flexible, but I'm not trading that for fast like I am with Rails. So what's the catch? What does a relational database give me that I can't do as well or at all with Mongo? In other words, why (other than immaturity of existing NoSQL systems and resistence to change) doesn't the entire industry jump ship from MySQL?

As I understood it, as you scale, you get MySQL to feed Memcache. Now it appears I can start with something equally performant from the beginning.

I know I can't do transactions across relationships... when would this be a big deal?

I read http://teddziuba.com/2010/03/i-cant-wait-for-nosql-to-die.html but as I understand it, his argument is basically that real businesses which use real tools don't need to avoid SQL, so people who feel a need to ditch it are doing it wrong. But no "enterprise" has to deal with nearly as many concurrent users as Facebook or Google, so I don't really see his point. (Walmart has 1.8 million employees; Facebook has 300 million users).

I'm genuinely curious about this... I promise I'm not trolling.

Answer 1

I am also a big fan of MongoDB. That having been said, it is absolutely not a wholesale replacement for RDBMS. Facebook has 300 million users but if some of your friends don't show up in the list one time, or one of the photo albums is missing on the occasional request, would you notice? Probably not. If your status update doesn't trickle down to all of your friends for a few minutes, does it matter? Hardly. If Wal-Mart's balance sheets are out of sync, would someone lose their head? Definitely.

NoSQL databases are great in "fuzzy" environments where relationships are not strict and data integrity can afford to be out of sync. RDBMS are still important when data sets are extremely complex and relational (hence the name), and they need to be kept pure.

The big push to NoSQL comes from the fact for the last 30 years, we have been using RDMBS systems for both scenarios. We now have a more appropriate tool for many situations. Some would argue most, in fact. But no one would argue all.