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 a

Answer 1

Remember, NoSQL isn't exactly new. After all, they had to use something before SQL and relational databases, right? In fact, systems like MUMPS and CODASYL work the same way and are decades old. What relational databases give you is the ability to query data in arbitrary ways.

Say you have a database with customers, their purchases, and what items they purchased. A NoSQL DB might have customers containing purchases and purchases containing items. This makes it easy to find out what items a given customer purchased, but hard to find out what customers purchased a given item. A relational DB would have tables for customers, purchases, items, and a table linking items to purchases. In SQL, both queries are trivial to formulate, and the database engine does all the hard work for you.

Also, keep in mind that part of the NoSQL trend is to sacrifice consistency or reliability for speed, scalability, and cost. Relational DBs can scale, but it's not cheap. If you go to http://tpc.org you can find RDBMSes that run on hundreds of cores simultaneously to deliver millions of transactions per minute, but they cost millions of dollars.

Answer 2

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.