What database do online games like Farmville use? [closed]

Question

Apart from graphical features, online games should have a simple Relational Database structure. I am curious what database do online games like Farmville and MafiaWars use?

Is it practical to use SQL based databases for such programs with such frequent writes ?
If not, how could one store the relational dependence of users in these games?

EDIT: As pointed, they use NOSQL databases like Couchbase. NOSQL is fast with good cuncurrency (which is really needed here); but the sotrage size is much larger (due to key/value structure). 1. Does't it slow down the system (as we need to read large database files from the disk)? 2. We will be very limited as we do not have SQL's JOIN to connected different sets of data.

Answer 1

These databases scale to about 500,000 operations per second, and they're massively distributed. Zynga still uses SQL for logs, but for game data, they presently use code that is substantially the same as Couchbase.

“Zynga’s objective was simple: we needed a database that could keep up with the challenging demands of our games while minimizing our average, fully-loaded cost per database operation – including capital equipment, management costs and developer productivity. We evaluated many NoSQL database technologies but all fell short of our stringent requirements. Our membase development efforts dovetailed with work being done at NorthScale and NHN and we’re delighted to contribute our code to the open source community and to sponsor continuing efforts to maintain and enhance the software.” - Cadir Lee, Chief Technology Officer, Zynga

Answer 2

To Answer your edit:

You can decrease storage size by using a non key=>value storage like MongoDB. This does still have some overhead but less than trying to maintain a key=>value store.

It does not slow down the system terribly since quite a few NoSQL products are memory mapped which means that unlike SQL it doesn't go directly to disk but instead to a fsync queue that then writes to disk when it is convient to. For those NoSQL solutions which are not memory mapped they have extremely fast read/write speeds and it is normally a case of trade off between the two.

As for JOINs, it is a case of arranging your schema in such a manner that you can avoid huge joins. Small joins to join say, a user with his score record are fine but aggregated joins will be a problem and you will need to find other ways around this. There are numerous solutions provided by many user groups of various NoSQL products.