Alex Payne (from twitter): Rails doesn't scale

Ok Ok Ok... first off, I apologize for the sliiightly misleading subject line. But, I thought I'd try to draw some attention and see if I could get some discussion going (hopefully no flames though!).

1) "At this point in time there's no facility in Rails to talk to more than one database at a time."

I have seen Rails deployments that use multiple databases, theres an illustration of one on the Ruby on Rails site for a german web community.

Basically there is two databases, each is attached to a farm of Rails Pizza boxes. Each Pizza box can only ever see one DB, but the DBs increment their PKs in 2s (one is even the other odd), so that they can periodically replicate/merge their data sets.

In principle you could use multiple DBs and scale horizontally, but replication overhead will probably eat into you very quickly.

2) "setting up multiple read-only slave databases [is not a quick fix to implement]"

3) Ruby + Rails' syntactical sugar = slow

Point #3 is pretty well known, the solutions always mentioned before is scale out. However, Alex says that they can't because of 1 & 2. I've been under the impression (and still am) that doing 1 & 2 really isn't that hard.

Well there are different options for 2. I am quite sure that you actually spend lots of money on your DB to achieve it.

He did say or imply though that scaling out incurs additional DB overhead per Rails instance. Which implies an optimal ratio between DBs & Rails instances.

So, the question is, what to do if you have a rails app and are in twitters place?

Pretty much what he is doing already. But then, its not like scaling is easy in the first place.

You have to ask yourself though - Twitter got up and running in ~ 9-12 months or so? In a Java/.NET version would he even be in production by now? How would the PHP equivalent fare?

I’m taking the multiple-databases idea even one further: reading from external databases and dynamically building an AR heirarchy to ease access to that database (this is an administration site used to administer many websites).

The real question needs to be: what server is he running? Is he using capistrano to help with distributed installs? I’ve heard good things about mongrel clusters and other bits of load balancing. Rails is most definitely scalable, it just may not be as easy as with other (read: older) technologies.


Preface: I've been working at Obvious (Twitter's parent company) since it was Odeo, and Alex is one of my co-workers.

We're definitely in a distributed setup, using capistrano, etc, etc. Our database is not currently a limiting factor, nor is ActiveRecord - scaling those components was certainly a challenge, but one that we were glad to have. That we have such a thriving and active community is, as many have suggested, a testament to the tools that Rails and the Ruby community in general offer us as developers and designers.

I'll be giving a talk at the SDForum Silicon Valley Ruby Conference next weekend on the subject of Scaling Twitter. More details at

Blaine It's Obvious -