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.

Jason

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
http://romeda.org/blog/2007/04/scaling-twitter-talk.html

Blaine
It's Obvious - http://twitter.com/blaine