Let's say you need to set up a replicated database with failover and monitoring.

First you set up the database (and make sure it works).

Then you set up monitoring (and make sure it works, too).

Followed by replication (which should also work).

And finally, automatic failover (there's a pattern here...)

This sounds obvious, but I've worked with developers that, when given this task, tried to set up the entire thing in one shot. It took them over a month to wrap up, and they didn't have time to properly test it, because they bit off too much in one go.

The same goes for migrations. You work along an incremental plan to move from Point A to Point B, where at every step of the way, the set of things you need to change is small enough to manage, and you have the option of rolling back.

I can't imagine that Github has a tiny codebase, so I could totally believe six months for a zero-downtime Rails 2.x -> 3.0 migration. The next big step (3.0 -> 3.2) will be easier from what they learned in the first step, as will the step after that (my guess is 3.2 -> 4.0).

The U.S. Healthcare system has tried to switch to ICD10 standards for a while now, but they keep bumping back the deadline. I think it's EOY 2015 right now.

ICD10 was released in 1992...

Those two and a half years were spread over dozens of tools, a svn->git migration, two rewrites of the core functionality, but it all stayed on track because I was building small pieces which did limited things, and chained them together (the unix-y philosophy). I could build some functionality, test it, integrate it into the tool, and use it for a while. Then I could add support for it to our distributed RPC-ish system, and then if that worked, I could add it to our automated scripts.

If I had to build the whole thing at once I probably would have ended up with a much nicer, cleaner design, a much worse product, and a far longer deadline, and we wouldn't have had useful automation tools for the two intervening years.

3.0 deprecates a ton of stuff in 2.3. 3.2 actually removes it all and adds in the asset pipeline.

Best to not bite off more than you can chew.

(The fact that 2->3 was, like for most, also ruby 1.8 to 1.9 probably contributed)

2 -> 3 migration is VERY VERY hard. 3 -> 4 should be relatively easy in comparison.

ActiveRecord 3.x still supports the query interface used in 2.3, I'm guessing this is how they're able to run both versions from the same codebase.

I'm not sure which Ruby version GitHub is on, but Rails 4 requires Ruby 1.9+. Running Rails 2.3 on Ruby 1.9+ is doable but tricky.

If I had to guess, I'd say it's easier to hit Rails 3 as an intermediate milestone before moving onto Rails 4.

Ruby 1.8.x -> Ruby 1.9.x requires quite some work by itself.