Quote:

1. Write code that can write to Cassandra in parallel to Mysql, but keep it disabled by the tool I mentioned above
2. Slowly turn up the writes to Cassandra (we can do this by user groups “turn this feature on for employees only” or by percentages “turn this feature on for 1.2% of users”)
3. Find a bug 🙂
4. Turn the feature off
5. Fix the bug and deploy
6. GOTO #2

In an interview Twitter developer Ryan King talks about how they plan to migrate their database system. Database is a great example for a very connected part of a system so this advice is useful in a lot of scenerios. Here is the source: http://nosql.mypopescu.com/post/407159447/cassandra-twitter-an-interview-with-ryan-king