Migrations
How do you rename a heavily used database column with no downtime?
Whether the candidate understands that during any rollout two versions of the code run against one database, and can sequence changes so both are always valid.
The situation behind the question
A
users table with 90 million rows has a column signup_source that the team wants to call acquisition_channel. It is read on every login, written by three services, and copied nightly into a reporting warehouse by a job whose owner left.