Changing the engine in flight (live schema and data migrations)

The ledger cut into books, drawn at rest. This lesson goes past it: what it takes to change the shape of a ledger that is being written to every second, with no night to close the shop.

Before you start. You need three ideas from earlier. A ledger can be cut into pieces so that more hands write at once, which is Splitting the ledger (sharding). A ledger can have copies that trail the original by a moment, which is The copy clerks (database replication). And different jobs want different kinds of book, which is The Right Book for the Job (databases, deeply). This lesson is about the day you must move from one book to another, or change what a page looks like, while people are still writing in it. What a page looks like is its schema: the list of fields a record has, such as a name, a price and a date.