MEPX
Chapter 9 of 10All chapters
  1. 1Why a database
  2. 2Tables, rows and types
  3. 3Keys
  4. 4Relationships
  5. 5Normalisation
  6. 6Transactions
  7. 7Indexes
  8. 8When not to use tables
  9. 9Changing the schema
  10. 10Backups and growth

Chapter 9 of 10

Changing the schema

Evolving a live database safely.

Migrations

A migration is a recorded change to the structure, stored in version control alongside the code. That makes the schema reproducible on every machine and reviewable like anything else.

  • Write the rollback as well, before you need it.
  • Adding a column is cheap; renaming or dropping one needs a staged plan.

Deploying without downtime

Add the new column, write to both, backfill, switch reads, then remove the old one. Each step keeps the running application valid, which is the whole point.