The Expand and Contract Pattern

1. One Shape, Not One Technique

The data modeling module introduced expand and contract for a column rename. That was one example. The pattern is general, and recognizing it as a shape rather than a recipe is what lets you apply it to migrations that look nothing like a rename.

The shape is always the same five moves.

1. Add the new thing alongside the old. Nothing reads it yet.
2. Write both, so the new thing starts accumulating correct data.
3. Backfill what already existed, so the new thing is complete.
4. Verify, then move reads across.
5. Remove the old thing.