You need to rename the column users.fullname to display_name with zero downtime, while old and new app versions run side by side during a rolling deploy. Describe the steps.

Use expand and contract, also called parallel change. Never rename the column in one step. During a rolling deploy, the old app version is still running, and it would break the moment fullname disappears.

Expand:

• Add display_name as a new nullable column. This breaks nothing.
• Deploy a version that writes both columns and still reads fullname.
• Backfill the existing rows, which means copying old values into the new column. Do it in batches, so the database does not hold long locks.
• Deploy a version that reads display_name and still writes both columns. Because it still writes both, a rollback stays safe.

Contract:

• When no running version reads fullname any more, deploy a version that stops writing it.
• Finally, in a later release, drop the old column.

Each step is backward compatible with the previous app version. So you can roll back any single deploy. Run migrations as a separate pipeline step, not when each instance starts up.