How do you perform database migrations without downtime?

Database migrations without downtime are achieved through gradual and careful changes to avoid blocking application operation. Main approaches:

• Backward-compatible changes: first add new columns or tables that do not affect current code.
• Dual read/write: the application reads and writes data to both old and new structures simultaneously to ensure consistency.
• Switching phase: after verifying that the new schema works correctly, switch the application to use the new data.
• Removing outdated elements: after a successful transition, delete old columns or tables.

Example: if you need to add a new field, first add a column with a default value, then update the code to read and write from the new field, after which the old field can be removed.

Migration tools supporting transactions and scripts are also used, which allow rolling back changes in case of errors.