Glossary
Two complete environments side by side: the new version runs quietly, traffic is switched over, and on failure you switch back.
The appeal is the way back. Instead of repairing a broken release while customers wait, you switch to the old environment and investigate calmly. The switch itself takes seconds.
The price is duplicated infrastructure during the change and the question of what happens to state: sessions, queues and above all database changes cannot be switched back as easily as application code.
How you notice it
Frequently asked
In three steps across two deployments: expand, switch, clean up. The new column is added and written to as well, then the application reads from the new place, and only in the next deployment does the old one disappear. That keeps the way back open.