Regan has been asked to apply updates to one of the company's web application server farms. He does not want to update all of the servers at once in case there is some sort of an issue that causes a crash or failure. He decides to direct traffic away from a few servers and update those first. Once he has tested that everything is working successfully, he'll allow them to handle traffic again and then do another small group of servers. He'll continue repeating this process until all servers have been updated. Which of the following describes the deployment method that he has chosen?