The first type of deployment that we'll cover is in-place upgrades. This style of deployment operates on an infrastructure that already exists, in order to upgrade the existing application. This model is a traditional model that was used when the creation of a new infrastructure was a costly endeavor, in terms of both time and money.
A general design pattern to minimize the downtime during this type of upgrade is to deploy the application across multiple hosts, behind a load balancer. The load balancer will act as a gateway between users of the application and the servers that run the application. Requests for the application will come to the load balancer, and, depending on the configuration, the load balancer will decide which backend server to direct the requests to.
To perform a rolling in-place upgrade of an application deployed with this pattern, each...