Updates initiated to Application Gateway are applied one update domain at a time. As each update domain's instances are being updated, the remaining instances in other update domains will continue to serve traffic1. Active connections are gracefully drained from the instances being updated for up to 5 minutes to help establish connectivity to instances in a different update domain before the update begins. During update, Application Gateway will temporarily run at reduced max capacity, which is determined by the number of instances configured. The update process will proceed to the next set of instances only if the current set of instances have been upgraded successfully.
1 A minimum instance count of 2 is recommended to be configured for Application Gateway v1 SKU deployments to ensure at least one instance can serve traffic while updates are applied.