A rolling upgrade in software engineering is when you deploy a new version of a piece of software to a few nodes at a time. Deploying to a few nodes at a time means deploying updates to a few running instances of your production build of your application.
"With server-side applications you may want to perform a rolling upgrade (also known as a staged rollout), deploying the new version to a few nodes at a time, checking whether the new version is running smoothly, and gradually working your way through all the nodes." -Designing Data Intensive Systems
Defined by cloudman on 24-7-2020