A deploy replaces a process. The replacement has a moment where two versions exist, and the moment is the reason a deploy is a decision rather than an action.
About 1 min of reading
Write the deploy order
Write the order
Before you run it, say out loud what it will print. Then open the answer.
Attempt a version that fails the suite
Before you run it, say out loud what it will print. Then open the answer.
Stamp the running version
Before you run it, say out loud what it will print. Then open the answer.
The running app must say which version it is. A version only in the file is a version that cannot be checked in place.
The route contract. Both versions are written against it, and it is the file that makes a break visible before it happens.
The new one starts, then the old one drains, then the old one exits. The order is the whole contract of a deploy.
A version that fails the suite never starts. A version that never starts cannot be the one serving.
The previous version is kept. Kept means it can be started, and started means the fix is a start rather than a build.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.