How does a production deployment for a cloud based software happen?

Lets take the example of a heavily used cloud based software.

When a deployment happens, let's say users are online.

1. Won't the server require stop & start after deploy? How is the service continuity maintained?
2. How will the ongoing user sessions / unsaved data be continued post deploy?
3. How is the risk managed? (Lets say an issue comes up after deploy and you need to revert to the older version, now imagine a user has already worked on the new version and saved some data with it, which is not compatible with previous versions)

Whether the server require stop & start after deploy depends on the technology being used. Any state information that is kept within the server can be externalized (i.e. written to disk) for the purpose of updating the application. Whether this is neccessary and whether this is done depends on the technology being used.