There are two methods for recovering node agents and sever instances.
Keep a backup zip file. There are no explicit commands to back up the node agent and server instances. Simply create a zip file with the contents of the node agents directory. After failure, unzip the saved backup on a new machine with same host name and IP address. Use the same install directory location, OS, and so on. A file-based install, package-based install, or restored backup image must be present on the machine.
Manual recovery. You must use a new host with the same IP address.
Install the Application Server node agent bits on the machine.
See the instructions for AS8.1 UR2 patch 4 installation
Recreate the node agents. You do not need to create any server instances.
Synchronization will copy and update the configuration and data from the DAS.