Loss of the Domain Administration Server (DAS) affects only administration. Application Server clusters and applications will continue to run as before, even if the DAS is not reachable
Use any of the following methods to recover the DAS:
Run asadmin backup commands periodically, so you have periodic snapshots. After a hardware failure, install App Server on a new machine, with the same network identity and run asadmin restore from the back up created earlier. For more information, see Recreating the Domain Administration Server.
Put the domain installation and configuration on a shared and robust file system (NFS for example). If the primary DAS machine fails, a second machine is brought up with the same IP address and will take over with manual intervention or user supplied automation. Sun cluster uses a similar approach for making DAS fault-tolerant.
Zip the Application Serverinstallation and domain root directory. Restore it on the new machine, assigning it the same network identity. This may be the simplest approach if you are using the file-based installation.
Restore from DAS backup. See the AS8.1 UR2 patch 4 instructions