You can use various techniques to manually recover individual subcomponents:
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 Application 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
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.
There are no explicit commands to back up only a web server configuration. Simply zip the web server installation directory. After failure, unzip the saved backup on a new machine with the same network identity. If the new machine has a different IP address, update the DNS server or the routers.
This assumes that the web server is either reinstalled or restored from an image first.
The load balancer plugin (plugins directory) and configurations are in the web server installation directory, typically /opt/SUNWwbsvr. The web-install/web-instance/config directory contains the loadbalancer.xml file.
Message Queue (MQ) configurations and resources are stored in the DAS and can be synchronized to the instances. Any other data and configuration information is in the MQ directories, typically under /var/imq, so backup and restore these directories as required. The new machine must already contain the MQ installation. Be sure to start the MQ brokers as before when you restore a machine.
If you have two active HADB nodes, you can configure two spare nodes (on separate machines), that can take over in case of failure. This is a cleaner method because backup and restore of HADB may result in stale sessions being restored.
For information on creating a database with spare nodes, see Creating a Database. For information on adding spare nodes to a database, see Adding Nodes. If recovery and self-repair fail, then the spare node takes over automatically.