C.3. Inheritance, the Block, and Reusing Resources
Some resources benefit by inheriting values from a parent resource; that is commonly the case in an NFS service. Example C.5, “NFS Service Set Up for Resource Reuse and Inheritance” shows a typical NFS service configuration, set up for resource reuse and inheritance.
Example C.5. NFS Service Set Up for Resource Reuse and Inheritance
<resources> <nfsclient name="bob" target="bob.example.com" options="rw,no_root_squash"/> <nfsclient name="jim" target="jim.example.com" options="rw,no_root_squash"/> <nfsexport name="exports"/> </resources> <service name="foo"> <fs name="1" mountpoint="/mnt/foo" device="/dev/sdb1" fsid="12344"> <nfsexport ref="exports"> <!-- nfsexport's path and fsid attributes are inherited from the mountpoint & fsid attribute of the parent fs resource --> <nfsclient ref="bob"/> <!-- nfsclient's path is inherited from the mountpoint and the fsid is added to the options string during export --> <nfsclient ref="jim"/> </nfsexport> </fs> <fs name="2" mountpoint="/mnt/bar" device="/dev/sdb2" fsid="12345"> <nfsexport ref="exports"> <nfsclient ref="bob"/> <!-- Because all of the critical data for this resource is either defined in the resources block or inherited, we can reference it again! --> <nfsclient ref="jim"/> </nfsexport> </fs> <ip address="10.2.13.20"/> </service>
If the service were flat (that is, with no parent/child relationships), it would need to be configured as follows:
- The service would need four nfsclient resources — one per file system (a total of two for file systems), and one per target machine (a total of two for target machines).
- The service would need to specify export path and file system ID to each nfsclient, which introduces chances for errors in the configuration.
In Example C.5, “NFS Service Set Up for Resource Reuse and Inheritance” however, the NFS client resources nfsclient:bob and nfsclient:jim are defined once; likewise, the NFS export resource nfsexport:exports is defined once. All the attributes needed by the resources are inherited from parent resources. Because the inherited attributes are dynamic (and do not conflict with one another), it is possible to reuse those resources — which is why they are defined in the resources block. It may not be practical to configure some resources in multiple places. For example, configuring a file system resource in multiple places can result in mounting one file system on two nodes, therefore causing problems.