14.2.2 Cascading Issues

In this section, we explain some of the design issues that are determined by the implementation of the cascading service.

14.2.2.1 Dynamic Cascading

When an MBean is unregistered from the subagent, the cascading service automatically removes its corresponding target CascadingProxy MBean from the master agent's target MBean server. When a new MBean is registered in the subagent's source MBean server, the cascading service registers a new CascadingProxy mirroring this source MBean with the master agent's MBean server. Note that if an ObjectName pattern was provided when performing the mount operation, only those source MBeans whose source ObjectName satisfy that ObjectName pattern are considered.

Both these mechanisms scale to cascading hierarchies. Adding or removing an MBean in the subagent will trigger a notification that any cascading service connected to the subagent will receive. This will start a chain reaction up to the top of the hierarchy. Removing an MBean from the middle of a hierarchy also triggers a similar reaction up to the top target MBean, which is finally removed. However, as explained in the previous section, corresponding source MBeans that are lower in the hierarchy will not be affected. The cascading service reflects the content of a source MBean server into a target MBean server, but it cannot be used to create or remove MBeans in the source MBean server. Calls to createMBean, registerMBean, and unregisterMBean only affect the local target MBean server.

14.2.2.2 MBean Patterns and Filtering

When the cascading service MBean is instantiated, you can pass it an object name pattern. This object name pattern is applied to the list of MBeans in the subagent's source MBean server to determine those MBeans that are to be cascaded into the master agent's target MBean server. The filtering is activated for the life of the of the mount operation, namely, until unmount is called for that mount point. Care must be taken when using the ObjectName pattern, so as not to implement cascading hierarchies that are not analogous to File System mount operations.

The object name pattern is used to filter any new MBean that is registered in the source MBean server. If the new MBean meets the filter criteria, it will become visible and be cascaded into the target MBean server.

14.2.2.3 Using Target Paths

Although the API also allows you to implement different cascading schemes, your applications should only implement those schemes that can be compared to a regular File System mount, as follows.

• When calling the CascadingServiceMBean.mount operation, always use a non null targetPath. The target path can be assimilated to a target mount point in the File System analogy.

• Never use a targetPath under which MBeans are already registered in the target MBean server. Using such a target path could cause a name conflict when mounting the source MBeans to the target MBean server.

• Never give the same targetPath to two different mount operations. Like in the file system analogy, you should not attempt to mount two sources to the same target path.

The present implementation does not enforce those rules, but applications that are concerned with naming consistency and coherency should make sure to respect them.