Cascading Issues

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

Dynamic Mirroring

Any changes in the subagent's MBeans are automatically applied to the set of mirror MBeans, to ensure that both master agent and subagent remain consistent.

When an MBean is unregistered from the subagent, the cascading service MBean removes its mirror MBean from the master agent. When a new MBean is registered in the subagent, the cascading service instantiates its mirror MBean, sets up its connection to the new MBean, and registers the mirror with the master agent's MBean server.

Both of these mechanisms scale to cascading hierarchies: adding or removing an MBean in the master agent will trigger a notification that any cascading service connected to the master agent 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 down to the original MBean that is finally removed.

Dynamic unregistration only applies to subagent MBeans that are actually mirrored. Dynamic registration is also subject to filtering, as described in the next section.

MBean Filtering

When the cascading service MBean is instantiated, you can pass it an object name pattern and a query expression. These will be applied to the list of MBeans in the subagent to determine those that will be mirrored in the master agent. The filtering will be in effect for the life of the cascading service connected to this MBean.

Filtering the mirrored MBeans reduces the number of MBeans in the master agent. It also provides a way of identifying mirror MBeans in the master agent, as in our example where cascading is limited to MBeans in the CascadedDomain.

Both the object name pattern and query expression will be used to filter any new MBean that is registered in the subagent. If the new MBean meets the filter criteria, it will become visible and mirrored in the master agent. Since the query expression applies to attribute values of the MBean, you must be careful to initialize the new MBean before registering it so that its values are significant when the filter is applied by the cascading service.

Naming in Cascading Agents

Mirror MBeans are registered in the master agent with the same object name as the mirrored MBean in the subagent. If the registration fails in the master agent's MBean server, no error is raised and no action is taken: the corresponding MBean will simply not be mirrored in the master agent.

The most likely cause for registration to fail is that the object name already exists in the master agent. An MBean cannot be registered if its chosen object name already exists in the MBean server.

If your management solution has potential naming conflicts, you will need a design that is guaranteed to assign unique object names throughout the cascade hierarchy. You can set the default domain name in your subagents or use the MBeanServerId attribute of the delegate MBean to give MBeans a unique object name.