This blog post explains the relational key – a little piece of event attribute metadata that is used to associate attributes in different event types with each other. This is a quick break in my series of posts about unsupported functionality, the reason for the break being that you will need to be armed with this knowledge for my next instalment in that series. 
The Relational Key
The relational key is a little piece of event type attribute metadata on the form of an URI. It is normally used as a marker to hint to JMC that it often makes sense to find all events (of different event types) with the same attribute value for attributes with the same relational key. For example, the Java Flight Recorder has quite a lot of different GC events. Almost all of them are related to a certain GC ID. It often makes sense to find all events related to the same GC ID. The picture below lists some common GC events, and their attribute metadata.
The relational key can be seen in action in how Mission Control presents the alternatives for the Operative Set. If you go to the GC view, pick a garbage collection and open the context menu, you will find that there is an option to add all the events with the same GC ID to the operative set.
Moving over to the events tab, and selecting to only show the operative set, a range of different events, all associated with that particular garbage collection, can be seen.
Another very useful example is the ECID attribute in the events produced by the WebLogic Diagnostics Framework (see the “Putting it All Together” section of my post about the Operative Set).
Conclusion
The relational key is a handy little piece of attribute metadata that is used to give hints to the JMC user interface that different event types are related to each other. It is used by Mission Control to, for example, control what attributes are listed in the Operative Set context menu.