Currently, we have a JSON encoded message being stored in the NOTIFICATION_LOG table.
If we want to be future proof, we need to allow for versioning of this message, since we might change what gets stored in the message. A prime example of what we'd want to change is as in HIVE-10393.
MessageFactory already has stubs to allow for versioning of messages, and we could expand on this further in the future. NotificationListener currently encodes the message version into the header for the JMS message it sends, which seems to be the right place for a message version (instead of being contained in the message, for eg.).
So, we should have a similar ability for DbEventListener as well, and the place this makes the most sense is to and add a version column to the NOTIFICATION_LOG table.