eXo JCR supports observation (JSR-170 8.3), which enables applications to register interest in events that describe changes to a workspace, and then monitor and respond to those events. The standard observation feature allows dispatching events when persistent change to the workspace is made.

eXo JCR also offers a proprietary Extension Action which dispatches and fires an event upon each transient session level change, performed by a client. In other words, the event is triggered when a client's program invokes some updating methods in a session or a workspace (such as: Session.addNode(), Session.setProperty(), Workspace.move() etc.

One important recommendation should be applied for an extension action implementation. Each action will add its own execution time to standard JCR methods (Session.addNode(), Session.setProperty(), Workspace.move() etc.) execution time. As a consequence, it's necessary to minimize Action.execute(Context) body execution time.

To make the rule, you can use the dedicated Thread in Action.execute(Context) body for a custom logic. But if your application logic requires the action to add items to a created/updated item and you save these changes immediately after the JCR API method call is returned, the suggestion with Thread is not applicable for you in this case.