JIRA have a nice feature called listeners. Easy-to-attach handler to JIRA events being happening are caught and processed.

However, the power of these listeners are too little, they do not allow to cancel the action or interact with it anyhow, they are already called when the action is happened and there is no chance to cancel the operation.

Why listeners: easy and pretty way to catch events without the need of modifying JIRA sources, since if we will modify that, our administrators will have many problems with upgrading to newer JIRA versions, making it very difficult to support.

Why we need this: we are going to use listeners service to maintain link with our server which itself maintains links with MS Project server in order to join the project planning with tracking and reporting. All changes in all systems joined together should be tracked and recorded, so if some problem occures on external system we should be able to cancel JIRA action, since it will be out-of-sync otherwise.

How this should work: same as currently, just listener should be able to throw exception that is processed in the same cycle as the event thus the error message from the exception will go to the error messages to the end user as the result of the operation and it will be canceled like normaly in JIRA if error occures.
Maybe some override function like requireInstantReply() should be added to listeners defaulting to false, thus only listeners that wish this instant processing (which takes time per each operation of course) will be executed in the same thread.

Here is same post on boards: http://forums.atlassian.com/thread.jspa?threadID=12731&tstart=0