Monday, April 27, 2009
More on Revision
Long day today, I got to the office around 8AM and left around 9PM. Since we have holiday this week I am trying to condense the remaining days of the week and the result is long day with plenty of conference calls. The picture above is a glance (from below) on the IBM Haifa Lab (the pair of connected building on the right hand side of the picture), my office is in the back building (known as the "banana" due to its shape), and is not really in the nice part of the building -- the one with the view to the Haifa Bay -- well, one can have everything in life -).
I still need to complete the previous posting on revision. I gave some explanation about the concept of revision, and now I still need to discuss implementation of revision in event processing. To recall -- a revision in event processing is getting later knowledge that asserts that either a reported event did not really happen, or some information associated with the event was wrong.
Let's look at two separate cases, one in which the processing has not gone out of the event processing network, and second that the results of the processing have gone out to the "outside world".
In the first case, there may be an opportunity to revise the impact of the revised event by doing kind of undo-redo for all the event processing agents that it passes directly or indirectly. Direct ones are easy -- those that the revised event participate as an input in them, indirect is more tricky, since we need to trace the causality among events, in this case, an event that is an output of an event processing agent in which the revised event participate (relate to the same context) has a causality relation to the original event, thus, an event processing agent, in which this event participates as an input, also needs to do an undo/redo, and causality is a transitive relation, so it continues as far as the EPN arrived so far. It should be noted that the fact that there is a causality may not require a real undo/redo, take as an example that an event of type E1 designates a bid, and the event of type E2 designates the bid with maximal value arrived in a certain time interval. Let's assume that a certain bid has been revised, however, neither the revised bid, or the revising bid change the selection of E2.
The second case is that the revised event has consequences that have been sent to an external consumer, thus, it may have triggered an action, a collection of actions, or a workflow that has been carried out, and this may propagate further ("the butterfly effect"), in this case, either we can treat it as "too late" and do nothing, however, there may be a cases that it can be critical to undo/redo also the consequences, e.g. the revised event has some financial meaning. In this case we'll need to issue compensation for the triggered action, which may be impossible (the consumer does not support compensation) or difficult. I'll blog again about revising the history and its aspects at a later phase.