Wednesday, August 09, 2006


Callbacks are the key to a good recognition system. A typical batch recognition system forgets about the importance of letting consumers know as soon as information is available. However, this is the lynchpin to a good recognition system.

Let's assume that a typical use case is the following:

1. Run a large file through the system to create a repository
2. Run the files through the repository to ensure correct linkage
3. Rinse and repeate monthly

You have to run the file through twice because you don't know what might happen later in the system to change one of your records. This is because the system is not set up to tell you about events.

If, instead, we allowed the sytem to tell you about important things that are happening, you would be able to complete your run in one pass. So, what needs events. Well, first let's say that we'll use a publish/subscribe mechanism so that only those events that we're interested in will be delivered to us. Second, let's say make the rule that anything that could have an impact on the end result should have an event fired. That means that any time an Entity or Group is created or deleted as well as any time an Entity is moved from one Group to another. I would say that Element updates should be allowed to have events, but not forced to. It could be that updating the salary field doesn't affect anything and you don't need that information to be disseminated.

There are lots of optimizations you can do to make this fast and I don't want to get into those right now, but suffice it to say that the event/callback mechanism can make for an extremely flexible (and efficient!) system.

Obviously, the code for the callback won't be in the same file as the code defining the entities. However, we may want to augment the event with some information at event generation time; therefore, we allow the override of the OnX methods (where X is something like Consolidation).

For example:

class Consumer < Group
def Consolidation(Group other):
if ...:
consolidation_reason = ...
elseif ...:
consolidation_reason = ...

def OnConslidation(ConsolidationEvent event):
event.reason = conslidation_reason

I'm not sure, but you might even be able to suppress events...I don't necessarily like that, but it could come in handy.

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