Revisiting EPN

This illustration, taken from the EPIA book, and drawn by Peter Niblett,  is a portion of the EPN that describes the "Fast Flower Delivery" example that accompanies this book.   In an internal discussion today somebody raised the question, why do we need EPN at all,  and not using the alternative that has been used in Amit, and other places:  each EPA subscribes to an event type, whenever an event from this event type is detected, the appropriate EPA listens to it and processes it,  and all the event flow is implicit and the person defining the system does not need to worry about it.


Since this question is actually a good question,  I wanted to share my response.  There are two main reasons why we have shifted in the thinking to the EPN model:  efficiency and usability.  


  I'll start with the usability, experience shows (and this observation is true also to inference based systems) that people feel more comfortable in ability to control the flow rather then having implicit flows, they understand better what it does, can better debug and validate it, and trust such systems more.   Note that EPN is not a workflow, it does not represent control flow, it represent event streaming flow (in a way similar to data flow, with some semantic distinctions).  


The other reason is efficiency.    If an EPA subscribes to event type then either an EPA has to process and filter out a substantial amount of irrelevant events, or the amount of event types might successfully be increased.   Imagine the following scenario:   An event of type ET1 arrives,  first it meets a filter that filters out much of the event using some assertion, and then there are various EPAs that process only the filtered-in events,  one of this EPAs is enrichment, adding some information from a database,  and then the enriched event is being sent to an aggregator for further processing.      If we use the "event type" subscription, there are two choices:  first -- create event type ET2 for the filtered-in events, identical to ET1, and create derived event of type ET2 for each filtered-in event of type ET1,  then create event type ET3 for the enriched event with added enriched attribute, and then indeed each EPA subscribes to a single event type.  The second choice is to use ET1 for all three cases, but add indication (using some derived attribute) which variation of ET1 it is, and filter inside the aggregator to have only the right type of ET1.  Both are inefficient, the first one due to the need to manage much more event types, the second is that much more events are transmitted to each EPA to filter out, and the order also becomes important here.   


The explicit EPN resolves it by the fact that each EPA sends it output to a channel and the channel can route according to source, type, assertion etc...   -  thus a specific  output terminal of a channel is really the topic which EPA subscribes to.     Note that all the possibilities mentioned before are just special cases of EPN and if one insists, such EPN can be constructed, in the extreme case, one can construct EPN with a single channel that routes every event to every EPA to decide whether it wants to use it or not,  but I would not recommend it as a good design pattern.     More - later.