Saturday, January 19, 2008

How Many Events?

Homeward bound, delayed in Zurich by the consequences of a crashed Boeing at Heathrow, Opher Etzion blogs On events in flight management. It seems to him "that more events related to flights happened relative to previous years".

For regular business travellers, the best thing we can say about a flight is that it was "uneventful". However good the catering, however large and comfortable the seats, however charming and sexy the air staff, none of this can make up for the inconvenience of delays or lost baggage. So when Opher counts the number of events, I assume he is referring to adverse events.

Of course there are countless events in flight management that are completely invisible to passengers - or even to the air crew - unless something goes wrong, and perhaps even then. In a distributed man-machine system, different parts of the system will be paying attention to different types of event, at different levels of granularity. (An air traffic controller deals with the event PlaneAwaitingLandingSlot; his manager deals with the aggregate event NumberOfPlanesAwaitingLandingSlotIsGreaterThanX.) We can think of this in terms of the architecture of attention - this calls for accurate modelling of events, leading to clear system design.

Furthermore, as Opher points out, there is an important distinction between attention (detection) and action. "In some cases ... the detection is very easy, the complexity is in the response." Last week, Opher apparently experienced a failure of response. "The captain told us several times that he is pushing them to send buses, but they are not responsive ..." It is often easy to blame Them, but it isn't always clear who exactly is responsible.

So the other challenge in designing complex event-driven systems-of-systems is to specify the architecture of response. There are many people and systems and organizations involved in flight management, and a complex event may call for a complex and collaborative response.

So is there a universal and homogeneous event model shared by all the participants in flight management? I don't think we can reasonably insist on this. Instead, we have to allow for some kind of amplification and attenuation - where different subsystems may have different event models, and there is some mechanism for translating and coordinating across these different models. I think this approach is more flexible, more robust, and compatible with loosely coupled SOA.

1 comment:

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