Sunday, September 13, 2009

Event and Content

This conversation with Nigel started innocently enough. Two people who have very similar views talking about architecture in the large. We described our current problem spaces - they looked very similar, but as is often the case we used the same words to mean different things. Of course once we realized this we got back on track quickly. I'll write this from my perspective, but in reality it would be better written by a dispassionate observer.

I set out to describe the separtion that I am seeing in VPEC-T especially around P-E-C. In my definition, I was thinking of Content as being everything about the Event. I was merrily proceeding down this path, thinking that the view was shared until Nigel spoke. We realized simultaneously that the word covered several concepts. There is the "stuff" about the event itself - the event properties like when it happened, what kind of event it is, what channel it was communicated on - essentially a kind of tag cloud for the event. I suppose in the current vernacular this might be the event meta-data. There is also the "state" that exists as a result of something doing a piece of work and generating the event. That may well need to be made available somehow. So for example, in my sales system the "event" of a sale being made has something useful like when it happened, however there is a whole lot of other information like who the parties to the sale were, what the value of the sale is, who the sales team is, what was sold... - in other word the "business document" representating the state as perceived by the event emitter.

In the Chris world, the tags (event meta data) were part of content. In the Nigel world they aren't part of content per se. Actually that is unfair, they aren't ONLY content. In other words, the event meta data can easily travel with the event. But the state information typically won't.

Reversing that point of view for a second, we get the notion that there is definitely a content "store" somewhere that the processor of the event might go to get the contents, and an event store - somewhere the event data would be stored. The content store doesn't have to be explicitly created by the event creating component, it could equally be some external source (e.g. a weather forecast). The event store's job is to store the events (duh) so that they can be replayed for a variety of reasons:
  • An event processor has been out of the loop for a hour and wants to catch up "get me all the sales events for the last hour"
  • A simulation wants to be done using some "live" state. What would the impact be if we had decided to close this airport at 2pm yesterday? So inject a new event into the event store and then replay forward with the existing events after that point. Of course there will quickly be diversion from the reality as processed, but that is OK. We have been able to run the simulation with a known starting point

Considering the second bullet above, it is quite convenient to use the same semantics for processing "missed events" as for processing the events in real time. Kind of rule - never pass the content with the event, always pass a reference to content, but make sure that the event tag (meta-) data do come with the event. Make the event store readable so that the event sender isn't responsible for hanging on to the event, and having to remember who has received it and who hasn't

At the end of our time together on this call we were, as expected, in agreement that for a proper separation of concerns, we should treat content as being the state data and the event store as being the place where the events and associated metadat are stored to allow for retries and simulations (among other things).

The Policy part of the call will come next.....

1 comments:

daveh said...

Thanks Nigel. Separation of event and content (with appropriate cross references) improves both processing domains. Events processing performance can be improved while focusing on event semantics. Meanwhile, content processes can be deployed such as validation and mediation without sacrificing performance or cluttering up the event handler.