Saturday, October 3, 2009

More on the nodes in the event processing network

This is the illustration of the event processing network for the Fast Flower Delivery example that accompanies the EPIA book we are writing, don't spoil your eyes reading it. We'll provide the readers with an open source editor to the EPN meta-language we have developed (developed by Ronen Vaisenberg, a Ph.D. student in UCI). I'll have posting about it with more instructions in one of the next postings.

In this posting I would like to go back to the nodes in EPN and discuss why do we need multiple nodes and what are these nodes. One version is that there is a single type of node called EPA -- event processing agent, but my preference is to have multiple nodes, since they have different roles. Here are the types of nodes in our EPN model:

  • Event Processing Agent (EPA): a node that gets one or more events as an input and produces one or more event as an output.
  • Event producer: An entry point to the event processing system that emits events to the EPN.
  • Event consumer: An exit point from the event processing system that receives events from the event processing system
  • Event channel: A node that make routing decisions within the EPN, I have recently discussed event channels in this Blog.
  • Global states: This is a node that holds various type of state (reference data, global variables)
  • EPN node: An EPA can represent an EPN, and thus make the EPN recursive.
One of the frequent questions is whether an event producer which is also an event consumer is considered an EPA. In my opinion, the answer is no. Event producers and consumers operate on the borderline of event processing network, and is actually have have two nodes one for each of its roles. The information that a consumer is also a producer is important for traceability and lineage, but we'll discuss this aspect another time.

Christoph Emmesberger, whom I met in Trento, has proposed to extend the notion of channel to provide aggregation and filtering, this idea is rooted in pub/sub systems where event processing seems as an extension of some event processing functionality. However, when we deal with event processing system, I prefer to make a clear distinction between event processing agents that is doing -- filtering, aggregation, transformation and event pattern detection , and leave channel as a routing mechanism. thus we'll ensure proper separation of concerns, and have a cleaner model. We are now finishing the chapter about pattern detection, and I'll write about patterns soon.

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