Thursday, January 10, 2008

On the classification of events

I am returning to the area of "classification of events", although it has been discussed in this blog several times regarding certain aspects, the reason is - some discussion in which I realized that the confusion is still alive and kicking.
Events can be classified according to their structure, semantics, or pragmatics. This classification is orthogonal.
Based on Structure:
  • Simple or atomic event - an event that reports on a single occurrence.

The event structure may be flat (like relational database), or semi-structured (XML) - which also can express non-flat structure (e.g. hierarchical). In some cases events are represented as objects. Typically events contain "header" - information about this event (event class, time stamp, source...), and "payload" - the content of the event.

  • Complex event - an event that is composed of multiple events

There is some difference between this and the term "composite event" from active databases which mixes structure and semantic aspect, but I don't see it useful to use this term any more.

Based on source

  • Raw event: Event that has been reported to the event processing system by an external event producer.

Note that a raw event can be simple or complex. The term "raw" is relative, since it can be result of some computation process in the event producer that is transparent to the event processing system.

  • Derived event: Event that is created by the event processing system as a result of some processing.

Again, the derived event can be simple/atomic - where the attributes are computed as functions of raw and derived events, or complex - in which the derived event is a concatenation of events.

Based on ontology:

  • Real event: An event that designates something that occurs in reality
  • Virtual event: Hypothetical, simulated or uncertain event.

Again, this is a separate dimension - can be in any structure and by any source.

Based on pragmatics

  • Reactionable event (AKA situation): An event that is consumed by an external consumer at the end of the event processing network (either notify or orchestrate).

We used the term "situation" for a phenomenon in the consumer's domain of discourse that the consumer wishes to react to. This can be raw event (and in this case the event processing system is reduced to "simple event processing" - i.e. filtering and routing) or derived event, it can also be real or virtual, and in any structure.

BTW - ECA - "Event-Condition-Action" rules are rules in which the events are situations, and they are executed within the consumer domain, but I'll continue to discuss the conceptual model in the next posting.

Tuesday, January 8, 2008

On the Event Processing Conceptual Model

The event processing conceptual model shown above is a result of a work by an IBM internal team lead by my colleague Mamdouh Ibrahim. I have presented it in the "reference architectures" session of the last EPTS meeting, we have decided not to use the term "reference architecture" since it has multiple implementations, but this represents the different parts of an event-based applications. The three major building blocks are - event prodcuers, event processing network, and event consumers.
  • Event producers - emit events in push or pull (periodic or on-demand).
  • Event Processing Networks - process the events, create derived events where applicable, route the event to event consumers at the edge of the network
  • Event consumers - consume the processed events and orchestrate/notify.

As you can also see the event processing network has two bridges - one to the producers and another one to the consumers, in which several of the functions of the EPN may reside.

We believe that this is a general enough conceptual model that can contain various variations.

Note that the "event processing network" requires an "event bus", this is a platform-independent term - in SOA environment - as noted in a previous posting the "event bus" converges with the "enterprise service bus". More deep description of the different ingredients - in later posts

Sunday, January 6, 2008

On Enterprise Service Bus and Event Processing

This is one of the variations of Enterprise Service Bus (ESB) illustrations that I have taken from an article by one of my IBM colleagues. The topic of today is -- what are the relations between ESB and Event Processing ?
  • An event processing functionality that runs Event Processing Agents in various sources, requires a that will take care of the routing and execution in a distributed environment. There are three alternatives here:
  1. No native platform -- an engine that can run in multiple platforms (thus need to be integrated to each platform that it runs in by adapters etc...).
  2. Dedicated event processing platform -- the event processing part has a dedicated platform that provides the infrastructure for the event processing functions.
  3. Event Processing is built as part of an already existing platform.

All of these variations exist in the market today, and there are pros and cons for each of them, smaller vendors may prefer the first alternative as my friend Marco noted in his Blog.

When getting to the third alternative, if the environment is a SOA environment, then the ESB is a natural place in the SOA middleware to be the principle carrier of event processing functionality:

  • It provides messaging infrastructure and routing capabilities
  • It provides mediations like - validation, transformation and enrichment that can be reused for event processing (have a large intersection with the "mediated event processing" functions)
  • It supports distributed environment.

While the principle usage of ESB in SOA has been to mediate between consumer and producer of services, being a carrier for event processing is now considered as a step in the evolution of ESBs.

This does not say that ESB is the ONLY place in which event processing functionality can run, which brings to a discussion about the Event Processing Conceptual Model- which I'll deal in a subsequent posting.

The ESB gets into the picture in alternative