What is Complex Event Processing? (Part 2)

(Originally Published by Tim Bass, TIBCO Software Inc. , April 23, 2007)

In a previous blog entry, What is Complex Event Processing? (Part 1), we introduced a few basic event processing concepts and a functional reference architecture for CEP based on the JDL model for multisensor data fusion. One of the most important concept in our reference architecture is the notion of events, which is the topic of this blog entry today.

What is an Event?

Similar to many topics in science and engineering, the term event has different meanings based on who is observing the event and the context of the observation. Let’s review of few of the different definitions from the point of various observers, keeping in mind that in CEP we are primarily interested in processing events related to business. First, we take a wider survey.

If you are a mathematician, you might view an event via the lens of event probability theory, which states than an event is a set of outcomes (a subset of the sample space) to which a probability is assigned. So, for example, if we were processing many banking application log files, in real-time, looking for fraud, there exists some conditional probability at any moment that a fraud is being orchestrated against the bank. The event is the fraud (detected or undetected outcome); and based on a number of factors, the probability of a fraudulent event against the bank changes over time.

On the other hand, if you are a particle physicist, an event is a single collision of two particles or a decay of a single particle! A collision, in particle physics, is any process which results in a deflection in the path of the original particles, or their annihilation. This view seems to imply that atomic and subatomic exceptions and state transitions are the foundation for events, which may be significant if you are a particle physicist. Assuming most of the readers of the blog are not particle physicists, you may be interested in the draft definition of an event from the Event Processing Technical Society (EPTS) CEP glossary working group, summarized below:

Event: Something notable that happens.


– a financial trade

– an airplane lands

– a sensor outputs a reading

– a change of state in a database, a finite state machine

– a key stroke

– a natural or historical occurrence such as an earthquake

– a social or historical happening, e.g., the abolition of slavery, the battle of Waterloo, the Russian revolution, and the Irish potato famine.

Event (also event object, event message, event tuple): An object that represents, encodes or records an event, generally for the purpose of computer processing. Notes: Events are processed by computer systems by processing their representations as event objects. Events are immutable objects. However, more than one event may record the same activity.


– a purchase order (records a purchase activity)

– an email confirmation of an airline reservation

– stock tick message that reports a stock trade

– a message that reports an RFID sensor reading

– a medical insurance claim document

Overloading: Event objects can contain data. The word “event” is overloaded so that it can be used as a synonym for event object. In discussing event processing, the word “event” is used to denote both the everyday meaning (anything significant that happens) and the computer science meaning (an event object or message). The context of each use indicates which meaning is intended.

As one can see, none of these definitions are completely satisfying! For example, if we look at financial market data, some might observe that it appears a bit pedestrian to say that each trade is an event. Why? Because the market data is the entire sample space and each trade is an element of the set of trades of that particular equity (for example) on a particular day. To call each trade an “event” may be unsatisfactory for some people.

On the other hand, when a business is processing market data using a VWAP algorithm, for some the event occurs, for example, when the price of a buy trade is lower than the VWAP. Conversely, if the price is higher than the VWAP the event would be an indication to sell.

This example tends to more closely align with the mathematicians view of events, an outcome from the sampled set with an assigned probability. The fact that the event is “significant” is due to the context of the theory and application of the VWAP strategy – without VWAP there might be no event, in this context. Similar analogies can be illustrated for fraud detection, supply-chain management, scheduling and a host of other CEP related business problems.

Events are Context Dependent

For example, if you have thousands of packages with RFID tags traveling the globe, is the event when the RFID reader registers the RFID tag? Or is the event when an exception occurs, for example, a lost package? One view is that the RFID reader is simply recording data and the associated RFID data is the sampled set (not necessarily the event). The outcome of interest, with an assignable probability based on the business context, are exceptions, which, in term, become business events. On the other hand, another view might be that each RFID recording is an event, and CEP is detecting “situations,” in this use case, the situation we refer to as “lost package”.

In you are interested in other terms related to CEP, please visit the Draft Event Processing Glossary. Your comments on the glossary are both welcome and much appreciated!

In What is Complex Event Processing (Part 3), I will begin to discuss the functional components of event processing based on the functional reference architecture introduced in What is Complex Event Processing? (Part 1).


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