Chapter 8. Filtering and transformation
There are painters who transform the sun to a yellow spot, but there are others who with the help of their art and their intelligence transform a yellow spot into the sun.
Pablo Picasso
We now return to the examination of event processing networks that we started in chapter 6, to take a deeper look at the event processing agents (EPAs) that an event processing network can contain. The most important kinds of EPAs are Filter, Transformation, and Pattern detect. In this chapter we look at the first two of these. Pattern detection is a large topic, so we dedicate the entire next chapter to that. We also look at the general question of how context is applied to EPAs of all kinds.
In this chapter we will cover the following:
- The idea of a filter, the places where filtering can be performed in an event processing agent, and languages used to specify filter expressions
- The use of event context to perform filtering
- The different types of stateful and stateless transformation
- The effect of filtering and transformation on event header elements
As usual we illustrate the concepts with examples from the Fast Flower Delivery application and examples from other fictional applications. We start with the discussion of filtering and then move on to transformation later in the chapter.