This page gives some advice to create a new Aggregation function.

This page is under construction.

Interfaces for a new function

An aggregation function needs to implement one of the following interfaces:

• de.uniol.inf.is.odysseus.aggregation.functions.IIncrementalAggregationFunction
  An incremental aggregation functions gets a notification when an element gets valid (enters the window) or gets invalid (leaves the window). You have to hold a state to return the current aggregation value on request. 
  • void addNew(T newElement);
    This method is called when a new element gets valid. You should add this value to the state.
    For example AVG function: State holds sum and count. sum += newElement and count++ 
  • void removeOutdated(Collection<T> outdatedElements, T trigger, PointInTime pointInTime);
    This method is called when a set of elements get invalid. You should remove these values from the state.
    For example AVG function: For each element in outdatedElements: sum -= element and count-- 
  • Object[] evalute(T trigger, PointInTime pointInTime);
    This method is called when a new aggregation value should returned.
    For example AVG function: return sum/count 
• de.uniol.inf.is.odysseus.aggregation.functions.INonIncrementalAggregationFunction
  An non-incremental function gets a set of all elements in the current window. It does not have to hold a state.
  • Object[] evaluate(Collection<T> elements, T trigger, PointInTime pointInTime);
    This method is called when a new aggregation value should returned. elements holds all elements in the window that starts at pointInTime. 

In de.uniol.inf.is.odysseus.aggregation.functions there are also abstract classes.

Function Registry and Function Factories

• Implement de.uniol.inf.is.odysseus.aggregation.functions.factory.IAggregationFunctionFactory to allow Odysseus to create a new function.
• Add OSGi service to add function to the registry (de.uniol.inf.is.odysseus.aggregation.AggregationFunctionRegistry).