Behavior Trees

Behavior trees are a formal, graphical modelling language used primarily in systems and software engineering. Behavior trees employ a well-defined notation to unambiguously represent the hundreds or even thousands of natural language requirements that are typically used to express the stakeholder needs for a large-scale software-integrated system. The amount of detail in the large number of natural language requirements for a large-scale system causes short-term memory overload and may create a barrier that prevents anyone from gaining a deep, accurate and holistic understanding of the system needs. Also, because of the use of natural language, there are likely to be many ambiguities, aliases, inconsistencies, redundancies and incompleteness problems associated with the requirements information. This adds further to the uncertainty and complexity. Generally, at best, a few people understand parts of the system or situation well, but no one has other than a superficial understanding of the whole – that is, the detailed integrated behavior of the system. The behavior tree representation, (with the help of the composition tree representation that resolves alias and other vocabulary problems with large sets of requirements) allows people to avoid short-term memory overload and produce a deep, accurate, holistic representation of system needs that can be understood by all stakeholders because it strictly uses the vocabulary of the original requirements. Because the behavior tree notation uses a formal semantics, for any given example, it already is, or can be made executable. Single and composite or integrated behavior tree forms are both important in the application of behavior trees in systems and software engineering. Having all the requirements converted to behavior trees (RBTs) is similar to having all the pieces for a jigsaw puzzle randomly spread out on a table - until we put all the pieces together we cannot see the emergent picture and whether any pieces are missing or do not fit. Constructing an integrated behavior tree (IBT) allows us to do this. Behavior trees and the concepts for their application in systems and software engineering were originally developed by Dromey with first publication of some of the key ideas in 2001. Early publications on this work used the terms 'genetic software engineering' and 'genetic design' to describe the application of behavior trees. The reason for originally using the word genetic was because sets of genes, sets of jigsaw puzzle pieces and sets of requirements represented as behavior trees all appeared to share several key properties: