Measuring complexity of domain standard specifications using XML schema entropy

XML schemas are used extensively in e-commerce standardization initiatives. Such XML-based standards define the structure and the semantics of messages that are used to implement business transactions in a particular industry domain (e.g. travel). The design of the document structures that form the message payloads is of key importance as once the specification is published it is difficult to re-design the documents without impacting on existing applications. Furthermore, such domain standards need to be maintained and evolved over long time periods, typically decades, without unduly increasing the complexity of the specification. The concept of software entropy has been used in the literature to estimate complexity and to express decline in quality, maintainability and understandability of software though its lifetime. In this paper we propose a Message Software Entropy (MSE) metric that estimates the complexity of XML message structures and we use this metric to study the complexity of a subset of the Open Travel Alliance Specification as it evolves over time