Goodbye ioco

Abstract

Model-based testing involves testing a system under test for conformance to a model that specifies its behaviour. An important aspect for model-based testing is the implementation relation that defines precisely when a system under test conforms to its model. The implementation relation ${\textbf{ioco}}$ has often been used and studied in model-based testing when models are expressed as labelled transition systems, and there are tools implementing ${\textbf{ioco}}$-based test generation. An alternative, slightly different implementation relation is ${\textbf{uioco}}$, which is more recent, has been less studied, and there are no tools for it. We will compare ${\textbf{ioco}}$ and ${\textbf{uioco}}$ on a couple of aspects, viz. intuition, the decision whether a test observation is correct, the definition of a consistent refinement relation, the construction of a canonical implementation for each specification, and the relation to other input-output implementation relations. For all these aspects, we conclude that ${\textbf{uioco}}$ is the preferred implementation relation, so, goodbye ${\textbf{ioco}}$, hello ${\textbf{uioco}}$.