  Context: Stack Overflow (SO) has won the intention from software engineers
(e.g., architects) to learn, practice, and utilize development knowledge, such
as Architectural Knowledge (AK). But little is known about AK communicated in
SO, which is a type of high-level but important knowledge in development.
Objective: This study aims to investigate the AK in SO posts in terms of their
categories and characteristics as well as their usefulness from the point of
view of SO users. Method: We conducted an exploratory study by qualitatively
analyzing a statistically representative sample of 968 Architecture Related
Posts (ARPs) from SO. Results: The main findings are: (1) architecture related
questions can be classified into 9 core categories, in which "architecture
configuration" is the most common category, followed by the "architecture
decision" category, and (2) architecture related questions that provide clear
descriptions together with architectural diagrams increase their likelihood of
getting more than one answer, while poorly structured architecture questions
tend to only get one answer. Conclusions: Our findings suggest that future
research can focus on enabling automated approaches and tools that could
facilitate the search and (re)use of AK in SO. SO users can refer to our
proposed guidelines to compose architecture related questions with the
likelihood of getting more responses in SO.
