This fact supports
the conjecture C2, which proposes that some common
technical challenges exist in multiple cloud service
architectural layers. In terms of covered testing topics, the
SaaS layer (9 topics) was more actively studied than the
layers of PaaS (5 topics) and IaaS (2 topics) [14]. The papers
in PS only involve 16 out of the 36 possible layer-specific
testing topics. Both the unbalanced layer-topic distribution
and the big proportion of un-investigated layer-specific
topics indicate that a generally-accepted clear understanding
of the CST-interface had not yet emerged.