  RESTful APIs are an increasingly common way to expose software systems
functionality and it is therefore of high interest to find methods to
automatically test and verify such APIs. To lower the barrier for industry
adoption, such methods needs to be straightforward to use with a low effort.
This paper introduces a method to explore the behaviour of a RESTful API. This
is done by using automatic property-based tests produced from OpenAPI documents
that describe the REST API under test. We describe how this method creates
artifacts that can be leveraged both as property-based test generators and as a
source of validation for results (i.e., as test oracles). Experimental results,
on both industrial and open source services, indicate how this approach is a
low effort way of finding real faults. Furthermore, it supports building
additional knowledge about the system under test by automatically exposing
misalignment of specification and implementation. Since the tests are generated
from the OpenAPI document this method automatically evolves test cases as the
REST API evolves.
