'GET /api/workflows/{workflow_id}/invocations/{invocation_id}/steps/{step_id}
:param  workflow_id:        the workflow id (required)
:type   workflow_id:        str
:param  invocation_id:      the invocation id (required)
:type   invocation_id:      str
:param  step_id:      encoded id of the WorkflowInvocationStep (required)
:type   step_id:      str
:param  payload:       payload containing update action information
for running workflow.
:raises: exceptions.MessageException, exceptions.ObjectNotFound'

@expose_api
def invocation_step(self, trans, workflow_id, invocation_id, step_id, **kwd):

    decoded_invocation_step_id = self.decode_id(step_id)
    invocation_step = self.workflow_manager.get_invocation_step(trans, decoded_invocation_step_id)
    return self.__encode_invocation_step(trans, invocation_step)
