def get_process_runtime_cache(self, key, pid=None):
        '''
        get_process_runtime_cache(self, key, pid=None)

        Get a pre-defined run time parameter value

        :Parameters:
        * *key* (`string`) -- Identifier of the runtime cache
        * *pid* (`string`) -- Identifier of an existing process

        '''
        value = None
        pid = self._get_pid(pid)
        value = self._call_rest_api('get', '/processes/'+pid+'/cache?key=%s'%key, error='Failed to fetch process runtime cache')
        return value