def set_process_runtime_cache(self, key, value, pid=None):
        '''
        set_process_runtime_cache(self, key, value, pid=None)

        Set a process run time parameter

        :Parameters:
        * *key* (`string`) -- parameter key
        * *key* (`value`) -- parameter value
        * *key* (`pid`) -- optional - Identifier of an existing process

        '''
        pid = self._get_pid(pid)
        self._call_rest_api('post', '/processes/'+pid+'/cache', data={'key': key, 'value': value}, error='Failed to modify process runtime cache')