public function bind($channel_ref, $affinity_key)
    {
        if (!array_key_exists($affinity_key, $this->affinity_key_to_channel_ref)) {
            $this->affinity_key_to_channel_ref[$affinity_key] = $channel_ref;
        }
        $channel_ref->affinityRefIncr();
        return $channel_ref;
    }