public function unbind($affinity_key)
    {
        $channel_ref = null;
        if (array_key_exists($affinity_key, $this->affinity_key_to_channel_ref)) {
            $channel_ref =  $this->affinity_key_to_channel_ref[$affinity_key];
            $channel_ref->affinityRefDecr();
        }
        unset($this->affinity_key_to_channel_ref[$affinity_key]);
        return $channel_ref;
    }