srs 边缘集群

srs官方关于边缘集群的介绍：

Edge Cluster | SRS

本篇分析一下边缘集群中上行边缘节点的处理逻辑。

关于上行的边缘节点：

SRS对于上行边缘，采取直接代理方式，并没有采取边缘缓存方式。所谓边缘缓存方式，即推流到边缘时边缘也会当作源站直接缓存（作为源站），然后转发给源站。边缘缓存方式看起来先进，这个边缘节点不必回源，实际上加大了集群的逻辑难度， 不如直接作为代理方式简单。

上行边缘SrsPublishEdge

上行边缘只做代理，收到流后直接转给源站

srs_error_t SrsRtmpConn::process_publish_message(SrsLiveSource* source, SrsCommonMessage* msg)
{
    srs_error_t err = srs_success;
    //对于边缘edge，收到流后直接发给源站
    // for edge, directly proxy message to origin.
    if (info->edge) {
        if ((err = source->on_edge_proxy_publish(msg)) != srs_success) {
            return srs_error_wrap(err, "rtmp: proxy publish");
        }
        return err;
    }
}

// TODO: FIXME: Use edge strategy pattern.
srs_error_t SrsLiveSource::on_edge_proxy_publish(SrsCommonMessage* msg)
{
    return publish_edge->on_proxy_publish(msg);
}
//消息转给forwarder模块，即：SrsEdgeForwarder类
//该类在SrsPublishEdge的构造函数中进行初始化
//并且在收到publish消息时，开启一个协程去队列中读取数据
srs_error_t SrsPublishEdge::on_proxy_publish(SrsCommonMessage* msg)
{
    return forwarder->proxy(msg);
}

srs_error_t SrsEdgeForwarder::proxy(SrsCommonMessage* msg)
{
    srs_error_t err = srs_success;
    
    if (send_error_code != ERROR_SUCCESS) {
        return srs_error_new(send_error_code, "edge forwarder");
    }
    
    //...
    //...
    SrsSharedPtrMessage copy;
    if ((err = copy.create(msg)) != srs_success) {
        return srs_error_wrap(err, "create message");
    }
    //forward模块收到消息后把消息放到队列中
    copy.stream_id = sdk->sid();
    if ((err = queue->enqueue(copy.copy())) != srs_success) {
        return srs_error_wrap(err, "enqueue message");
    }
    
    return err;
}

SrsEdgeForwarder转发消息给源站

真正的业务场景中，一般源站会部署多台组成一个集群，所以当一个边缘收到流后，需要选择向集群中的哪台源站进行推流。srs选取源站的负载均衡算法是RoundRobin

string SrsLbRoundRobin::select(const vector& servers)
{
    srs_assert(!servers.empty());
    
    index = (int)(count++ % servers.size());
    elem = servers.at(index);
    
    return elem;
}
//通过负载均衡算法选取了源站后，边缘节点会创建一个rtmp client然后把流转推给该源站节点
srs_error_t SrsEdgeForwarder::start()
{
    std::string url;
    if (true) {
        //从配置文件中读取源站地址列表
        SrsConfDirective* conf = _srs_config->get_vhost_edge_origin(req->vhost);
        srs_assert(conf);
        //通过 roundrobin算法选取合适的源站节点
        // select the origin.
        std::string server = lb->select(conf->args);
        int port = SRS_CONSTS_RTMP_DEFAULT_PORT;
        srs_parse_hostport(server, server, port);
        
        // support vhost tranform for edge,
        std::string vhost = _srs_config->get_vhost_edge_transform_vhost(req->vhost);
        vhost = srs_string_replace(vhost, "[vhost]", req->vhost);
        //生成向源站推流所用的url地址
        url = srs_generate_rtmp_url(server, port, req->host, vhost, req->app, req->stream, req->param);
    }
    //创建一个rtmp client向源站推流
    sdk = new SrsSimpleRtmpClient(url, cto, sto);
     if ((err = sdk->connect()) != srs_success) {
        return srs_error_wrap(err, "sdk connect %s failed, cto=%dms, sto=%dms.", url.c_str(), srsu2msi(cto), srsu2msi(sto));
    }

    // For RTMP client, we pass the vhost in tcUrl when connecting,
    // so we publish without vhost in stream.
    string stream;
    if ((err = sdk->publish(_srs_config->get_chunk_size(req->vhost), false, &stream)) != srs_success) {
        return srs_error_wrap(err, "sdk publish");
    }
}

SrsEdgeForwarder协程中发送

srs_error_t SrsEdgeForwarder::do_cycle()
{
     while (true) {
        if ((err = trd->pull()) != srs_success) {
                return srs_error_wrap(err, "edge forward pull");
        }
         // forward all messages.
        // each msg in msgs.msgs must be free, for the SrsMessageArray never free them.
        //首先从队列中取出要发送的消息
        int count = 0;
        if ((err = queue->dump_packets(msgs.max, msgs.msgs, count)) != srs_success) {
            return srs_error_wrap(err, "queue dumps packets");
        }
        //通过sdk把数据发给源站节点
         // sendout messages, all messages are freed by send_and_free_messages().
        if ((err = sdk->send_and_free_messages(msgs.msgs, count)) != srs_success) {
            return srs_error_wrap(err, "send messages");
        }
    }
}