“大学堂”:一个分布式课程视频共享系统

Realcourse: A Distributed Course Video Sharing System

"大学堂"是第一个采用分布式系统技术实现的、应用在中国多所高校的互联网教学视频发布和分享系统。大学堂系统通过树状的分布式文件系统目录(课程目录树)来表达系统中的整棵学科课程树。每一台系统服务器本地保存着全部或部分课程目录树,服务器间通过异步消息中间件来维持课程目录树的一致性。系统中通过冗余备份的策略来屏蔽服务器故障,保证系统视频资源可以被正常访问;通过对课程目录树进行分区管理,实现了对系统服务器的有效管理及服务器间负载均衡,提高了系统的可扩展性。系统采用基于带宽探测的服务器选择机制,选择出与用户连接带宽最大的服务器为用户提供服务,促进了用户访问体验的改善。自从2003年系统发布以来,大学堂已经稳定运行了七年时间,其间为数以百万计的用户提供过课程视频服务,收到了良好的社会效果。

＂Realcourse＂ is the first educational video publishing and sharing system based on Internet in China, it has been deployed onto a cooperative network covering several Colleges. Realcourse organize subjects and courses with hierarchical structure. It uses the course directory and related index files, the combination of which is called course directory tree, to represent the real hierarchical knowledge structure of the whole course. Each course directory tree is wholly or partially maintained by one of Realcourse servers, and the asynchronous message communication mechanism between the servers gives a consistently wholistic view of the course directory tree. To maximize availability and scalability, the global course directory tree is partitioned in Realcourse, and video files are orderly backuped to prevent server failures. The system adopts a server choosing strategy based on bandwidth-probing to pick out the server which has the highest bandwidth access to a certain end user to serve his requests, this technique greatly enhanced user experiences. Since it was released in 2003, the Realcourse system has been running stably for 7 years while providing video delivery service for millions of users, which has benefited the whole society a lot.